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DXP Saturn User`s Manual

1. Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 Parameters Eg FIPPI RunTasks DACs JELEN MM Update Slope FineGan 73 Cance SlowGap f3 M Update Baseline VenFineGain fe Peaklnt 30 M Subtract Baseline Offset fist FastLen 4 V Residual Baseline SlopeDac Rio a fo TT Plot DeltaBaseline SlopeRef f Tere E T FIR Baseline fiter SlowThresh 0 I Dynamic ThreshAdj PeakSamp Ge Reset Time fo microseconds Minwidth 4 Baseline running average length ee samples Maxwidth 20 Baseline histogram scaling factor 20 Deden a Baseline cut PWM x 0 0499877 J Enable cut Slow fiter bins per MCA bin 7 ADC trace Sampling 0 15 microseconds Bins in MCA 1oz4 Figure 2 10 Edit gt Params dialog At this point you may wish to fine tune some of the parameters These can be found on the Edit gt Params dialog shown in Figure 2 10 What follows is description of some of parameters e FiPPI Parameters These parameters are described in the DXP users manual except Slow Threshold Some of these may require small amounts of tweaking to get best performance If you are working at low energies you may want to adjust FastLen default value 4 Increasing this value will allow you to lower your energy threshold Here are the rules to keep in mind If FastLen gt C FastLen then Threshold gt C Threshold keeps energy threshold unchanged MaxWidth gt MaxWidth 2 C 1 FastLen Subject to the f
2. rrrrrrererere rece reeziziz iz en ieeeriezezenene i Contents lisci iaia Aneta Cue Ana he i Guide for installation se ninoi aie n EAE A AEA E ceialeiaindeieh ile ls i Notes EN A R R E NRA E ANER i SAL CUY AE EE AEE EEA NEE casio EE EATE EEE ETE v SYM DOlS oia iie EER ERN EEEE EER E NE v Specitic PrecautionSs cicalino v WARRANTY AA vi ROAA a a A E zeniseenizizenionizeonizinezoneseonizinezzonezecnee 1 T1 DXPS turn Features cacti N dll R E EN 1 1 2 Unit Specifications aeieeiaii a 1 2 Getting Started with the DXP Saturn sesesesesrereerereereseseseesereeresesesnesescesesesesneseseeresesesseses 3 2 1 Setting up the DXP Saturn iii 3 2 2 Making Connections eci 5 23 DXP Saturn Firmware hanse iosas eui i a tes E E A tein Ee a Eia ea 5 2 4 Installing and using the DxpDemo Program 6 3 Digital Filtering Theory DXP Structure and Theory of Operationi 17 3 1 X ray Detection and Preamplifier Operation i 17 3 2 X ray Energy Measurement amp Noise Filteringi iii 17 3 3 Trapezoidal Filtering in the DXPi 19 34 Baseline Issues eraci onneani eiae aa i arie 20 3 5 X ray Detection amp Threshold Setting ii 22 3 6 Energy Measurement with Resistive Feedback Preamplifiers 23 3 7 Pilexap specion eien a n E E EE E AR E 26 3 8 Input Count Rate ICR and Output Count Rate OCR n 27 3 9 Throughputi nisa
3. 42152 SPLO 0 FASTGAP 0 SPECTLEN 14000 REALTIMEO 77 SPHI 0 THRESHOLD 22 BASESTART 3072 REALTIME1 12478 FPLO 0 MINWIDTH 4 BASELEN 1024 EVTSINRUNO 0 FPHI 0 MAXWIDTH 20 EVTBSTART 1024 EVTSINRUNI 17635 BASEEVTSO 10 SLOWTHRESH 0 EVTBLEN 1024 I Unsigned UNDRFLOWSO BASEEVTS1 44489 PEAKSAM 29 TCALSTART 400 UNDRFLOWS1 16 BASEMEANO 108 SLOPENOM 512 TCALLEN 256 OVERFLOWSO 0 BASEMEANI 36099 SLOPEMIN 100 RCALSTART 656 Download OVERFLOWS1 73 LIVETIME2 0 SLOPEMAX 1000 RCALLEN 256 FASTPEAKSO 0 REALTIME2 0 SGRANULAR 1638 HSTSTART 8192 FASTPEAKS1 22775 INTRCNTD WN 0 BLMIN 63484 HSTLEN 8000 NUMASCINTO 0 INTRFROCY 0 BLMAX 2048 USERI 0 NUMASCINT1 314 READY 0 OFFDACVAL 154 USER2 0 NUMRESETSO 0 SCALIMLO 0 FINEGAIN 73 USER3 0 NUMRESETS1 147 SCALIMHI 0 VRYFINGAIN 8 USER4 0 NUMUPSETSO 0 WHICHTEST 8 SLOPEVAL 31 USERS 0 NUMUPSETS1 0 RUNTASKS 106 SDACREF 16 USERE 0 NUMDRUPSO 0 BINFACTI 7 TRKDACVAL 246 USER 0 NUMDRUPS1 80 MCALIMLO 0 GRSAVE 8 USERS 0 NUMDRDOSO 0 MCALIMHI 1024 RAMPGEN 1 Figure 2 11 View gt Parameters shows all DXP control parameters and internal code variables Input Data Quality If there is some problem with the detector the DXP card or the connection between the two particularly ground loop problems or high frequency noise pickup then no amount of refining parameter settings will produce a good spectrum ADC Trace The first test of data quality is to look at an ADC trace Use View gt ADC to capture a trace of input signal values as seen at t
4. Because the FiPPI is implemented in a Xilinx field programmable gate array FPGA it may be reprogrammed for special purposes although this process is non trivial and would probably require XIA contract support 4 4 The Digital Signal Processor DSP The Digital Signal Processor acquires and processes event data from the FiPPI controls the ASC through DACs and communicates with the host The processor is an Analog Devices ADSP 2181 16 bit Fixed Point DSP optimized for fixed point arithmetic and high I O rates Different DSP program variants are used for different types of data acquisition and different preamplifier types Section 5 describes in detail the DSP operation its tasks and parameters which control them The ADSP 2181 has 16K words of 16 bit wide data memory and 16K words of 24 bit wide program memory part of which is used as data memory to hold the MCA spectrum If more memory is required for special purposes up to 4 Mbytes of extended memory can be added by specifying option M Transferring data to from these memory spaces is done through the DSP s built in IDMA port which does not interfere with the DSP program operation 32 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 4 5 Interface to the Host Computer Communications between the DXP and host computer occur through the Enhanced Parallel Port EPP and complies with IEEE specification 1284 Such a port is included in most Pentium class PCs
5. Figure Gains of 8 to 16 are possible thus reducing the required number of bits necessary to achi3eve the same resolution from 14 to 10 The generator required to produce this sawtooth function is quite simple comprising a current integrator with an adjustable offset The current which sets the slope is controlled by a DAC SLOPEDAC while the offset is controlled by adding a current pulse of either polarity using a second DAC TRACKDAC The DAC input values are set by the DSP which thereby gains the power to adjust the sawtooth generator in order to maintain the ASC output i e the Amplified Sawtooth Subtracted Data of Figure 4 2 within the ASC input range ADC Max Input Amplified Sawtooth Subtracted Data 1 0 ADG Mininput 5 2 5 007 Preamp a e Output o E Ici 10 L 20 7 ia Sawtooth Function j e 30 J Reset a Pa e mia maie e E a e a o a E a a aa a Level Preamp Sawtooth kfig 960923 T 0 1 2 3 4 5 Time ms Figure 4 2 A sawtooth function having the same average slope as the preamp output is subtracted from it and the difference amplified and offset to match the input range of the ADC 31 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 Occasionally as also shown in Figure 4 2 fluctuations in data arrival rate will cause the conditioned signal to pass outside the ADC input range This condition is detected by the FiPPI which has digital discrimination level
6. PCB Diagram 2 2 Making Connections 2 2 1 Connecting to the Detector Preamplifier The detector bias voltage connection is made using a standard SHV cable If your detector uses either MHV or BNC for this connection an adapter will be required The bias voltage inhibit and preamplifier signal connections are made using standard BNC cables Preamplifier power is supplied via the NIM standard DB 9 Preamplifier Power interface The DXP Saturn is specified for operation with cables under three meters in length 2 2 2 Connecting to the Computer The DXP Saturn connects to the parallel port of a standard PC The parallel port must be set to run in EPP mode you may need to adjust the port setup in your BIOS CMOS setup If this option is not available in the BIOS setup and there are problems communicating with the DXP Saturn the BIOS may need to be upgraded 2 3 DXP Saturn Firmware The DXP Saturn requires several firmware files to run these include the code for the onboard digital signal processor DSP as well as configuration files for the programmable logic chip used for the energy filtering the FiPPI for Filter Peak and Pileup Inspector The most recent versions of these files can be obtained from the XIA Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 ftp site ftp xia com in the pub DXP X10 directory The DSP code has file names of the form X10Pnnnn hex where nnnn is a version number FiPPI configuratio
7. Setting the LN Inhibit Mode and Default Level The Inhibit function shuts down the detector bias voltage under a user defined condition The TTL CMOS compatible input is typically connected to an external liquid nitrogen sensor that monitors the temperature of the detector and outputs a logic level or alternatively closes a switch ie contact closure reflecting whether the detector is safe to operate The polarity of the asserted Inhibit signal is set with dual pole jumper JP21 The two positions are labeled 0 and 1 corresponding to the logic level at which the bias voltage is inhibited Default operation for the open circuit condition eg contact closure systems or Inhibit simply not used is selected w JP20 It is essentially a pull up pull down switch for the Inhibit logic input Again the two positions are labeled 0 and 1 corresponding to the default logic level applied The DXP Saturn is shipped w the Inhibit assert level set to 0 and the default level set to 1 NOTE as many types of sensors are commercially available it is difficult to guarantee proper performance with all types It is strongly recommended that the functionality of the chosen configuration be tested prior to making connections to the detector While the DXP Saturn top panel is removed proceed to the Internal Jumper Settings section below Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 gt Setting the Bias Voltage The DXP Satu
8. amount of charge Qx produced when the x ray is absorbed in the detector This Fano noise of adds in quadrature with the electronic noise so that the total noise o in measuring Vx is found from G sqrt of oe Equation 3 4 The Fano noise is only a property of the detector material The electronic noise on the other hand may have contributions from both the preamplifier and the amplifier When the preamplifier and amplifier are both well 20 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 designed and well matched however the amplifier s noise contribution should be essentially negligible Achieving this in the mixed analog digital environment of a digital pulse processor is a non trivial task however In the general case however the mean baseline value is not zero This situation arises whenever the slope of the preamplifier signal is not zero between x ray pulses This can be seen from Equation 3 2 When the slope is not zero the mean values of the two sums will differ because they are taken over regions separated in time by L G on average Such non zero slopes can arise from various causes of which the most common is detector leakage current When the mean baseline value is not zero it must be determined and subtracted from measured peak values in order to determine Vx values accurately If the error introduced by this subtraction is not to significantly increase ot then the error in the baseline esti
9. sets the gain of the amplifier stages 30 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 preceding the ADC Then if the preamplifier s full scale voltage range is Vmax it must digitize to N bits where N is given by N log 10 Vmax AV log 10 2 Equation 4 1 For a typical high resolution spectrometer N must be 14 to 15 However 14 bit ADCs operating in excess of 10 MSA are very expensive particularly if their integral and differential non linearities are less than 1 least significant bit LSB At the time of this writing a 10 bit 20 MSA ADC costs less than 10 while a 14 bit 5 MSA ADC costs nearly 500 which would more than triple the parts cost per channel The ASC circumvents this problem using a novel dynamic range technology for which XIA has received a patent which is indicated in Figure 4 2 Here a resetting preamplifier output is shown which cycles between about 3 0 and 0 5 volts We observe that it is not the overall function which is of interest but rather the individual steps such as shown in Fig 3 1b which carry the x ray amplitude information Thus if we know the average slope of the preamp output we can generate a sawtooth function which has this average slope and restarts each time the preamplifier is reset as shown in Figure 4 2 If we then subtract this sawtooth from the preamplifier signal we can amplify the difference signal to match the ADC s input range also as indicated in the
10. the basewidth 2L G This may be compared to analog filtered pulses which have tails which may persist up to 40 of the peaking time a phenomenon due to the finite bandwidth of the analog filter As we shall see below this sharp termination gives the digital filter a definite rate advantage in pileup free throughput o Filtered Step S kfig 960920 4 S 2 E gt 2 o 5 0 joooonononopanan0 E o ooooo000 6 lt L gt boooooooo000000 gt L G 2 D daga 246 gt Preamp Output mV Filter Output mV 24 26 28 30 32 Time us Figure 3 3 Trapezoidal filtering the Preamp Output data of Figure 3 2 with L 20 and G 4 3 4 Baseline Issues Figure 3 4 shows the same event as is Figure 3 3 but over a longer time interval to show how the filter treats the preamplifier noise in regions when no x ray pulses are present As may be seen the effect of the filter is both to reduce the amplitude of the fluctuations and reduce their high frequency content This signal is termed the baseline because it establishes the reference level from which the x ray peak amplitude Vx is to be measured The fluctuations in the baseline have a standard deviation ce which is referred to as the electronic noise of the system a number which depends on the peaking time of the filter used Riding on top of this noise the x ray peaks contribute an additional noise term the Fano noise which arises from statistical fluctuations in the
11. the preamplifier output If the preamp signal is carried on the DB9 connector then you could simply plug the scope directly into the DXP s BNC without a tee If the preamp output passes through zero volts you can keep the scope DC coupled otherwise AC coupling will help Set the time base to 10 usec per box and the vertical gain to say 10 mV box Trigger the scope on edge transitions with the appropriate polarity Measure the step height for x rays of known energy For example if the step height for an Fe 55 source is 30 mV then the detector gain would be 30 5 9 5 1 mV keV Region of Interest ROD The ROI is used to choose bins for fitting peak positions and widths FWHMs The Menu item ROI gt Delete All will delete all ROI s from the spectrum If no ROI s are defined the program will use the largest peak to define a ROI To add an ROI use ROI gt Add a dialog will appear for entering the energy you want to associate with the ROI e g energy ofa line The dialog is dismissed when lt Return gt is typed Then select the ROI with the mouse as follows Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 1 Position the mouse ABOVE the middle of the peak you want to select 2 Press and hold the left mouse button 3 SLOWLY move the mouse towards the left edge of the ROI Bins will acquire a yellow fill as they are added to the ROI 4 Move the mouse to the right edge of the ROI again bins added to the ROI will acq
12. the very best filtering is accomplished by using cusp like weights and time variant filter length selection There are serious costs associated with this approach however both in terms of computational power required to evaluate the sums in real time and in the complexity of the electronics required to generate usually from stored coefficients normalized w sets on a pulse by pulse basis A few such systems have been produced but typically cost about 13K per channel and are count rate limited to about 30 Kcps Even time invariant systems with cusp like filters are still expensive due to the computational power required to rapidly execute strings of multiply and adds One commercial system exists which can process over 100 Kcps but it too costs over 12K per channel The DXP processing system developed by XIA takes a different approach because it was optimized for very high speed operation and low cost per channel It implements a fixed length filter with all wj values equal to unity and in fact computes this sum afresh for each new signal value k Thus the equation implemented is k L G k LV VT 2 Vi i k 20 G 1 i koL 14 Equation 3 2 where the filter length is L and the gap is G The factor L multiplying Vx k arises because the sum of the weights here is not normalized Accommodating this factor is trivial for the DXP s host software In the DXP Equation 3 2 is actually implemented in hardwired logic by noting the recursion relationship b
13. time 3 30s 1 2 5 Power Requirements e AC Line Voltage Frequency 115V 60Hz 230V 50Hz e Current Draw 200mA 100mA Supply voltage fluctuations are not to exceed 10 of the nominal value All DC voltages necessary for operation are generated internally In addition the DXP Saturn produces 12V and 24V up to 100mA per voltage to power an external preamplifier 1 2 6 Environment e Temperature Range 0 C 50 C e Maximum Relative Humidity 75 e Maximum Altitude 3 000 meters e Pollution degree 2 e Installation Category II Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 2 Getting Started with the DXP Saturn 2 1 Setting up the DXP Saturn 2 1 1 Line Voltage Selection and Fusing The DXP Saturn can be set up to run on either 115 VAC or 230 VAC at 50 60 Hz The recessed Line Select switch on the rear panel must be set to the appropriate position prior to powering the unit CAUTION Failure to properly set the Line Select switch before powering the unit can result in damage to the DXP Saturn and connected equipment Use only Time Lag 5mm x 20mm IEC 127 2 IIT 250mA fuses rated for 250V A spare fuse is provided in the fuse drawer located at the power entry point 2 1 2 Detector Bias Voltage Settings The DXP Saturn provides a detector bias voltage of up to 1000V The output impedance is 10MegQ due to low pass filtering thus the current is limited to a few microamperes Do not connect the
14. 5535 HIGHGAIN Parameter High gain relay setting INPUTENABLE Parameter Input Enable relay setting ASC Control Parameters and Calibrations pulsed reset variants RESETWAIT Parameter Quick Reset time 25ns units RESETINT Parameter Reset time 0 25 usec units SLOPEDAC Calibration Current Slope DAC value 16 bit serial DAC range 0 65535 SLOPEZERO Calibration Slope DAC zero value approximately center of range SLOPEVAL Calibration Abs SLOPEDAC SLOPEZERO SGRANULAR Parameter Slope DAC step size TRKDACVAL Parameter Tracking DAC value 12 bit parallel TDACWIDTH Parameter Track DAC pulse width 50 ns units TDQPERADC Calibration TDQPERADCE Calibration ASC Control Parameters and Calibrations RC feedback variants OFFSETDAC Parameter Current offset DAC value 16 bit serial DAC range 0 65535 OFFSETSTEP Parameter Offset DAC step size TAURC Parameter Preamplifer decay constant in 25 ns units RCF variant only RCFCOR Calibration Preamplifer decay correction RCF variant only Miscellaneous Constants SPECTSTART Constant Address of MCA spectrum in program memory SPECTLEN Constant Length of MCA spectrum buffer BASESTART Constant Address of baseline histogram in data memory offset by 0x4000 BASELEN Constant Length of baseline histogram EVTBSTART Constant Address of event buffer in data memory offset by 0x4000 EVTBLEN Constant Length of baseline histogram HSTSTART Constant Address of history buffer in data memory offset by 0x4000 HST
15. 8 r FPGAErr Set if FiPPI configuration download error 9 f DSPErr Set of DSP error condition exists ll r Active Set if data acquisition is in progress 0x8001 0x8002 Diagnostic or special purpose registers 0x8003 FiPPI configuration register 33 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 DXP Saturn DSP Code Description 5 1 Introduction and Program Overview The following sections are intended to provide the DXP user with a good understanding of the various tasks performed by the DSP in the DXP Saturn The DSP performs several functions 1 Respondto input and output calls from the host computer to start and stop data collection runs download control parameters and upload collected data 2 Perform system calibration measurements by varying the various DAC settings under its control and noting the output change at the ADC 3 Make initial measurements of the slow filter baseline and preamplifier slope value at the start of data taking runs to assure optimum starting parameter values 4 Collect data a Read energy values Ex from the FiPPI under interrupt control and store them in DSP buffer memory in less than 0 25 us b Adjust the ASC control parameters under interrupt control to maintain its output within the ADC s input range c Process captured Ex values to build the x ray spectrum in DSP memory d Sample the FiPPI slow filter baseline and build a spectrum of its values in order to compute the bas
16. FFSETDAC 1 Acquire ADC trace in history buffer 2 Gain calib measure TDACPERADC 3 Slope calibration measure SLOPEMULT 4 Measure ADC non linearity 5 Not currently used 6 Put DSP to sleep while FPGA logic is downloaded 7 RESET calibration measure TRACKRST 8 OffsetDAC calibration measure OFFDACVAL 9 10 Not currently used 11 Program Fippi 12 Set internal polarity to current value of POLARITY parameter 13 Close input relay 14 Open input relay 15 RC feedback calibration trace of baseline filter and decimator values 16 RC feedback calibration trace of event filter and decimator values Table 5 4 Special tasks and test segments that can be selected with the DSP parameter WHICHTEST 43 Manual DXP Saturn Digital X ray Processor 5 10 DSP Parameter Descriptions mdo DXP Saturn UM001 5 As noted above DSP operation is based on a number of parameters Some are control parameters required to operate the DXP some are calibration values determined by the DSP and others are run statistics Variable Type Description Reference PROGNUM Constant Program variant number CODEREV Constant Current DSP program revision HDWRVAR Constant Hardware variant DSP reads this from interface FPGA FIPPIREV Constant FiPPI design revision DSP reads this from FiPPI FPGA FIPPIVAR Constant FiPPI design variant DSP reads this from FiPPI FPGA DECIMATION Constant Slow filter decimation factor DSP reads this from FiPPI FPGA RU
17. LEN Constant Length of history buffer NUMSCA Parameter Number of SCA regions defined mapping variants only SCAXLO x 0 23 Parameter Lower MCA channel for SCA region x mapping variants only SCAXHI x 0 23 Parameter Upper MCA channel for SCA region x mapping variants only USER1 USER8 User User variables Host software can use these for any purposes Table 5 5 Summary of DSP parameter definitions 45 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 10 1 Specifying fixed run lengths PRESET PRESETLENO 1 By default the DXP Saturn acquires data until a stop command is received from the host A fixed run length can be specified using the parameters PRESET and PRESETLENO 1 as follows PRESET specifies the type of run 0 indefinite default 1 fixed realtime 2 fixed livetime 3 fixed output events 4 fixed input counts PRESETLENO PRESETLENI specifies the length of preset fixed length run as a 32 bit quantity For fixed real time or live time the units are 800 nanosecond intervals 5 10 2 Setting the slow filter parameters SLOWLEN SLOWGAP In general the user does not modify these parameters directly but through the host software routine SetAcquisitionValues See Appendix The DXP uses a trapezoidal filter characterized by the peaking time Tp and flat top time Tf The peaking time is determined by the SLOWLEN and DECIMATION values DECIMATION is automatically sensed by the DSP and should n
18. NIDENT Returned Run identifier RUNERROR Returned Error code if run is aborted 0 for success BUSY Returned DSPs current acquisition status Values listed below Acquisition Statistics LIVETIME0 1 2 Statistic DAQ live time in 800 nsec units REALTIMEO 1 2 Statistic Elapsed acquisition time in 800 nsec units EVTSINRUNO 1 Statistic Number of events in MCA spectrum UNDRFLOWSO0 1 Statistic Number of MCA underflow events OVERFLOWS0 1 Statistic Number of MCA overflow events FASTPEAKSO 1 Statistic Number of input events detected by FiPPI NUMASCINTO 1 Statistic Number of ASC interrupts NUMRESETSO 1 Statistic Number of reset events seen NUMUPSETSO 1 Statistic Number of upset events seen NUMDRUPS0 1 Statistic Number of drift up events seen NUMDRDOS0 1 Statistic Number of drift down events seen NUMZIGZAG0 1 Statistic Number of zigzag events seen BASEEVTSO 1 Statistic Number of baseline events acquired BASEMEANQO 1 Statistic Updating mean baseline value Control parameters WHICHTEST Parameter Which test segment to execute RUNTASKS Parameter Which tasks will be executed in run sequence BINFACTI Parameter MCA binning factor MCALIMLO Parameter Lower limit of MCA spectrum MCALIMHI Parameter Upper limit of MCA spectrum TRACEWAIT Parameter ADC trace time factor ASCTIMOUT Parameter Timeout for ASCSetup in tenths of seconds YELLOWTHR Parameter Medium rate throughput threshold for front panel LED REDTHR Parameter High rat
19. Note To use this variant the Ramp Offset jumper should be in the Ramp position 5 11 2 MCA acquisition with resistive feedback preamplifiers variant 2 This firmware variant is intended for use with resistive feedback preamplifiers described in Section 3 6 Firmware files Program file name X10PRC0103 HEX Fippi file names F02X10PxG FIP x 0 2 4 6 Additional parameters described in Section 5 10 TAURC Exponential decay time in 50 ns units RCFCOR Correction factor calculated automatically at start of run if TAURC not 0 Note To use this variant the Ramp Offset jumper should be in the Offset position 48 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 Appendix A DPP OEM Revision D 2 This addendum to the DXP Saturn User s Manual is provided for DXP OEM customers It summarizes jumper settings connector locations part numbers and pinouts and power consumption calculations for the DPP X10P Revision D 2 digital x ray processor circuit board GATE BNC SYNC BNC IEEE 1284 EPP DSUB 25 F PREAMP POWER DSUB 9 F INPUT BNC 0 nocccccscencoce STATUS I O RATE LED Indicators Jumper Settings Reference Jumper Label Position Labels Description Chassis and internal ground not connected JP1 _ GND symbol Chassis connected to internal ground Improves signal integrity in some cases but can introdu
20. SINRUN LIVETIME Equation 3 7 3 9 Throughput Figure 3 9 shows how the values of ICRm and OCR vary with true input count rate for the DXP and compare these results to those from a common analog shaping amplifier plus SCA system The data were taken at a synchrotron source using a detector looking at a CuO target illuminated by x rays slightly above the Cu K edge Intensity was varied by scanning a pair of slits across the input x ray beam so that its harmonic content remained constant with varying intensity 200 e DXP OCR s DXPICR Analog OCR Analog ICR 150 True ICR ry 2 o KI 4 100 S o oO 5 S gt e 50 ICR OCR Plot kfig 960922 o4 0 50 100 150 200 Input Count Rate kcps Figure 3 9 Curves of ICRm and OCR for the DXP using 2 us peaking time compared to a common analog SCA system using us peaking time Table 3 1 Comparing the deadtime per event for the DXP and an analog shaping amplifier Notice that that the DXP produces a comparable output count rate even though its peaking time is nearly twice as long Functionally the OCR in both cases is seen to initially rise with increasing ICR and then saturate at higher ICR levels The theoretical form from Poisson statistics for a channel which suffers from paralyzable extending dead time Ref 4 is given by 28 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 OCR ICR exp ICR ta Equatio
21. Saturn Documentation version 1 5 Guide for installation The host software runs on Windows 95 98 Me and NT If you are updating from a previous version you should not need to uninstall the previous version before installing this new version Please direct your questions or comments to software_support xia com Installation from CD rom To install from CD rom run the program setup exe in the Installation directory You will be prompted for the location for the program files If not already installed the DriverLinx Port IO driver DLPORTIO needs to be installed also The DLPORTIO driver is included in the release with the file name PORT9SNT EXE To install the driver just run the program PORT9SNT EXE If installing under WindowsNT you should be logged on as Administrator Notes The DSP program HEX file format has been changed in the parameter table section Where the previous version had a comment header in the first column followed by the number of parameters and the list of parameters one per row in the new format the list of parameters includes access codes for read only for read write and allowable bounds for the read write parameters For example PROGNUM the program number PROGNUM is read only WHICHTEST 0 15 the test segment WHICHTEST is read write with valid range 0 15 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 Release Notes for the DXP Saturn Version 3 0
22. User s Manual Digital X ray Processor Saturn Revision A Release 3 0 X ray Instrumentation Associates 8450 Central Ave Newark CA 94560 USA Tel 510 494 9020 Fax 510 494 9040 http www xia com Information furnished by X ray Instrumentation Associates XIA is believed to be accurate and reliable However no responsibility is assumed by XIA for its use nor for any infringements of patents or other rights of third parties which may result from its use No license is granted by implication or otherwise under any patent or patent rights of XIA XIA reserves the right to change specifications at any time without notice Patents have been applied for to cover various aspects of the design of the DXP Digital X ray Processor Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 9 15 02 Release Notes for the DXP Saturn Version 3 0 Contents This note accompanies release 3 0 of the host software and firmware for the DXP Saturn Please check the DXP Software page of XIA s website www xia com DXP_Software html for the upcoming release of This release includes the following components Firmware files DSP programs version 1 0 X10P HEX Standard pulsed reset variant3 Fippi FPGA firmware revision J FXPDxG FIP x 0 2 4 6 Standard pulsed reset variant Host software XiaSystems DLL XiaSystems interface library XiaDemo exe version 3 0 Demo program for setting up and acquiring data Documentation DXP
23. acquisition when asserted polarity selectable Bomar P N 364A595BL J8 Sync Input BNC timing signal for time resolved spectroscopy and other special modes Bomar P N 364A595BL J9 DC Power Entry 0 100 Header with lock ramp input from the DC power supply Molex P N 22 23 2121 Pin Name Description 1 GND Internal ground connection NOT chassis ground 2 VCC_RAW 5V DC supply regulated on board to 3 3V DC for digital components 3 GND Internal ground connection NOT chassis ground 4 5V_RAW 5V DC supply for on board analog components Optional preamplifier supply individually or substituted for 12V both options require soldering 5 5V_RAW 5V DC supply for on board analog components Optional preamplifier supply substituted for 12V requires soldering 6 12V_RAW 12V DC supply for on board analog components Standard supply for preamplifier 7 12V_RAW 12V DC supply for on board analog components Standard supply for preamplifier 8 24V_RAW 24V DC analog supply for preamplifier not used by DPP 9 24V_RAW 24V DC analog supply for preamplifier not used by DPP 10 HV_INHIBIT AV Inhibit output only used in conjunction with PWR X10P supply 11 EXT_INHIBIT AV Inhibit input only used in conjunction with PWR X10P supply 12 GND Internal ground connection NOT chassis ground 50 Manual DXP Saturn Digi
24. and if not a very inexpensive card can be added The DXP Saturn interface is implemented in an FPGA which can be thus be relatively easily modified by a PROM upgrade Access to the DXP Saturn is supported through the XiaSystems DLL on Windows 95 98 NT platforms In this way a programmer needs to know very little about the interface specifics The host application is responsible for downloading firmware to the FiPPI software to the DSP program memory segment and parameters to the DSP data memory segment The Control Status Register CSR is used to control the downloading of firmware and the starting and stopping data acquisition Reading and writing to the DSP program download parameter download spectrum upload takes place directly through an IDMA transfer These transfers involve first writing an address to the EPP address port followed by one or more reads writes from to the EPP data port The following is the address space of the DXP Addresses 0x0000 0x7FDF map directly into the on board DSP while those addresses greater the 07FFF are decoded by the DXP interface circuit 0x0000 0x3FFF DSP Program memory Contains the DSP instructions and 24 bit data 0x4000 0x7FDF DSP Data memory 16 bit data including the parameters memory 0x7FEO 0x7FFF reserved 0x8000 Control Status Register CSR bit access Name Meaning 0 rw RunEnable Disable 0 or Enable 1 data acquisition 1 rw NewRunUpdate 0 or Reset 1 spectra statistics at run start
25. ble at the top followed by the code generated by the Analog Devices 218x development system The internal data memory area is subdivided into three sections The first section starting at location 0x4000 contains DSP parameters and constants both those used for controlling the DSP s actions and those produced by the DSP during normal running These parameters and their addresses are listed and described in the following sections When these parameters are referred to they will be denoted by all capital letters e g RUNTASKS The locations of parameters can and for forward compatibility should be determined from the symbol table The second section of data memory contains acquired monitoring data such as the baseline event histogram The third section of internal data memory is used as a circular buffer for storing events from the FiPPI Note that future hardware revisions may eliminate the need for this buffer area in which case it could be switched to more histogramming area 35 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 2 Program Flow The flow of the DSP program is illustrated in Figure 5 1 It is essentially identical for all program variants The structure is very simple after initialization the DSP enters an idle phase waiting for a signal from the host to start a run During this idle phase the DSP is continuously collecting baseline events from the FiPPI as well as monitoring the Analog Signal Conditi
26. buffer large enough to hold all N samples is necessary For this reason the length of the finite response filter is limited to 1024 The filter length is stored in the parameter BLFILTERF 5 5 3 Baseline Histogram As part of the baseline processing all valid baseline samples are entered into the baseline histogram which occupies 1024 words of data memory The baseline histogram can be very useful in monitoring or evaluating the performance of the DXP Saturn The parameter BASESTART contains the pointer to the location of the histogram in data memory and the length nominally 1K is contained in the parameter BASELEN The baseline histogram is centered about a zero baseline The parameter BASEBINNING determines the granularity of the histogram 2 BASEBINNING baseline values are combined into one bin of the baseline histogram The default value of BASEBINNING is 2 i e the baseline value is divided by 4 to determine the bin All valid baseline values are included in the histogram even if there is a baseline cut in use 39 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 The baseline histogram is only filled during a normal datataking run when the DSP is idle the baseline average is calculated but the histogram is not filled Since the baseline histogram is stored in data memory 16 bit words are used to record the bin contents As a result the histogram overflows quite often the time to overflow depends on the baseli
27. ce a ground loop JP9 amp INPUT DSUB 9 Signal entry via DSUB 9 connector JP10 BNC Signal entry via BNC connector HV INHIBIT 0 HV inhibited when BNC input is LO JP20 ASSERT 1 HV inhibited when BNC input is HI LEVEL JP21 HV INHIBIT 0 If BNC disconnected asserted level is LO DEFAULT 1 If BNC disconnected asserted level is HI JP100 amp INPUT SINGLE ENDED Single ended input configuration standard JP101 DIFFERENTIAL Differential input configuration rare 12dB ATTEN input signal divided by four 12dB attenuation P 0dB ATTEN input signal not divided Odb attenuation nie INERTE The 12dB setting should be selected if the preamplifier output voltage exceeds 10V RAMP Setting for pulsed reset preamplifiers SEA MODE OFFSET Setting for resistive feedback preamplifiers 49 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 LED Indicators DI Status LED Red illuminated when an error condition is present QT Optoelectronics P N MV67539 MP7 D2 I O LED Yellow illuminated during EPP transfers QT Optoelectronics P N MV63539 MP7 D2 Rate LED Red Green bi color LED flashes at a frequency proportional to x ray event rate Flashes green yellow or red depending on the processor dead time Gilway P N E250 Connectors J101 Signal Input BNC connects preamplifier output to the DPP Bomar P N 364A595BL J7 Gate Input BNC halts data
28. e Finite Impulse Response FIR filter to Use Infinite Impulse Response IIR filter to calculate baseline average calculate baseline average DEFAULT 3 Acquire baseline values for histogramming and Disable baseline acquisition averaging DEFAULT 4 Adjust fast filter threshold to compensate for Disable fast filter threshold adjustment rate shifts 5 Correct for baseline shift either in FiPPI pulse Disable baseline correction reset or DSP RC feedback DEFAULT 6 Apply residual baseline correction DEFAULT No residual baseline correction 7 Continuously write baseline values to baseline Disable writing baseline values to baseline history history circular buffer DEFAULT circular buffer 8 Indicates special task or calibration run specified Indicates normal acquisition run by WHICHTEST 9 Histogram DeltaBaseline Histogram raw baseline DEFAULT baseline lt baseline gt 10 Enable baseline cut Disable baseline cut DEFAULT 11 15 Reserved set to 0 Reserved set to 0 Table 5 3 Data acquisition tasks controlled by the DSP parameter RUNTASKS 42 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 9 Special Tasks WHICHTEST Special tasks are selected by starting a run with bit 8 of the RUNTASKS parameter set The following tasks are currently supported Number Test Segment 0 Set ASC DAC values to current value of GAINDAC SLOPEDAC and or O
29. e ful MCA spectrum and use the spectrum as an aid in choosing the limits for each SCA The reduced amount of data storage in SCA mapping mode is very useful in time resolved spectroscopy or scanning applications where separate spectral data are desired for many different time or spatial points 38 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 5 Baseline Measurement The DSP collects baseline data from the FiPPI whenever there are no events to process both during a run and between runs when there are never events to process The DSP keeps a running average of the most recent baseline samples this average is written back into the FiPPI where it is subtracted from the raw energy filter value to get the true energy The baseline data read from the FiPPI is just the raw output of the energy filter One bit of the baseline register is used to indicate whether the sample occurred while an event was in progress in which case it is not used Two methods are available to determine the average baseline value By default an infinite impulse response IIR filter is used where the baseline average is calculated by combining a new baseline sample with the old average using weights x and 1 x respectively where x is typically 1 64 By setting the appropriate bit in the parameter RUNTASKS see below a finite impulse response FIR filter is used where the baseline mean is just the straight average of the N most recent baselin
30. e menu item ROI gt Delete All and then click Update Otherwise you will need delete the ROI and redefine it see Appendix You should acquire reasonable statistics gt 100k events in peak If the fitted peak position under lt E gt in the text box agrees with the calibration energy the DXP is said to be calibrated Otherwise you must again perform DXP gt Reconfigure This time there is no need to change any of the fields accepting the values simply iterates on the gain calibration After calibration with 10 eV MCA choice the display may look something like the following linear scale Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 sie DxpDemo Figure 2 7 Main window after DXP gt Reconfigure Linear Scale Figure 2 8 Main window after DXP gt Reconfigure Log Scale Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 View gt Baseline The baseline histogram shows what the FiPPI output looks like when there are no X rays present Figure 2 9 shows the baseline histogram for a good quality detector The high energy tail arises from partial charge collection events that do not satisfy the X ray selection requirements Crudely speaking that which is not an X ray can be considered baseline Ideally this display should be Gaussian shaped and this sample shows Gaussian behavior over almost 3 decades oi DxpDemo Figure 2 9 Baseline histogram Log scale
31. e of 5 x ray pulses separated by various intervals to show the origin of both slow channel and fast channel pileup and demonstrate how the two cases are detected by the DXP 3 8 Input Count Rate ICR and Output Count Rate OCR During data acquisition x rays will be absorbed in the detector at some rate This is the true input count rate which we will refer to as ICR Because of fast channel pileup not all of these will be detected by the DXP s x ray pulse detection circuitry which will thus report a measured input count rate ICRm which will be less than ICR This phenomenon it should be noted is a characteristic of all x ray detection circuits whether analog or digital and is not specific to the DXP Of the detected x rays some fraction will also satisfy both fast and slow channel pileup tests and have their values of Vx captured and placed into the spectrum This number is the output count rate which we refer to as the OCR The DXP normally returns in addition to the collected spectrum the actual time LIVETIME for which data was collected together with the number FASTPEAKS of fast peaks detected and the number of Vx captured events 27 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 EVTSINRUN From these values both the OCR and ICRm can be computed according to Equation 3 7 These values can then be used to make deadtime corrections as discussed in the next section ICR FASTPEAKS LIVETIME OCR EVT
32. e samples Both averaging methods are described in more detail in the following sections The baseline mean is stored with 32 bit precision in the parameters BASEMEANO high order word and BASEMEANI 5 5 1 IIR Infinite Impulse Response Filter By default the baseline mean is calculated using an infinite impulse response filter characterized in the following way lt Bi gt LRRD N Equation 5 1 where lt Bi gt is the baseline mean after the ith baseline sample Bi is the ith baseline sample and lt Bi gt is the baseline mean before the ith sample With this filter the most recent baseline samples are weighted the most but up to the precision of the stored mean value all baseline values have a small effect on the mean hence the infinite in the name The length of the filter is controlled by the parameter BLFILTER which holds the value 1 N in 16 bit fixed point notation which has sign bit and 15 binary bits to the right of the decimal point Expressed as a positive integer BLFILTER 1 N 2 15 The default value for BLFILTER corresponds to N 64 Interpreting BLFILTER as an integer gives 1 64 2 15 2 9 512 5 5 2 FIR Finite Impulse Response Filter By setting the appropriate RUNTASKS bit it is possible to choose a finite impulse filter to calculate the baseline mean With this filter a straight average of the N most recent valid baseline samples is used to calculate the mean To implement this filter a
33. e throughput threshold for front panel LED PRESET Parameter Preset type 0 none l real time 2 live time 3 output cts 4 input cts PRESETLENO 1 Parameter Preset run length FiPPI Digital Filter Event selection parameters SLOWLEN Parameter Slow filter length SLOWGAP Parameter Slow filter gap PEAKINT Parameter Peak interval FASTLEN Parameter Fast filter length FASTGAP Parameter Fast filter gap THRESHOLD Parameter Threshold value for fast filter trigger range 1 255 0 disables MINWIDTH Parameter Minimum peak width MAXWIDTH Parameter Maximum peak width SLOWTHRESH Parameter Threshold for slow filter trigger range 1 255 0 disables PEAKSAM Parameter Peak sampling time Baseline Related Parameters 44 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 BLFILTER Parameter Filtering parameter for baseline IIR filtering BLFILTERF Parameter Filtering parameter for baseline FIR filtering BASEBINNING Parameter Baseline binning for histogram 0 finest to 6 coarsest BLCUT Parameter DSP baseline cut cut at BLCUT FWHM units defined below BLMIN Calibration Min baseline value accepted in average calculated from BLCUT BLMAX Calibration Max baseline value accepted in average calculated from BLCUT ASC Control Parameters and Calibrations all variants POLARITY Parameter Preamplifier signal polarity O negative step l positive step GAINDAC Parameter Current Gain DAC value 16 bit serial DAC range 0 6
34. eline offset for Ex values Several DSP program variants are available to cover a range of applications The standard program Variant 0 provided with the DXP Saturn is for typical x ray fluorescence spectroscopy using a pulsed reset preamplifier Additional program variants listed in Table 5 1 are available for other applications including hardware diagnostics Other specialized measurements including 1 x ray mapping 2 Quick XAFS scanning 3 switching between multiple spectra synchronously with an experimentally derived signal e g Phased locked EXAFS and 4 time resolved spectroscopy e g multi channel scaling The variants which have been released to date are described in Section 5 11 Several other variants have been developed for particular customers and may be made available upon request Variant Name Standard Application Configuration 0 X10P General x ray spectroscopy data acquisition for pulsed reset preamplifiers Single MCA per channel with up to 8K bins 1 X10PRC General x ray spectroscopy data acquisition for RC feedback preamplifiers Single MCA per channel with up to 8K bins 2 X10PDIAG Hardware diagnostics for testing purposes Table 5 1 DSP Software Variants By convention the DSP programs are named NAMEmmnn HEX where NAME is the variant name listed in the table mm and nn are major and minor version numbers respectively The hex file format is an ascii with the parameter ta
35. en the excursion is classified as a true peak and not a noise fluctuation Once the MINWIDTH criterion has been satisfied the DXP finds the arrival of the largest value from the fast filter which becomes the pulse s official arrival time and starts a counter to count PEAKSAMP clock cycles to arrive at the appropriate time to sample the value of the slow filter Because the digital filtering processes are deterministic PEAKSAMP depends only on the values of the fast and slow filter constants and the risetime of the preamplifier pulses The slow filter value captured following PEAKSAMP is then the slow digital filter s estimate of Vx 3 6 Energy Measurement with Resistive Feedback Preamplifiers In previous sections the pulse height measurement was shown for the case of pulsed reset preamplifiers The pulsed reset scheme is most often used for optimum energy resolution x ray detectors Other detectors use a continuous reset which we refer to as resistive feedback or RC feedback where the reset switch in Figure 3 1a is replace by a large value resistor giving a exponential decay time of typically 50 usec The RC feedback is most often used for gamma ray detectors which cover a larger dynamic range and where the electronic noise is not as significant a contribution to energy resolution Where analog shaping amplifiers typically have a pole zero adjustment to cancel out the exponential decay the DXP uses a patented exponentia
36. ergy Measurement amp Noise Filtering Reducing noise in an electrical measurement is accomplished by filtering Traditional analog filters use combinations of a differentiation stage and multiple integration stages to convert the preamp output steps such as Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 shown in Figure 3 1b into either triangular or semi Gaussian pulses whose amplitudes with respect to their baselines are then proportional to Vx and thus to the x ray s energy Digital filtering proceeds from a slightly different perspective Here the signal has been digitized and is no longer continuous but is instead a string of discrete values such as shown in Figure 3 2 The data displayed are actually just a subset of Figure 3 1b which was digitized by a Tektronix 544 TDS digital oscilloscope at 10 MSA megasamples sec Given this data set and some kind of arithmetic processor the obvious approach to determining Vx is to take some sort of average over the points before the step and subtract it from the value of the average over the points after the step That is as shown in Figure 3 2 averages are computed over the two regions marked Length the Gap region is omitted because the signal is changing rapidly here and their difference taken as a measure of Vx Thus the value Vx may be found from the equation Va EE 2a WM i before i after Equation 3 1 where the values of the weighting cons
37. ersonal injury and or damage to the DXP Saturn always disconnect power before removing the top panel Servicing and Cleaning To avoid personal injury and or damage to the DXP Saturn do not attempt to repair or clean the unit The DXP hardware is warranted against all defects for 1 year Please contact the factory or your distributor before returning items for service To avoid personal injury and or damage to the DXP Saturn do not attempt to repair or clean the unit The DXP hardware is warranted against all defects for 1 year Please contact the factory or your distributor before returning items for service Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 WARRANTY Xray Instrumentation Associates XIA warrants that this product will be free from defects in materials and workmanship for a period of one 1 year from the date of shipment If any such product proves defective during this warranty period XIA at its option either repair the defective products without charge for parts and labor or will provide a replacement in exchange for the defective product In order to obtain service under this warranty Customer must notify XIA of the defect before the expiration of the warranty period and make suitable arrangements for the performance of the service This warranty shall not apply to any defect failure or damage caused by improper uses or inadequate care XIA shall not be obligated to furnish service under this warra
38. es as in the ballistic deficit phenomenon the slow filter gap time can be extended to accommodate that range The shortest value of tps 0 5 us is set by the response time of the DSP to the FiPPI when a value of Vx is captured At this setting however with a gap time of 100 ns the dead time would be about 1 2 us and the maximum throughput according to Eqn 3 9 would be 310 kcps The FiPPI also includes a livetime counter which counts the 20 MHz system clock divided by 16 so that one tick is 800 ns This counter is activated any time the DSP is enabled to collect x ray pulse values from the FiPPI and therefore provides an extremely accurate measure of the system livetime In particular as described in 3 2 the DSP is not live either during preamplifier resets or during ASC out of ranges both because it is adjusting the ASC and because the ADC inputs to the FiPPI are invalid Thus the DXP measures livetime more accurately than an external clock which is insensitive to resets and includes them as part of the total livetime While the average number of resets sec scales linearly with the countrate in any given measurement period there will be fluctuations in the number of resets which may affect counting statistics in the most precise measurements All FiPPI parameters including the filter peaking and gap times threshold and pileup inspection parameters are all externally supplied and may be adjusted by the user to optimize performance
39. et ASC DACs to initial default values 4 Initialize FiPPI and download default filter parameters 5 Perform initial calibrations for controlling the ASC a Find the SlopeDAC setting corresponding to zero slope b TrackDAC Calibration determine TrackDAC step needed to move the ADC input signal from the edge of the range to the center of the range c Measure conversion factor used to calculate the contribution of the slope generator to the FiPPI baseline 6 Enable the input relay and enable the ASC and timer interrupts After the interrupts are enabled the DSP is alive and ready to take data After completing the initialization phase the DSP enters the idle phase In the idle phase the DSP continuously samples the FiPPI baseline and updates the baseline subtraction register in the FiPPI so that the FiPPI is always ready to take data 5 4 Event Processing There are two primary tasks performed during a normal data taking run event processing and baseline processing These tasks are described in detail below 5 4 1 Run Start Prior to the start of a normal the run the DSP performs several tasks 7 Sets the desired gain by setting the GAINDAC and the HIGHGAIN relay If the gain has changed the TrackDAC calibration is redone for reset detectors only 8 Sets the desired polarity the internal DSP polarity and the FiPPI polarity must be changed simultaneously to avoid ASC instability Only applicable if the desired polarity differs from
40. etween Vx x and Vx k 1 which is L Vx k L Vegert Vk VieL ViL G Vk 2L G Equation 3 3 While this relationship is very simple it is still very effective In the first place this is the digital equivalent of triangular or trapezoidal if G 0 filtering which is the analog industry s standard for high rate processing In the second place one can show theoretically that if the noise in the signal is white i e Gaussian distributed above and below the step which is typically the case for the short shaping times used for high signal rate processing then the average in Equation 3 2 actually gives the best estimate of Vx in the least squares sense This of course is why triangular filtering has been preferred at high rates Triangular filtering with time variant filter lengths can in principle achieve both somewhat superior resolution and higher throughputs but comes at the cost of a significantly more complex circuit and a rate dependent resolution which is unacceptable for many types of precise analysis In practice XIA s design has been found to duplicate the energy resolution of the best analog shapers while approximately doubling their throughput providing experimental confirmation of the validity of the approach 3 3 Trapezoidal Filtering in the DXP From this point onward we will only consider trapezoidal filtering as it is implemented in the DXP according to Equation 3 2 and Equation 3 3 The result of applying such a fi
41. he ADC It should look approximately as shown below without any spikes or other odd behavior High frequency spikes are often due to picking up computer noise particularly from monitors with large EMF emissions 14 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 Figure 2 13 Same as Figure 2 12 but showing a tracking step correcting a drift out of range Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 Baseline History The next test is to look at View gt Baseline History which will capture about 50 ms of baseline values Ideally this should be a curve of white noise whose width is the same as the baseline width Most detectors are not particularly ideal as may be seen from a pulsed optical reset detector below Each time two instances in Figure 2 13 the preamplifier resets it goes through a significant change in leakage current due to optical stimulation of traps in the first FET Gate Oxide which causes the baseline to move At these relatively slow frequencies the baseline tracking can keep up pretty well and there is not too much degradation in resolution At much higher counting rates however the problem becomes more serious and can significantly degrate resolution Problems caused by resetting are best addressed using the Reset Time and the Baseline Running Average parameters described above oe DxpDemo OF x File Edit View DXP Download ROI Special Help Baseline hist
42. i bilie lenta 28 3 10 Dead Time Correctionsi tutua rca ziale niua ille ai 29 4 DXP Saturn Hardware Description rrrrrrrrerereezezezizesese rece ceceezinizizezin iz iz iz ee zionicenee 30 4 1 Organizational Overview i 30 4 2 The Analog Signal Conditioner ASC iii 30 4 3 The Filter Pulse Detector amp Pile up Inspector FiPPI n 32 4 4 The Digital Signal Processor DSP 32 4 5 Interface to the Host Computer i 33 D2 Program Flow mille dle 36 b ScInitializationa aGfuria iaia sini lana nananana Rana 37 DA Event Processing titan eda diri 37 5 5 Baseline Measurement iriiai alari 39 5 6 Interrupt Routines it waitin ieee aii aan anaes 40 D7 Error Handling eil aaa Lele bano 41 5 8 Specifying Data Acquisition Tasks RUNTASKS ii 42 5 9 Special Tasks WHICHTEST iene 43 5 10 DSP Parameter DeScriptions i 44 ill Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 11 DSP Program Variants oiia epen a e R AA EEE AE RE AEEA 48 Appendix A DPP OEM Revision D 2 esesseresseresesessereeceresesessereeeesesesrosercoreresesroseseoreseseseesee 49 Jumper Settings alia lalla ria 49 LED Indicatofs uao aLaaa 50 CONNECtOrS c cirie ea EA cSt stale A elas A at N vent Aad dust eens 50 Power CONSUMPtON iii hdi lara 51 Manual DXP Saturn Digital X ray Processor md
43. ion is symmetrical this also means that pulse 1 s trailing edge point 1c also does not fall under the peak of pulse 2 For this to be true the two pulses must be separated by at least an interval of L G 2 Peaks 2 and 3 which are separated by only 1 8 us are thus seen to pileup in the present example with a 2 0 us peaking time This leads to an important first point whether pulses suffer slow pileup depends critically on the peaking time of the filter being used The amount of pileup which occurs at a given average signal rate will increase with longer peaking times We will quantify this in 3 6 Because the fast filter peaking time is only 0 4 us these x ray pulses do not pileup in the fast filter channel The DXP can therefore test for slow channel pileup by measuring for the interval PEAKINT after a pulse arrival time If no second pulse occurs in this interval then there is no trailing edge pileup PEAKINT is usually set to a value close to L G 2 1 Pulse 1 passes this test as shown in the figure Pulse 2 however fails the PEAKINT test because pulse 3 follows in 1 8 us which is less than PEAKINT 2 3 us Notice by the symmetry of the trapezoidal filter if pulse 2 is rejected because of pulse 3 then pulse 3 is similarly rejected because of pulse 2 Pulses 4 and 5 are so close together that the output of the fast filter does not fall below the threshold between them and so they are detected by the pulse detector as only being a si
44. it with a source of known energy x rays and a scope See the Appendix below for details The detector polarity positive polarity means that an x ray produces a positive step in voltage at the preamplifier output YA known source of X rays for example an Fe 55 source produces 5900 eV X rays When you start up the program the first time the following screen should appear OK Use this dialog box to configure the DXP x whenever hardware changes e g detector Cancel of coarse gain jumper The ADC tule is the fraction of the ADC range spanned by an x ray at the calibration energy Detector Gain 4 myke Detector Polarity Negative Threshold Energy 800 ev Slow Threshold 0 eV 11 Calibration Energy 5900 ev Peaking Time 20 x usec x ADC tule at Calibration Energy 45 Figure 2 2 Setup Dialog Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 v Enter the detector gain and polarity in the fields provided v Enter the detector polarity v Enter the Threshold energy This depends upon several factors Eventually you will want to set this as low as possible but to start use a conservative value say 2000 eV or 1500 eV below the calibration energy peak which ever is lower This is only a starting point it can be changed later Y Leave the Slow Threshold field at 0 v Enter the known x ray energy of your source as the calibration energy Choose a peaking time the defau
45. ith external setup for x ray mapping and other specialized applications 1 2 Unit Specifications 1 2 1 Performance The following quantities are specified for a particular detector i e the Ortec Iglet x and may vary somewhat for other detectors e Count rate precision lt 0 1 or limited by statistics e Peak stability with count rate lt 0 1 up to highest counting rates e Energy Scale Integral Nonlinearity lt 0 1 of full scale e Gain stability with temperature lt 0 05 degree C At high event rates for a particular detector the resolution and non linearity may degrade somewhat For POR preamplifiers this is primarily due to time dependent leakage currents within a preamplifier reset interval These produce baseline shifts which occur too rapidly for the DXP to track perfectly 1 2 2 Host Interface The DXP Saturn communicates w the host computer via the Enhanced Parallel Port defined by IEEE specification 1284 The interface is described further in Section 4 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 1 2 3 Preamplifier Interface The following specifications describe the preamplifier power supply port and signal input connection e 24V supply rating 100mA each e 12V supply rating 100mA each e Input signal range 10V e Input impedance 10KOQ 1 2 4 Detector Interface e Bias voltage range 1000V e Bias voltage output impedance 10MegQ e Bias voltage turn on off
46. l decay correction to achieve good energy resolution without a pole zero correction Figure 3 6 and Figure 3 7 illustrate the method used The first shows the output voltage of a RC 23 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 feedback preamplifier with a x ray or y ray step of amplitude A appearing at t 0 Ve is the voltage just before the step pulse arrives and Vo is the asymptotic value that the signal would decay to in the absence of steps t is the earliest time used in the slow filter L and G are the length and gap of the trapezoidal filter in clock units and At is the clock period In addition to the slow filter measurement the ADC amplitude Vp is made at time tp In the following discussion it is assumed that the signal rise time is negligible t t t 2 D 2 lt GAt t 0 Figure 3 6 RC preamplifier output voltage An x ray step occurs at time t 0 24 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 ICR 41 kcps Ge escape peaks 0 4000 8000 12000 Amplitude Figure 3 7 Correlation between step size and amplitude for Zr Ka x ray events measured with the DXP 4C As Figure 3 7 makes clear there is a linear correlation between the step height from the trapezoidal filter and the ADC amplitude for pulses of a given energy This is due to the fact that the exponential decay causes a deficit in the measured step height which grow
47. l increase susceptibility to electromagnetic radiation See section 4 2 1 Signal Grounding for further information gt JP102 and JP103 Input Voltage Range The default input range of the DXP is 10V sufficient for nearly all preamplifiers thus far encountered Larger voltage ranges up to 15V can be accommodated by reducing the gain of the first amplifier stage Align jumpers JP102 and JP103 with R100 and R103 respectively to use the extended input range Note the extended input range is not currently supported by the host software gt JP104 Preamplifier Type RC Feedback vs Pulsed Reset The DXP employs different methods of analog signal conditioning depending on the preamplifier type The position of jumper JP104 determines the digitally controlled signal to be subtracted from the preamplifier signal In the default position labeled RAMP a saw tooth signal is employed accommodating pulsed reset preamplifier types In the position labeled OFFSET a low frequency offset suitable for resistive feedback preamplifier types is used At this point the top panel of the DXP Saturn can be reassembled Proceed to Setting the Bias Voltage above Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 GATE SYNC IEEE 1284 EPP PREAMP POWER INPUT BNC BNC DSUB 25 F DSUB 9 F BNC sa a i o km gr kh S iosa SE AE Ma i tt ea ee STATUS I O RATE LED Indicators Figure 2 1
48. line noise 3 5 X ray Detection amp Threshold Setting As noted above we wish to capture a value of Vx for each x ray detected and use these values to construct a spectrum This process is also significantly different between digital and analog systems In the analog system the peak value must be captured into an analog storage device usually a capacitor and held until it is digitized Then the digital value is used to update a memory location to build the desired spectrum During this analog to digital conversion process the system is dead to other events which can severely reduce system throughput Even single channel analyzer systems introduce significant deadtime at this stage since they must wait some period typically a few microseconds to determine whether or not the window condition is satisfied Digital systems are much more efficient in this regard since the values output by the filter are already digital values All that is required is to capture the peak value it is immediately ready to be added to the spectrum If the addition process can be done in less than one peaking time which is usually trivial digitally then no system deadtime is produced by the capture and store operation This is a significant source of the enhanced throughput found in digital systems In the DXP the peak detection and sampling is handled as indicated in Figure 3 5 In the DXP two trapezoidal filters are implemented a fast filter and a slo
49. lt is 20 usec which will work fine for count rates up to 20 30 kcps If you are initially starting higher rates choose a shorter peaking time v The ADC rule is used to set the DXP conversion gain a typical value is 5 this means that one X ray of the calibration energy spans 5 of the ADC range 10 bits or 51 2 ADC counts A value larger than 5 may improve the resolution by decreasing digitization errors but at long peaking times where many ADC samples are used in the energy measurement this effect is usually quite small The disadvantage of using a large value of the ADC rule say 20 is that small fluctuations in the arrival times of X rays can push the signal out of the ADC range this will degrade system livetime Once all of the fields have been selected click OK You may be prompted to change the ADC rule or the gain jumper on the DXP or both if you request an ADC rule and detector gain that is outside the DXP s gain ranges At this point the DXP should be set up to take data Click on the Update button and a spectrum should appear oe DxpDemo olx File Edit View DXP Download ROI Special Help Decimation 4 MCA AT 3 971 LT 3 9349 CNTS 17328 OVER 2 UNDER 11 MO mo RO E EI E2 lt E gt FWHM Sum ied hal SI La ete er clear 0 5900 0 5346 9 5508 2 54315 1392 11964 Threshold 800 eV eV ADC 128 0 MCA bins 1024 ICR 5 673 keps OCR 4 363 keps Bin size 23 04 eV Save to Save Saves AquireSave Energy Spectrum fi 0 Sec
50. lter with Length L 20 and Gap G 4 to the same data set of Figure 3 2 is shown in Figure 3 3 The filter output Vx is clearly trapezoidal in shape and has a risetime equal to L a flattop equal to G and a symmetrical falltime equal to L The basewidth which is a first order measure of the filter s noise reduction properties is thus 2L G This raises several important points in comparing the noise performance of the DXP to analog filtering amplifiers First semi Gaussian filters are usually specified by a shaping time Their peaking time is typically twice this and their pulses are not symmetric so that the basewidth is about 5 6 times the shaping time or 2 8 times their peaking time Thus a semi Gaussian filter typically has a slightly better energy resolution than a triangular filter of the same peaking time because it has a longer filtering time This is typically accommodated in amplifiers offering both triangular and semi Gaussian filtering by stretching the triangular peaking time a bit so that the true triangular peaking time is typically 1 2 times the selected semi Gaussian peaking time This also leads to an apparent advantage for the analog system when its energy resolution is compared to a digital system with the same nominal peaking time Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 One extremely important characteristic of a digitally shaped trapezoidal pulse is its extremely sharp termination on completion of
51. mate op must be small compared to og Because the error in a single baseline measurement will be ce this means that multiple baseline measurements will have to be averaged In the standard DXP operating code this number is 64 which leads to the total noise shown in Equation 3 5 0 sqrt of 1 1 64 o Equation 3 5 This results in less than 0 5 eV degradation in resolution even for very long peaking times when resolutions of order 140 eV are obtained In practice the DXP initially makes a series of 64 baseline measurements to compute a starting baseline mean It then makes additional baseline measurements at quasi periodic intervals to keep the estimate up to date These values are stored internally and can be read out to construct a spectrum of baseline noise This is recommended because of its excellent diagnostic properties When all components in the spectrometer system are working properly the baseline spectrum should be Gaussian in shape with a standard deviation reflecting op Deviations from this shape indicate various pathological conditions which also cause the x ray spectrum to be distorted and which should be fixed 21 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 6 Filtered Step L kfig 960920 4 S 2 E 5 0 O 2 Preamp Output mV Filter Output mV 4 5 10 15 20 25 30 35 40 45 Time us Figure 3 4 The event of Figure 3 3 displayed over a longer time period to show base
52. n 3 8 where tq is the dead time Both the DXP and analog systems OCRs are so describable with the slow channel dead times tq s shown in Table 3 1 The measured ICRm values for both the DXP and analog systems are similarly describable with the fast channel dead times taf as shown The maximum value of OCR can be found by differentiating Equation 3 8 and setting the result to zero This occurs when the value of the exponent is 1 i e when ICRt equals 1 tg At this point the maximum OCRmax is 1 e the ICR or OCR max 1 e tg 0 37 tg Equation 3 9 These are general results and are very useful for estimating experimental data rates Table 3 1 illustrates a very important result for using the DXP the slow channel deadtime is nearly the minimum theoretically possible namely the pulse basewidth For the shown example the basewidth is 4 6 us 2Lg Gs while the deadtime is 4 73 us The slight increase is because as noted above PEAKINT is always set slightly longer than Lg Gg 2 to assure that pileup does not distort collected values of Vx The deadtime for the analog system on the other hand is much larger In fact as shown the throughput for the digital system is almost twice as high since it attains the same throughput for a 2 us peaking time as the analog system achieves for a 1 us peaking time The slower analog rate arises as noted earlier both from the longer tails on the pulses from the analog triangular filter and on additio
53. n become quite serious at rates approaching the point of maximum throughput In other words under these conditions peak counts in the low energy range should not be directly compared with peak counts at higher energies For this reason we recommend disabling the Slow Threshold by setting it equal to 0 in most cases e Other Parameters RunTasks Normally the top four boxes should be checked Y DACs These should not be adjusted under normal conditions v Reset Time Default 10 usec this is the blanking time data not used baselines not acquired after resets are detected Maximum 4000usec Use a scope to determine the best value for your detector preamp system e Baseline adjustments how the baseline is collected can have a significant effect on energy resolution v Baseline running average Default 64 This is the number of baseline samples that are averaged for the baseline correction Minimum value is 2 maximum value is 32767 Smaller values of this variable track changes in the baseline better but suffer from larger statistical variations Larger values do not track real shifts in the baseline well You can use View gt Baseline History to evaluate the effect of this parameter Non optimal value of this parameter will hurt resolution Optimal value is detector dependent v Baseline histogram scale factor Default 2 This parameter controls the scaling of the baseline histogram View gt Baseline see Figure 2 9 The baseline spectrum bin
54. n of the parameter SLOPEVAL determined by the value of the parameter SGRANULAR By default the slope adjustment granularity is 5 which is a good compromise between adjusting the slope quickly to match quickly changing input rates and being able to set the SlopeDAC just right For an RC feedback detector the offset added to the input signal is adjusted such that the signal stays in range as much as possible 5 6 2 Timer Interrupt Every 500 usec the DSP is interrupted to take care of the regular maintenance type tasks These tasks include 16 Update the run statistics EVTSINRUN LIVETIME REALTIME and FASTPEAKS only during a run 17 Control the Rate LED This LED flashes whenever a reset is detected reset detector only and during a tun the color indicates the current output input ratio By default the LED flashes green for OCR ICR gt 0 5 flashes yellow green plus red for 0 5 gt OCR ICR gt 1 e and flashes red for OCR ICR lt 1 e The thresholds are determined by the parameters YELTHR and REDTHR 18 Handle fixed length runs During a fixed length run the current value of EVTSINRUN output events FASTPEAKS input events LIVETIME or REALTIME is compared to the desired run length Once the value exceeds the desired value the run is ended 5 7 Error Handling When the DSP detects an error in the operation of the DXP Saturn the red Status LED is turned on and the source of the error is stored in the parameter RUNERROR The p
55. nal deadtime introduced by the operation of the SCA In spectroscopy applications where the system can be profitably run at close to maximum throughput then a single DXP channel will then effectively count as rapidly as two analog channels 3 10 Dead Time Corrections The fact that both OCR and ICRy are describable by Equation 3 8 makes it possible to correct DXP spectra quite accurately for deadtime effects Because deadtime losses are energy independent the measured counts Nmi in any spectral channel i are related to the true number Ntj which would have been collected in the same channel i in the absence of deadtime effects by Nti Nmi ICRy OCR Equation 3 10 Looking at Figure 3 9 it is clear that a first order correction can be made by using ICRm in Equation 3 7 instead of ICR particularly for OCR values less than about 50 of the maximum OCR value For a more accurate correction the fast channel deadtime tgf should be measured from a fit to the equation ICRm ICT exp ICR taf Equation 3 11 Then for each recorded spectrum the associated value of ICRm is noted and Equation 3 11 inverted there are simple numerical routines to do this for transcendental equations to obtain ICR Then the spectrum can be corrected on a channel by channel basis using Equation 3 8 In experiments with a DXP prototype we found that for a 4 us peaking time for which the maximum ICR is 125 kcps we could correct the area of a reference peak t
56. ne sample rate typically several 100 kHz and the width of the baseline distribution When the DSP detects an overflow all bins are scaled down by a factor of 2 and histogramming continues The baseline distribution should be very gaussian the width of the distribution reflects the electronic noise in the system including the effects of the energy filter A tail on the positive side of the distribution indicates the presence of energy in the baseline resulting from undetected pileup or energy depositions that did not satisfy the trigger threshold The tail should be very small compared to the peak of the histogram it will grow with rate If this tail is too large it can have a noticeable effect on the baseline mean leading to negative peak shifts Under these circumstances enabling the baseline cut is useful in eliminating the bias A tail on the low energy side of the baseline distribution is usually caused by baseline samples just after a preamplifier reset the effects of the reset can last quite a while tens of microseconds especially for optical reset preamplifiers It is usually best not to take data while the reset is in effect the dead time associated with a reset can be adjusted using the parameter RESETWAIT which sets the dead time in units of 250 ns 5 5 4 Residual Baseline When operating with a reset type preamplifier the raw baseline measured in the FiPPI which is just the output of the energy filter comes from two sources
57. ngle x ray pulse Indeed only a single though somewhat distorted pulse emerges from the slow filter but its peak amplitude corresponds to the energy of neither x ray 4 nor x ray 5 In order to reject as many of these fast channel pileup cases as possible the DXP implements a fast channel pileup inspection test as well The fast channel pileup test is based on the observation that to the extent that the risetime of the preamplifier pulses is independent of the x rays energies which is generally the case in x ray work except for some room temperature compound semiconductor detectors the basewidth of the fast digital filter i e 2Lf Gf will also be energy independent and will never exceed some maximum width MAXWIDTH Thus if the width of the fast filter output pulses is measured at threshold and found to exceed MAX WIDTH then fast channel pileup must have occurred This is shown graphically in the figure where pulse 3 passes the MAXWIDTH test while the piled up pair of pulses 4 and 5 fail the MAX WIDTH test Thus in Figure 3 8 only pulse passes both pileup inspection tests and indeed it is the only pulse to have a well defined flattop region at time PEAKSAMP in the slow filter output 26 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 Digitized MultiPile kfig 960921 Passes PEAKINT Test Fast Filter PEAKSAMP gt Slow Filter 5 10 15 20 25 30 Time us Figure 3 8 A sequenc
58. ns are in files of the form fx10pnx fip where n is the decimation value 0 2 4 and 6 different decimation values are needed to cover the full range of peaking times and x is the version letter 2 4 Installing and using the DxpDemo Program The DxpDemo program is provided for getting started quickly with the DXP Saturn It is intended for demonstration purposes and making some basic spectrum measurements with the unit XIA does not intend to support this program in the long term and is working on replacing DxpDemo with a more comprehensive software program The DxpDemo program works on Windows 95 98 and NT machines If not provided with the unit the latest version of DxpDemo can be copied from the XIA FTP site ftp xia com User Anonymous in the area ftp pub DXP X10 HostSoftware The installation file is a ZIP archive which can be un zipped with WinZip or a similar program Then the program Setup exe can be used to install the program using the familiar InstallShield For Windows NT machines you may need to be logged into the Administrator account to install the software DxpDemo and the other XIA software uses a share ware parallel port driver called DLPORTIO from Driver Linx The installation file for this is called PORT9SNT exe which can also be copied from the XIA ftp site The quickest way to get started is if you have the following information v The detector preamplifier gain in mV keV If you don t know it you can measure
59. nty a to repair damage resulting from attempts by personnel other that XIA representatives to repair or service the product or b to repair damage resulting from improper use or connection to incompatible equipment THIS WARRANTY IS GIVEN BY XRAY INSTRUMENTATION ASSOCIATES XIA WITH RESPECT TO THIS PRODUCTI IN LIEU OF ANY OTHER WARRANTIES EXPRESSED OR IMPLIED XIA AND ITS VENDORS DISCLAIM ANY IMPLIED WARRANTIES OF MERCHANTABILITYOR FITNESS FOR A PARTICULAR PURPOSE XIA S RESPONSIBILITY TO REPAIR OR REPLACE DEFECTIVE PRODUCTS IS THE SOLE AND EXCLUSIVE REMEDY PROVIDED TO THE CUSTOMER FOR BREACH OF THIS WARRANTY XIA AND ITS VENDORS WILL NOT BE LIABLE FOR ANY INDIRECT SPECIAL INCIDENTAL OR CONSEQUENTIAL DAMAGES IRRESPECTIVE OF WHETHER XIA OR THE VENDOR HAS ADVANCE NOTICE OF THE POSSIBILITY OF SUCH DAMAGES Contact Information Xray Instrumentation Associates 8450 Central Ave Newark CA 94560 USA support xia com 510 494 9020 vi Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 1 Overview The Digital X ray Processor DXP is a high rate digitally based multi channel analysis spectrometer designed for energy dispersive x ray or y ray measurements using semiconductor detectors The DXP offers complete computer control over all amplifier and spectrometer controls including gain filter peaking time and pileup inspection criteria The DXP s digital filter typically increases throughput by a factor of two or more
60. o better than 0 5 between 1 and 120 kcps The fact that the DXP provides highly accurate measurements of both LIVETIME and ICR yy therefore allows it to produce accurate spectral measurements over extremely wide ranges of input counting rates 29 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 4 DXP Saturn Hardware Description 4 1 Organizational Overview The DXP channel architecture is shown in Figure 4 1 showing the three major operating blocks in the DXP the Analog Signal Conditioner ASC Digital Filter Peak Detector and Pileup Inspector FiPPI and Digital Signal Processor DSP Signal digitization occurs in the Analog to Digital converter ADC which lies between the ASC and the FiPPI In the DXP Saturn the ADC is a 12 bit 40 MSA device which is currently being used as a very linear 10 bit 20 MHz ADC The functions of the major blocks are summarized below Analog Signal Digital Filter Pulse Digital Signal Conditioner Detector amp Pile up Inspector Processor ASC FiPPI DSP i ee ee _ e e e o e a a a o Filter I l I I I I i Sawtooth Peak Measure I Function MCA Binning amp Generator ASC Control I I I I I I I I I I I I lt Gain DAC T een ore Bao et Interface to C Tracking DAC Tracking DAC k AE Control Computer lt TDACPulse I System Dwg e RRR Figure 4 1 Block diagram of the DXP channel architec
61. o DXP Saturn UM001 5 Safety Please take a moment to review these safety precautions They are provided both for your protection and to prevent damage to the digital x ray processor DXP and connected equipment This safety information applies to all operators and service personnel Symbols These symbols appear on equipment as required for safety t S DANGER Protective ATTENTION High Voltage ground earth Refer to the Terminal manual Specific Precautions Observe all of these precautions to ensure your personal safety and to prevent damage to either the DXP Saturn or equipment connected to it Power Source The DXP Saturn is intended to operate from a mains supply voltage of either 115V or 230V at 50 60Hz however THE REAR PANEL LINE VOLTAGE SELECTION SWITCH MUST BE SET before the system is powered on Refer to the Getting Started section of the user manual for instructions on supply selection Supply voltage fluctuations are not to exceed 10 of the nominal value A protective ground connection through the grounding conductor in the power cord is essential for safe system operation Use the Proper Fuse To avoid a fire hazard use only Time Lag 5mm x 20mm IEC 127 2 III 250mA fuses rated for 250V A spare fuse is provided in the fuse drawer located at the power entry point User Adjustments Disassembly All user adjustments are accessible via the top panel Do not attempt to remove any other panels or components To avoid p
62. ollowing limits Fastlen lt 32 Threshold lt 256 Maxwidth lt 250 LASA After making these adjustments you can go back to DXP gt Reconfigure and enter a smaller Threshold Energy value For example suppose the energy threshold is 1000 eV and the DXP gain is set for 142 9 eV ADC this is displayed on the upper left hand side of the main page Figure 2 3 Here are the default FiPPI values FastLen 4 Threshold 56 MaxWidth 20 The following set of parameters are equivalent in terms of Threshold Energy still 1000 eV but the fast filter peaking time used in peak detection and pileup inspection 800 nsec instead of 200 nsec FastLen 16 Threshold 226 MaxWidth 44 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 Now you can acquire spectra at reduced values of the threshold by adjusting Threshold Energy in the DXP gt Reconfigure dialog Warning if you use DXP gt Setup the default values of FastLen 4 and MaxWidth 20 will be restored Slow Threshold invokes a second discriminator that is applied to the slow trapezoidal filter output extending the detection range well below 1000 eV The dead time per event significantly higher for events detected in this manner and also can vary significantly between events of the same energy Because of this soft x ray throughput will be attenuated and the statistical accuracy within this range degraded This will be a very small effect at low count rates but ca
63. onds 2200 2000 1800 1600 1400 inea 1200 Cursor 1000 X 5860 Y 3003 e _ v Zoom Fs aka r Full Scale Figure 2 3 Main window after DXP gt Setup Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 If the setup procedure worked properly you should see a 1024 bin spectrum with the calibration peak centered near of full scale If you see a peak near 0 this indicates that the threshold energy is too low To adjust the threshold energy select the DXP gt Setup menu item and the Setup Dialog will reapear Note if the threshold is really low no events will be in the spectrum but the ICR Input Count Rate in kcps field on the right side of the display will show a non zero value A Region of Interest ROI is automatically selected it is based on the largest peak and is the contiguous set of MCA bins with contents greater then half the peak bin s contents These bins appear with yellow fill If the largest peak is NOT the calibration energy you must delete this ROI and define one that spans the calibration peak See the Appendix on defining ROIs The ROI is used to fit the peak position and FWHM needed to fine tune the DXP gain to achieve the proper calibration Note The text box in Figure 2 3 shows that after these first startup approximations the first ROI is 5431 5 eV which is quite different from the calibration energy of 5900 eV The next
64. oner ASC to keep the ADC input signal in the proper range and to adjust the slope generator to match the current input rate When the Begin Run signal is received from the host through the CSR register the DSP first determines whether the run is a normal data taking run or a special run DXP X10P DSP Code Flow Chart Error Condition RUNERROR 0 Error Loop Turn on Red STATUS LED _ Wait for RUNERROR 0 Error Condition Timer Interrupt Update Statistics Update LEDs lt Handle fixed length runs Figure 5 1 DSP code flow diagram Startup Initialize variables Initial calibrations Begin monitoring ASC Enable Timer Interrupt Idle Loop Continuously measure baseline Wait for Begin Run signal _ Begin Run from Host Special Run Yes Bit 8 of RUNTASKS No Begin Normal Run Download FiPPI Parameters Perform gain Calibration Zero Statistics if new run Normal Run Collect data events Fill spectrum or ROIs Collect baseline events End Run Internal or from Host Finish Run Update Statistics ASC Monitoring Detect Resets Keep signal in ADC range Interrupt driven End Run Internal Run Test Segment determined by WHICHTEST Error Condition To Error Loop 36 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 In a normal run ASC monitoring and baseline collection continue as in the idle phase Event interrupts are enabled when the FiPPI detect
65. ory I 20 25 30 milliseconds Approx Figure 2 14 Baseline history from a pulsed optical reset preamplifier This figure shows a particularly bad case Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 3 Digital Filtering Theory DXP Structure and Theory of Operation The purpose of this section is to provide the DXP user with an explanation of its operation which is deep and complete enough to allow the module to be used effectively yet not so filled with detail as to become cumbersome A further level of detail is required for those who wish to engage in developing control programs for the DXP and this is provided in subsequent sections This introduction is divided into three parts In the first we examine the general issues associated with using a digital processor to extract accurate x ray energies from a preamplifier signal and detect and eliminate pile ups In the second section we then describe how these general functions are specifically implemented in the DXP This leads rather naturally to a discussion of the parameters used to control the DXP s functions that is those digital values which replace knob positions in analog systems In the third section we the proceed to describe strategies both for selecting reasonable starting parameter values and for adjusting their values to optimize performance in particular situations 3 1 X ray Detection and Preamplifier Operation Energy dispersive detectors
66. ossible values for RUNERROR are listed below RUNERROR Value Meaning 0 No Error 1 FiPPI communication error 2 ASC setup failure 3 5 Reserved 6 TrackDAC calibration error Table 5 2 Identification of DXP errors according to the DSP parameter RUNERROR 4l Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 A FiPPI communication error could mean that the FiPPI configuration was not successful An ASC calibration error can indicate a hardware problem or possibly that a jumper is not set properly for example the DSP code for reset preamplifiers will generate an error if the jumper is set to run in OFFSET mode Once the source of the error has been located and cleared the host can set RUNERROR to 0 to force the DSP to exit the error loop and reinitialize the system Note that all system settings are saved when initialization is performed coming out of the error loop Of course another valid method for clearing the error is to redownload the DSP code after fixing the problem 5 8 Specifying Data Acquisition Tasks RUNTASKS Many aspects of the operation of the DXP Saturn are controlled by individual flag bits of the parameter RUNTASKS The meaning of each RUNTASKS bit is described below Bit Meaning if set 1 Meaning if cleared 0 Reserved set to 0 Reserved set to 0 1 Update SlopeDAC or OffsetDAC value to match SlopeDAC or OffsetDAC adjustments disabled current rate DEFAULT 2 Us
67. ot be modified For Tp and Tr in usec the following gives the value of SLOWLEN and SLOWGAP SLOWLEN 20 Tp 9 DECIMATION SLOWGAP 20 T 2 DECIMATION The user will want to be able to choose the peaking time based on resolution and throughput requirements Throughput output count rate OCR divided by input count rate ICR is given by exp t ICR where to a good approximation the pulse processing deadtime t 2Tp T fis the pulse basewidth The dependence of resolution on peaking time is detector specific Typical values of SLOWGAP are 6 for the 0 bit decimation FiPPI and 3 for the others Different detectors may work best with slightly different values 5 10 3 Setting the fast filter parameters FASTLEN FASTGAP In general the user does not modify these parameters directly but through the host software routine SetAcquisition Values See Appendix The fast filter is also trapezoidal but has a decimation of 0 for all FiPPI designs The values of FASTLEN and FASTGAP are given for Tp fast peaking time and Tf fast flat top time in usec FASTLEN 20 T FASTGAP 20 Tf Typical values of these parameters are FASTLEN 4 and FASTGAP 0 While these are reasonable values for most users some may want to make FASTLEN larger in order to trigger at lower values see beloe or shorter to run at higher rates 5 10 4 Setting the pulse detection parameter THRESHOLD MINWIDTH In general the user does not modify these parameters directly b
68. over available analog systems at comparable energy resolution but at a lower cost The DXP is easily configured to operate with a wide range of common detector preamplifier systems including pulsed optical reset transistor reset and resistive feedback preamplifiers The DXP Saturn is a single channel unit which is controlled with the Enhanced Parallel Port EPP an upgraded version of the IBM compatible PC standard parallel printer port 1 1 DXP Saturn Features e Single unit replaces spectroscopy amplifier shaping amplifier multi channel analyzer and detector bias HV supply at significantly reduced cost e Operates with a wide variety of x ray or y ray detectors using preamplifiers of pulsed optical reset transistor reset or resistor feedback types e Instantaneous throughput up to 500 000 counts second e Digital trapezoidal filtering with programmable peaking times between 0 25 and 80 usec e High precision internal gain control e Pileup inspection criteria computer settable including fast channel peaking time threshold and rejection criterion e Accurate ICR and live time reporting for precise dead time corrections e Multi channel analysis allowing for optimal use of data to separate fluorescence signal from backgrounds e Supplies preamplifier power on a NIM standard DB 9 connector e Supplies detector bias HV up to 1000V with LN sensor HV inhibit input e External Gate and Sync signals synchronize data acquisition w
69. r fixed length runs see Section 0 below The event processing involves either binning the energy into an MCA or determining whether the event falls into a defined SCA window depending upon the DSP code variant If there is no event to process the DSP reads a baseline value from the FiPPI see below for a detailed description of the baseline processing Once the run is over the statistics are finalized and the DSP returns to the idle state where it continuously samples baseline and waits for a command to start a new run 5 4 4 Spectrum Binning The primary event processing task is to use the energies measured in the FiPPI to build up a full energy spectrum MCA The MCA bin width is determined by the analog gain the FiPPI filter length and the binning parameter BINFACT1 The DSP determines the spectrum bin by multiplying the FiPPI energy output by I BINFACTI If the bin is outside the range determined by the parameters MCALIMLO and MCALIMHI the event is classified as an underflow or overflow Otherwise the appropriate bin is incremented A 24 bit word is used to store the contents of each bin allowing nearly 16 8 million events per MCA channel 5 4 5 SCA Mapping An alternate variant of the DSP code allows the user to define up to 24 SCA regions and count the number of events that fall into each region The regions are defined in terms of MCA bin number and can overlap A useful method for defining the SCA windows is to take a run with th
70. rn power must be turned on in order to set the bias voltage magnitude Do not power the DXP Saturn until the line voltage has been properly selected all internal settings have been made and the top panel has be re installed The magnitude of the high voltage is set with a potentiometer accessible through the front panel using a small screwdriver The front panel LDC display indicates the set value of the bias voltage Note once enabled the high voltage ramps slowly to its set value at the user defined rate see above 2 1 4 Internal Jumper Settings gt JP9 and JP10 Input Signal Connection BNC or DSUB9 In the typical configuration power for the detector preamplifier is supplied by the DXP via the NIM standard DB 9 Preamplifier Power interface The output signal and its reference return via BNC coaxial cable to the DXP Some manufacturers instead route the signal back through the preamplifier power cable in order to save space The DXP Saturn accommodates either configuration Jumpers JP9 and JP10 connect pins 8 and 3 of the DB9 connector to the BNC shield and inner conductor respectively These jumpers are not stuffed by default gt JP101 Signal Reference DXP ground vs Detector ground JP101 connects the signal reference to the DXP ground It is stuffed by default If JP101 is removed the preamplifier signal and reference are amplified differentially This configuration may improve performance if a ground loop is present but wil
71. rupt is used to handle such housekeeping chores as updating statistics These routines are described in more detail below 5 6 1 ASC Monitoring There are four main tasks performed by the ASC interrupt routine 12 Detects Resets pulsed reset detectors only 13 Adjusts the slope generator to match the event rate pulsed reset detectors only 14 Adjusts the offset value to keep the signal in range RC feedback detectors only 15 Moves the signal back to the center of the ADC range whenever it drifts out of range high or low The ASC interrupt routine is triggered whenever the FiPPI detects the ADC going out of range If the out of range is due to the signal drifting out of range instead of a reset the DSP triggers a TrackDAC step to bring the signal back to the center of the ADC range and data taking resumes If the DSP determines that the out of range is due to a reset then the DSP holds the signal at the center of the ADC range for a time determined by the parameter RESETINT which specifies the dead time after a reset in 0 25 usec units After the reset interval the signal is released and data taking resumes The DSP keeps track of how many times the signal drifts out of range in both directions and adjusts the slope such that the number of drifts high DriftUps roughly matches the number of drifts low DriftDowns If the DSP determines that the slope must be changed to match the rate the SlopeDAC value is modified by a constant fractio
72. s set to ADC zero and full scale which then interrupts the DSP demanding ASC attention The DSP remedies the situation by pulsing the TRACKDAC until the conditioned signal returns into the ADC s input range During this time data passed to the FiPPI are invalid Preamplifier resets are detected similarly When detected the DSP responded by resetting the current integrator with a switch 4 3 The Filter Pulse Detector amp Pile up Inspector FiPPI The FiPPI is implemented in a field programmable gate array FPGA to accomplish the various filtering pulse detection and pileup inspection tasks discussed in 3 As described there it has a fast channel for pulse detection and pileup inspection and a slow channel for filtering both with fully adjustable peaking times and gaps The fast filter s tp tpf can be adjusted from 100 ns to 1 25 us while the slow filter s tp tps can be adjusted from 0 25 us to 80 us Adjusting tpf allows tradeoffs to be made between pulse pair resolution and the minimum x ray energy that can be reliably detected When tpf is 200 ns for example the pulse pair resolution is typically less than 200 ns When tpf is us x rays with energies below 200 eV can be detected and inspected for pileup To maximize throughput tps should be chosen to be as short as possible to meet energy resolution requirements since the maximum throughput scales as 1 tps as per Eqn 3 9 If the input signal displays a range of risetim
73. s an event it interrupts the DSP which quickly responds and reads the energy value from the FiPPI into an internal buffer in data memory The events in the buffer are then used to build the x ray spectrum or fill regions of interest In a special run the action is determined by the value of the parameter WHICHTEST The special runs include calibration tasks such as collecting an ADC trace as well as ways of putting the DSP code into a special state such as putting it into a dormant state to allow reprogramming the FiPPI on the fly Normally the special runs end on their own and the DSP returns to the idle state After the initialization phase the Timer interrupt is enabled This interrupt is used to handle the housekeeping type chores such as updating the statistics during a run controlling the rate LED and handling fixed length runs The Timer interrupt occurs with a period of 500 psec If the DSP encounters an error condition the DSP turns on the red status LED and waits for the host to set the parameter RUNERROR to 0 after finding and fixing the problem that resulted in the error condition Each phase of the DSP program is discussed in more detail below 5 3 Initialization The DSP code starts running immediately after the DSP download is complete During the initialization phase several tasks are performed 1 Setup internal DSP control registers 2 Zero spectrum and data memory then initialize parameters to default values 3 S
74. s linearly with the distance from the asymptotic ADC offset at zero count rate The DSP reads these two values for each event that passes the FiPPI s trigger criteria and makes a correction of the form E k Sx k Vx lt Sg k Vg gt Equation 3 6 Here the quantities Sx and Vx are the step height and ADC amplitude measured for the step and the corresponding values with the B subscript are baseline values which are measured frequently at times when there is no trigger The brackets lt gt indicate that the baseline values are averaged over a large enough number of events to not introduce additional noise in the measurement The constant k the DSP parameter called RCFCOR is inversely proportional to the exponential decay time this correction factor is a constant for a detector channel at a fixed gain and shaping time The constant k is effectively a gain factor and is taken into account with a detector gain calibration The parameter RCFCOR is a function of the digital filter parameters SLOWLEN SLOWGAP and DECIMATION and the preamplifier decay time the DSP parameter TAURC The decay time TAURC is in units of 50 ns clock ticks and is measured with an exponential fit for example using the program DxpRCSetup At the start of an acquisition run the DSP calculates RCFCOR using the following approximate expression RCFCOR 2 LEN GAP TAURC LEN GAP 2 3 2 25 Manual DXP Saturn Digital X ra
75. se baseline gain 0 0285 Overall the internal gain can range from 0 057 V V to 45 56 V V The coarse gain is set either low HIGHGAIN 0 or high HIGHGAIN 1 this setting controls a relay which offers a factor of 4 difference in gain between the high and low gain settings The fine gain control is a 16 bit DAC which sets the gain of a variable gain amplifier which is linear in dB The gain setting accuracy is approximately one bit or 0 00061 dB 0 007 The relationship between Gvar and GAINDAC is Gain in dB GAINDAC 65536 40 dB Gvar 10 Gain in dB 20 In addition to the programmable gain control a jumper JP102 located in the corner of the board next to the input BNC connector chooses an input stage gain of either roughly 2 with the jumper away from the edge of 47 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 the board or roughly 4 with the jumper towards the edge of the board The exact gain of this stage depends on the output impedance of the detector preamplifier Gain 2k 1k or 500 Output Impedance 5 11 DSP Program Variants 5 11 1 MCA acquisition with pulsed reset preamplifiers variant 0 Variant 0 is the standard firmware variant supplied with the DXP Saturn as described in this manual It is intended for use with pulsed reset preamplifiers described in Section 3 Firmware files Program file name X10P0106 HEX Fippi file names FXPDxx0_ST FIP x 0 2 4 6
76. size scales like 2 bin factor If the Baseline cut next item is not enabled the DXP performance does not depend upon this factor Minimum value 0 maximum 16 Y Baseline cut FWxM x Default disabled This cut restricts the baseline values used in the running average to be close to average This cut is based on the baseline histogram For example a cut made at x 0 1 on the baseline histogram shown in Figure 2 9 would limit baseline samples in the range 344 768 eV from being used in the running average This cut eliminates the tail events from entering the running average calculation Warning At the present time the baseline cuts e g 344 768 eV are computed only when the peak bin in the baseline histogram reaches 64k When this happens the histogram contents are scaled down by a factor of 2 The rate at which this updating occurs depends on the baseline histogramming scale factor and the actual value of the baseline FWHM this can range from once per second to once a minute This is usually only a problem when the cut is first enabled or disabled when a parameter that affects baseline is changed e g gain or peaking time or if the detector baseline wanders around on a time scale comparable to the reset time This will be addressed in the future Measuring the Detector Preamp Gain The best way to measure this is to put a tee on the scope input use high impedance connect one end of the tee to the DXP and the other end of the tee to
77. step is to perform an energy calibration and refine some of the DXP parameters Select the menu item DXP gt Reconfigure Use this dialog to reconfigure the DXP Note This procedure uses an existing ere spectrum to calculate new gain and binning parameters The largest peak is assumed to be to lie at the calibration energy Spectrum size 1024 v bins MCA bin size 23 046875 ev Calibration Energy 5300 ev Threshold Energy 2000 eV SlowThreshold o eV Peaking Time 20 microseconds Figure 2 4 DXP gt Reconfigure Dialog You may fill the fields with values you want Spectrum size can be up to 14000 it does not need to be a power of 2 The MCA bin size can be any value perhaps 10 eV The other fields are the same as described for the Setup dialog The Peaking Time can be any value between 0 5 and 80 usec the software will use the nearest match Accepting the values click OK adjusts the internal DXP parameters to achieve the desired MCA bin size threshold energy and peaking time For example adjusting to a convenient 10 eV MCA channel gives Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 Reconfigure Figure 2 5 DXP gt Reconfigure Dialog after adjustment Use the Update button to display the new spectrum The following alert box may appear Figure 2 6 An annoyance to ignore If the highest peak in the spectrum is the calibration peak you can simply select th
78. supply to your detector until the following settings have been confirmed 2 1 3 Bias Voltage Polarity The polarity of the high voltage is indicated by LEDs in the lower right of the front panel the LED corresponding to the chosen polarity will glow when the unit is turned on Yellow indicates that the high voltage is disabled red indicates the supply is enabled To change the high voltage polarity the power must be turned off and the top panel of the DXP Saturn removed The polarity is set using the HV Control Key the small printed circuit card with a finger hole that extends down through a slot in the top printed circuit board PCB to a card edge connector in the bottom PCB Align the proper polarity on the card with the arrow on the surface of the top board As shipped the units are set to 500V negative CAUTION Applying high voltage of the wrong polarity will almost certainly damage your detector gt Enable Disable Ramp Rate Shorting jumpers JP10 and JP11 on the HV Control Key determine the rate at which the detector bias voltage ramps up and down upon enabling and disabling the supply Text on the key itself indicates the selected ramp duration The default is ten seconds selected by a solder short across JP11 A three second ramp duration is selected by shorting the terminals of JP10 together and removing the solder from JP11 if present A thirty second duration is achieved by removing solder shorts from both JP10 and JP11 gt
79. tal X ray Processor mdo DXP Saturn UM001 5 P1 Preamplifier Power Exit DSUB 9 Female output DC voltages to preamplifier AMP P N 745781 4 Pin Name Description 1 GND Internal ground connection NOT chassis ground 2 GND Internal ground connection NOT chassis ground 3 IN ALT Alternate signal input selected with jumper JP10 BNC standard 4 12V_OUT 12V 5V solder option DC for preamplifier 5 NC No connection solder option 5V connection 6 24V_OUT 24V DC for preamplifier 7 24V_OUT 24V DC for preamplifier 8 REF ALT Alternate signal reference selected with jumper JP9 BNC standard 9 12V_OUT 12V 5V solder option DC for preamplifier P2 IEEE 1284 Standard EPP Port DSUB 25 Female parallel communications port standard pinout AMP P N 745783 4 Power Consumption DPP X10P Only preamplifier power consumption NOT INCLUDED Standby Active Voltage Current mA Power mW Current mA Power mW 120 600 240 1200 220 1100 230 1150 100 500 100 500 40 480 40 480 30 360 40 480 3040 3810 51
80. tants w determine the type of average being computed The sums of the values of the two sets of weights must be individually normalized N Length Gap Preamp Output mV o Digitized Step 960919 4 20 22 24 26 28 30 Time us Figure 3 2 Digitized version of the data of Figure 3 1b in the step region The primary differences between different digital signal processors lie in two areas what set of weights wi is used and how the regions are selected for the computation of Equation 3 1 Thus for example when the weighting values decrease with separation from the step then the equation produces cusp like filters When the weighting values are constant one obtains triangular if the gap is zero or trapezoidal filters The concept behind cusp like filters is that since the points nearest the step carry the most information about its height they should be most strongly weighted in the averaging process How one chooses the filter lengths results in time variant the lengths vary from pulse to pulse or time invariant the lengths are the same for all pulses filters Traditional analog filters are time invariant The concept behind time variant filters is that since the x rays arrive randomly and the lengths between them vary accordingly one can make maximum use of the available information by setting Length to the interpulse spacing Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 In principal
81. the default negative polarity and then only for the first run 9 Downloads the specified FiPPI parameters SLOWLEN SLOWGAP etc to obtain the desired peaking time 10 Updates the internal calibrations with the new gain and FiPPI values 11 If desired the run statistics and the MCA are cleared determined by the NewRun bit in the CSR Otherwise the run is treated as a continuation of the previous run Note that for a run continuation no 37 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 gain or FiPPI changes are performed In either case the run number parameter RUNIDENT is incremented 5 4 2 Event Interrupt When the FiPPI detects a good event it triggers a high priority interrupt in the DSP Upon receiving the interrupt the DSP immediately reads the event energy from the FiPPI into an internal circular buffer and increments the write pointer into that buffer The normal event loop compares the write pointer to the read pointer to determine that there is a new event to process 5 4 3 Event Loop The processing that takes place during a normal collection run is very simple in order to allow high event rates The structure of the event loop is illustrated below in pseudocode while RunInProgress if EventToProcess ProcessEvent else CollectBaseline endif RunFinish goto IdleLoop The run can be stopped by the host by clearing the RunEnable bit in the CSR or can be stopped internally fo
82. the detector preamplifier and the slope generator in the DXP Saturn itself At high rates the slope gets rather large in order to balance the high energy deposition rate in the detector under these conditions the baseline due to the slope is by far the dominant factor in the baseline By default the DSP continually adjusts the slope to match the current rate these slope adjustments result in an instantaneous change in the baseline If the baseline due to the slope generator is included in the baseline mean the change in the calculated mean would be delayed relative to the change in the slope due to the effect of all the baseline samples prior to the slope change For this reason the baseline due to the slope is subtracted out of the overall baseline prior to calculating the mean value and added back in prior to loading the FiPPI baseline subtraction register The residual baseline included in the mean reflects the detector leakage current and should be fairly constant with rate to the extent that the leakage current does not depend on rate The calibration procedure used to determine the baseline due to the slope generator is performed during the initial startup procedure By default the baseline due to the slope generator is taken out of the baseline average The user can choose to include the slope baseline in the mean by clearing the residual baseline bit 6 in RUNTASKS 5 5 5 Baseline Cut As specified above a baseline cut is available
83. to exclude baseline samples that include real event energy which can lead to peak shifting at high event rates The cut is expressed as a fraction of the peak value of the baseline distribution by default the baseline cut is set to 5 The cut values are based on the baseline histogram and are recalculated every time the histogram overflows every few seconds The DSP searches on either side of the peak of the baseline distribution for the first bin whose contents are less than the cut 05 by default times the peak value these bin numbers are used to calculate the actual baseline cut The cut fraction is stored in the parameter BLCUT expressed in 16 bit fixed point notation Interpreted as an integer BLCUT cut fraction 2 15 the default 5 cut corresponds to BLCUT 1638 decimal or 666 hex The actual cut values determined by the DSP code are stored in BLMIN and BLMAX The baseline cut is enabled or disabled by setting or clearing a bit 10 in the RUNTASKS parameter 5 6 Interrupt Routines There are several tasks performed under interrupt control within the DSP on the DXP Saturn The event interrupt routine which just transfers event data from the FiPPI to an internal buffer is described above in Section 5 4 above There are two other interrupt routines the ASC interrupt is used to keep the analog signal within the 40 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 input range of the ADC and the timer inter
84. ture showing the major functional sections 4 2 The Analog Signal Conditioner ASC The ASC has two major functions to reduce the dynamic range of the input signal so that it can be adequately digitized by a 10 bit converter and to reduce the bandwidth of the resultant signal to meet the Nyquist criterion for the following ADC This criterion is that there should be no frequency component in the signal which exceeds half of the sampling frequency Frequencies above this value are aliased into the digitized signal at lower frequencies where they are indistinguishable from original components at those frequencies In particular high frequency noise would appear as excess low frequency noise spoiling the spectrometer s energy resolution The DXP Saturn therefore has a 4 pole Butterworth filter with a cutoff frequency of about 8 MHz The dynamic range of the preamplifier output signal is reduced to allow the use of a 10 bit ADC which greatly lowers the cost of the DXP This need arises from two competing ADC requirements speed and resolution Speed is required to allow good pulse pileup detection as described in 3 5 For high count rates pulse pair resolution less than 200 ns is desirable which implies a sampling rate of 10 MSA or more The DXP uses a 20 MSA ADC On the other hand in order to reduce the noise o in measuring Vx see Fig 3 1 experience shows that o must be at least 4 times the ADC s single bit resolution AV This effectively
85. uire a yellow fill 5 Release the left mouse button You are done You may add up to 20 ROIs The first one labeled 0 in the text box is the calibration energy used in DXP gt Reconfigure To delete a single ROI use the cursor Click on the Cursor button located on the upper left side of the main window to select a ROI and then use ROI gt Remove Each time the spectrum is updated the bins in each ROI are fit to a Gaussian and the peak position and FWHM are calculated and displayed in the text box Also displayed are the ROI bin limits and counts in each ROI The full parameter set For debugging purposes it is useful to be able to report all the DXP s control parameters and internal code variables used to record runtime statistics These can be found from the View gt Parameters menu This view is also useful in determining sets of operating parameters to download to the DXP when operating from other systems than DxpDemo program 2 NUMDRDOS1 96 RESETINT DACPERADC CODEREV 116 NUMZIGZAGO 0 ASCTIMEOUT 50 DACPERADCE 65531 HDWRVAR 1 NUMZIGZAGI 0 BLFILTER 512 TRACKRST 234 FIPPIREY 5 EVTSINSCAO 0 BLFILTERF 64 TRACKCEN 126 FIPPIVAR 0 EVTSINSCAI 0 BASEBINNING 2 TRACKLST 23 DECIMATION 4 RTLO 0 BLCUT 63899 POLARITY 0 RUNIDENT 10 RTHI 0 TRACEWAIT 0 SLOPEMULT 17636 RUNERROR 0 LTLO 0 SLOWLEN 25 SLOPEMULTE 7 ERRINFO 0 LTHI 0 SLOWGAP 3 BINFACT 1024 BUSY 6 SCALO 0 PEAKINT 30 CIRCULAR 12117 LIVETIMEO 76 SCAHI 0 FASTLEN 4 SPECTSTART 2384 LIVETIME1
86. ulse pile up when the pulses are well resolved by the fast channel This value should be set as PEAKINT SLOWLEN SLOWGAP N where N 2 or 3 5 10 6 Setting Gain parameters HIGHGAIN GAINDAC In general the user does not modify these parameters directly but through the host software routine SetAcquisition Values See Appendix The DXP internal gain is chosen to set the ADC dynamic range appropriately for the signals of interest If it is set too low the energy resolution may be compromised while if is set too high there may be excessive deadtime The ADC range is one volt full scale Two guidelines are suggested for the internal gain setting 19 This is appropriate when there is a single peak of interest Set the gain such that the typical pulse height is between 5 and 10 of the ADC range for 10 bit ADC or between 2 and 10 of the ADC range for 12 bit ADC 20 This is appropriate when looking at a fixed energy range with no particular peak of interest Set the gain such that the maximum energy pulses are around 300 400 ADC channels The parameters HIGHGAIN and GAINDAC set the coarse and fine internal amplifier gain The overall gain can be expressed as follows Gtot Gin Ghigh Gvar Gbase where Gin input stage gain roughly 2 or roughly 4 depending on setting of jumper JP102 Ghigh High Gain relay setting 1 for low gain 4 for high gain Gvar variable gain setting 1 to 100 depending on GAINDAC setting Gba
87. ut through the host software routine SetAcquisition Values See Appendix X rays are identified when the fast filter output in units of AADC FASTLEN goes above threshold This threshold can be expressed in energy units once the DXP conversion gain Gpxp number of ADC counts per keV at the DXP input is known For an energy threshold Eth in keV THRESHOLD Gpxp Etph FASTLEN The conversion gain is discussed below The MINWIDTH parameter is used for noise rejection It is the minimum number of time bins the fast filter is above threshold A typivsl value that works with FASTLEN 4 is MINWIDTH 4 46 Manual DXP Saturn Digital X ray Processor mdo DXP Saturn UM001 5 5 10 5 Setting the Pile up inspection parameters MAXWIDTH PEAKINT In general the user does not modify these parameters directly but through the host software routine SetAcquisitionValues See Appendix MAXWIDTH is used to reject pulse pile up on a time scale that is comparable to FASTLEN The idea is that wide pulses are really two or more normal width pulses that are separated in time but not enough for the fast filter to go below threshold A typical value is MAXWIDTH 2 FASTLEN FASTGAP N where N is in the range 4 8 If the signal rise time depends on the x ray energy e g bandwidth limited preamplifier or low field regions of the detector that are preferentially sampled at some energy this cut can bias the spectrum if it is too small PEAKINT is used to reject p
88. w filter The fast filter is used to detect the arrival of x rays the slow filter is used to reduce the noise in the measurement of Vx as described in the sections above Figure 3 5 shows the same data as in Figure 3 1 Figure 3 4 together with the normalized fast and slow filter outputs The fast filter has a filter length Lf 4 and a gap Gf 0 The slow filter has Lgs 20 and Gg 4 Because the samples were taken at 10 MSA these correspond to peaking times of 400 ns and 2 us respectively 22 Manual DXP Saturn Digital X ray Processor mdo DXP SATURN UM001 5 F S Filtered Data kfig 960920 Di A oe eoat tooe 0000000 oe eo et oe 20 Preamp 1 6 eeatt s oe anrenat Ponte ett ony Da Arrival Time s y 12 MINWIDTH 3 gt 2 2 5 Fast Filter pa O 8 ver eette see 0000 30 wetter tens 09 00 000000 000 000 900 0 00009200 00000000 00 0 000 cere 3 vo e e 5 PEAKSAMP gt lt Sampling Time 4 Slow Filter 0 smaneccaneessseoonecenssosnsconesgensgenssesesee 00000000000000000000000000 20 22 24 26 28 30 32 Time us Figure 3 5 Peak detection and sampling methods in the DXP digital processor The arrival of the x ray step in the preamp output is detected by digitally comparing the fast filter output to the digital constant THRESHOLD which represent a threshold value Once the threshold is exceeded the number of values above threshold are counted If they exceed a minimum number MINWIDTH th
89. which include such solid state detectors as Si Li HPGe HgI2 CdTe and CZT detectors are generally operated with charge sensitive preamplifiers as shown in Figure 3 1a Here the detector D is biased by voltage source V and connected to the input of amplifier A which has feedback capacitor Cr In resetting preamplifiers a switch S is provided to short circuit Cf from time to time when the amplifier s output voltage gets so large that it behaves nonlinearly Switch S may be an actual transistor switch or may operate equivalently by another mechanism In pulsed optical reset preamps light is shined on the amplifier A s input FET to cause it to discharge Cr In transistor reset preamps the input FET may have an additional electrode which can be pulsed to discharge Cr The output of the preamplifier following the absorption of an x ray of energy Ex in detector D is shown in Figure 3 1b as a step of amplitude Vy When the x ray is absorbed in the detector material it releases an electric charge Qx Ex s where e is a material constant Qx is integrated onto Cr to produce the voltage Vx Qx Cf Ex eC Measuring the energy Ex of the x ray therefore requires a measurement of the voltage step Vx in the presence of the amplifier noise o as indicated in Figure 3 1b 4 z 2 8 0 e 2 E 2 L o 4 0 00 0 02 0 04 0 06 a b Time ms Figure 3 1 a Charge sensitive preamplifier with reset b Output on absorption of an x ray 3 2 X ray En
90. y Processor mdo DXP SATURN UM001 5 The above expression is valid for peaking times less than about TAURC 2 Alternatively RCFCOR can be determined empirically in a special test run from a linear fit of data as in Figure 3 7 3 7 Pile up Inspection The value Vx captured will only be a valid measure of the associated x ray s energy provided that the filtered pulse is sufficiently well separated in time from its preceding and succeeding neighbor pulses so that their peak amplitudes are not distorted by the action of the trapezoidal filter That is if the pulse is not piled up The relevant issues may be understood by reference to Figure 3 8 which shows 5 x rays arriving separated by various intervals Because the triangular filter is a linear filter its output for a series of pulses is the linear sum of its outputs for the individual members in the series In Figure 3 8 the pulses are separated by intervals of 3 2 1 8 5 7 and 0 7 us respectively The fast filter has a peaking time of 0 4 us with no gap The slow filter has a peaking time of 2 0 us with a gap of 0 4 us The first kind of pileup is s ow pileup which refers to pileup in the slow channel This occurs when the rising or falling edge of one pulse lies under the peak specifically the sampling point of its neighbor Thus peaks 1 and 2 are sufficiently well separated so that the leading edge point 2a of peak 2 falls after the peak of pulse 1 Because the trapezoidal filter funct

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