A Flexible Waveform Generator Using OmniSys and Instrumentation
Contents
1. Kong 852 2599 7889 fax 852 2506 9233 China 86 10 505 0149 fax 86 10 505 0394 for more information on HP EEsof solutions please visit us at our World Wide Web site http Awww hp com go hpeesof Data subject to change 1995 Hewlett Packard Company Printed in USA PN E4600 8 3 97 5965 8120E2. MHz 4 3 1 0 2034505208333 0 6 Therefore external filters are needed because neither of the ARB filters will get rid of the image frequencies The Start and Stop parameters for the ARB2 element were chosen to cover exactly one period of the short code to avoid any discontinuity between the last and first points The Start parameter was set at 12 0035807 usec which corresponds to the 60th sample which is larger than the startup transient of the FIR filters The short code is 32767 chips long at 4 samples per chip and a sampling period of 0 2034505208333 usec this implies that the Stop parameter should be set at 26677 65288 usec 26677 65288 32767 4 1 0 2034505208333 12 0035807 A Flexible Waveform Generator Using OmniSys and Instrumentation The Filter parameter is set to N one because external filters were used The MaxAmplitude parameter is set at 0 16187V the minimum value that will be accepted by the ARB which satisfies the ESG D4000A requirement that 1 Q lt 0 5V rms The spectrum of the output of the ARB modulated up to 100 MHz is shown in Figure 8 FREQ 0 5 MH2 OIV Figure 8 Spectrum of an IS 95 CDMA Pilot Signal A Flexible Waveform Generator Using OmniSys and Instrumentation Example 2 The example in Figure 9 takes the ideal CDMA signal modulates it and transmits it through a 3 path Rayleigh faded channel The inphase and quadrature components of the channel output are extracted and sent t
3. PrimAddr specifies the primary address of the ARB SecAddr specifies the secondary address of the ARB The ARB is addressed using the primary and secondary addresses iopen hpib primary address secondary address where the iopen function is defined in the SICL library To check the address of the ARBs remove the card and check the switch settings If the switch settings for the master and slave ARBs are 80 and 88 respectively then the SecAdadr for the master slave ARBs should be specified as 10 and 11 respectively in OmniSys In general the secondary address specified in OmniSys equals the switch setting divided by 8 Refer to figure 1 2 on page 1 5 of the ARB manual 3 for details on the switch settings The OmniSys ARB2 device has the same parameters except that the primary and secondary addresses of both ARBs must be specified it does not matter which ARB is designated as the master or the slave Practical Considerations for Using the Link If multirate elements are not used the minimum value of TStep that can be specified in an OmniSys simulation is 1 21 47483648 usec if multirate elements are used ensure that the sampling period of the signal at the input to either the ARB1 or ARB2 devices is greater than 1 21 47483648 usec This is because the maximum clock rate for the ARB is 21 47483648 M Hz Given this maximum sampling rate the largest bandwidth signal that can be theoretically generated equals 10 73741824 MHz However
4. an IS 95 CDMA Pilot Signal The schematic shows a clock that clocks two linear feedback shift registers that generate the short codes for the and Q channels of the pilot channel the pilot channel of the forward link contains only the short codes actually the Oth Walsh code which is all zeros is used and therefore is not included in the schematic To generate a traffic channel the Walsh code must be considered The sampling period of the simulation TStep is set such that there are 4 samples per chip The down sample element discards 3 of the samples and the up sample inserts 3 zeros The input to the FIR filter now consists of 4 samples per chip where one sample is the actual value of the chip and the remaining 3 are zeros The PN sequences are then filtered with FIR filters whose tap coefficients are listed in the IS 95 specification The outputs of the filters are the inphase and quadrature phase components of the final RF signal The FIR outputs are then fed to an ARB2 device whose outputs arein turn connected to an HP ESG D series signal generator and modulated up to the required frequency The sampling period TStep in the simulation is chosen to be 0 2034505208333 usec which gives 4 samples per chip The FIR filters have been specified for this sampling rate The bandwidth of the signal at the output of the FIR filters is approximately 600 kHz This implies that the lower end of the image of the spectrum will lie at approximately 4 3
5. due to practical filtering considerations the actual bandwidth that can be accommodated will be smaller Filtering can be done external to the ARB if the 250 kHz and 10 MHz filters available with the ARB arenot suitable for a particular application A Flexible Waveform Generator Using OmniSys and Instrumentation The D A will introduce a sinc f distortion to the spectrum of the output signal If the sampling rate is much greater than the bandwidth of the signal this distortion is typically not significant If this is significant an inverse 1 sinc f filter can be created in OmniSys to compensate for the distortion This can be done by using the LPF RCI lowpass raised cosine filter located in the Functional Filter Elements category of the OmniSys library Set the parameters of this filter as follows F bandwidth of filter set equal to the bandwidth of the signal A roll off factor set equal to 0 Type set equal to Pulse or mpulse with eg this adds a 1 sinc f term to the transfer function of the filter E shaping exponent set equal to 1 Dday delay added to ideal response to make it causal set to beat least 4 F WindowType _ select the type of window required The schematic in Figure 6 shows how the filters are used in the CDMA design LOGICTONRZ C_DSample C_USample C4 c6 CJ CLOCK1 f SRC1 A VOLTS1 CONTROL TimeControl LOGICTONRZ C_DSamp leC_USample C_FIRT LPFRCI CTRL L4 C7 cs c3 L6 Figure 6 Using OmniSys F
6. Solutions from HP EEsof A Flexible Waveform Generator Using OmniSys and Instrumentation Product Note E 4600 9 Overview The Series IV OmniSys software now has the capability of interfacing to instruments that can generate test signals The flexibility of OmniSys allows a wide variety of signals to be simulated which can then be generated by the instruments This allows systems to be tested even when some subsystems have not been completed Applications include generation of I S 95 CDMA signals both ideal and those impaired by fading channels This product note describes the software and hardware requirements and how they should be configured The C code that implements the interface is described Examples of how to generate an ideal S 95 pilot signal and one that is faded are also provided Introduction This product note describes the interface between the OmniSys system simulator and the HP 1445A Arbitrary Function Generator hereafter referred to as the ARB This hardware software link provides a flexible method for generating waveforms OmniSys generates samples of the desired waveform the samples are then fed to the ARB which generates the corresponding analogue waveform Baseband signals can be generated directly by this method RF waveforms such as the IS 95 CDMA or the GSM 0 3GMSK can be generated using OmniSys to compute the inphase and quadrature samples of the waveform using twoARBs to generate the corresponding analogue wa
7. be sent to the ARB If there is no sample corresponding to this time index then the sample with the largest time index less than Stop is the last sample sent The Start and Stop parameters should satisfy the condition O lt Start lt Stop lt TStop where T Stop is specified in the control item MaxAmplitude specifies the maximum amplitude of the output signal of the ARB The ARB is automatically programmed so that the gain of its output attenuator equals this value The amplitude of the ARB output is independent of the amplitude of the signal at the input to the ARB devicein the simulation The input samples are automatically scaled so that the entire dynamic range of the DAC is used and the final output of the ARB is then controlled with the output attenuator Filte specifies the output filter if any of the ARB 250 kHz or 10 MHz Impedance specifies the output impedance of the ARB 50 or 75 ohms DataAvg specifies the method by which the sample points are to be averaged Smoothing the data may help in mitigating the effects of any discontinuity between the first and last points in the data e The 3 point average modifies all data points as follows x i x G 1 mod L x i mod L x i 1 mod L 3 where 0 lt i lt L and L is the length of the data array e Theend point average modifies only the first and the last data points as follows x 0 x 0 x 0 x L 1 2 and x L 1 x L 1 x 0 x L 1 2
8. dress If large amounts of data are being sent saving the data to file can take a considerable amount of time Malloc memory to store data points Store data point Reset ARBs Send commands to initialize ARBs Rescale data convert to DAC codes and send to ARBs Send commands to ARBs to start generating waveforms function setup_arb for arb1 setup_marb and setup_sarb for arb2 functions store_signal scale_signal double_to_dac and send_data function run_arb Figure 11 ARB Interface Device C Code Flow functions arb1 or arb2 A Flexible Waveform Generator Using OmniSys and Instrumentation 11 A Flexible Waveform Generator Using OmniSys and Instrumentation 12 Configuring OmniSys In order to use the ARB link you must obtain a new executable for the OmniSys program this file is omnisys bin and the AEL file that contains the definitions for the ARB1 and ARB2 symbols this file is arb ad These files can be obtained via anonymous ftp to hpeesof external hp com IP address 192 6 21 2 use anonymous as the login name and your email address as the password cd to the directory distribution example and get the file test_prj tar Z This compressed tar file contains omnisys bin and arb ad and an OmniSys project that contains the example for generating the I S 95 CDMA signal The C code file arbl c is also included You can either replace the OmniSys executable that you alrea
9. dy have in EESOF_DIR bin with the new executable or you can place the new executable in the project directory from which you will be running the program Correspondingly you can place the arb ad filein the E ESOF_DIR ad omnisys directory or in the networks subdirectory of the project directory for details refer to Configuring the Environment in the Series IV U ser Defined Elenents manual Hardware and software requirements e HP UNIX workstation e OmniSys E4604A e HPIB card E2071C e VXI cardcage E1421B e VXI controller card E1406A e 2ARB cards E1445A e Signal generator ESG D1000A ESG D2000A ESG D3000A or ESG D4000A e Filters as needed References 1 A Oppenheim and R Schaefer Discrete Time Signal Processing Prentice Hall 1989 2 HP Standard Instrument Control Library Installation and Users s Guide for HP UX Third Edition 1995 3 HP 1445A Arbitrary Function Generator User s Manual Hewlett Packard 1992 NOTES A Flexible Waveform Generator Using OmniSys and Instrumentation 13 Ez HEWLETT PACKARD For more information contact a regional HP office listed below Or check your telephone directory for a local HP sales office United States 1 800 452 4844 Canada 905 206 4725 Europe 31 20 547 9900 J apan 0120 421 345 Latin America Miami Florida 305 267 4245 4220 Australia 1 800 629 485 toll free fax 61 3 9899 3727 Asia Pacific Hong
10. g period sampling rates can be changed only by using multi rate devices such as an upsampler or a downsampler The simulation runs from time 0 to TStop and the samples of the signal at every node are indexed with the values 0 e TStep 1 e TStep 2 e TStep N e TStep where N is the largest integer such that N e TStep lt TStop The DSP portion of the schematic processes samples of baseband signals These samples can be sent directly to the ARB to generate the corresponding waveform In the RF portion of the schematic signals are represented by the inphase x t and quadrature components x t and the carrier frequency The corresponding RF waveform can be generated by the following equation X t X t cos t X t sin t Therefore to generate the corresponding RF waveform the inphase and quadrature components must be first generated with the ARBs and then modulated up to the carrier frequency using an HP ESG D series signal generator The inphase and quadrature components of an RF signal can be extracted in OmniSys by using a DEM QAM element For example to generate the signal at the output of the bandpass filter labeled BPFC a DEMQAM element must be placed as shown in Figure 3 A Flexible Waveform Generator Using OmniSys and Instrumentation FSR LOGICTONRZ C_OSompie C_USaomplie C_FIRT 1 ca c6 c2 H LISTI LI g Be ji ODQAM BPFC GAINI SpecAnalyz G1 SpecA2 Fc 10 l E oa 2 In phase component TimeC
11. ilters to Provide 1 sinc Compensation Because the ARB outputs the data cydically any discontinuities between the last and the first data points must be avoided A discontinuity can cause spurious frequencies in the analogue signal Manually examine the waveform in OmniSys and pick the start and end data points so that they match The DataAvg parameter provides some simple algorithms to smooth the data refer to the previous section for DataAvg details Asan aid to debugging the ARB link the commands sent to the ARB are saved in the file ArbCommandsDebug_hpibaddr log where hpibaddr equals the primary address followed by the secondary address for example the commands sent to the ARB with primary address 9 and secondary address 10 is saved in the file ArbCommandsDebug_910 109 Error messages returned by the ARB are saved in the ArbE rrorMsgs log file A Flexible Waveform Generator Using OmniSys and Instrumentation Example 1 The example in Figure 7 illustrates how an ideal pilot tone CDMA signal for the forward link of the I S 95 standard can be generated LOGICTONRZ C_DSample C_USample c4 c6 Rotio 4 Type zero Ratio 4 CLOCK1 SRC1 A Period 0 81 IS 2 MSecAddr 10 Delay 0 S SPr imAddr 9 DutyCycle 0 50 i SSecAddr 11 TimeControl LOGICTONRZ C_DSomp1eC_USamp le C_FIRT MPrimAddr 9 CTRL L4 c7 cs c3 TStop 26680 Ralio 4 Type zero inserlion File bb_fil TStep 0 20 Ratio 4 Interpol linear Figure 7 OmniSys Schematic for Generating
12. ions is turned off by using the isetbuf function This slows down the rate of data transfer between OmniSys and the ARB The commands for the ARB are set up as an array of strings which are then sent using the command_exe function To program the ARB with options not provided in the interface modify the code in the functions setup_arb stup_marb and setup _sarb When compiling the code and linking it tothe OmniSys archive the SICL library sicl sl or sicl a must also be linked in for details refer to the SICL Users manual 2 The make file OmniSys make should be modified for this The algorithm for scaling the input signal simply goes through all the data points finds the maximum value of the absolute value of all the data points say MAX and multiples all data points by 5 11875 MAX the value 5 11875 is the maximum value that can be output by the DAC The data points are then converted into DAC codes which are unsigned integers with O representing 5 12V 4096 representing OV and 8191 representing 5 11875V see page 7 3 of the ARB manual for details 3 The double to_dac function converts the scaled data points to the DAC codes To examine the DAC codes sent to the ARB change the line at the top of the arbl c ile define DATA_DEBUG 0 tO define DATA_DEBUG 1 and recompile the code The data will be saved to the file ArbDataDebug_address log where address is the primary address followed by the secondary ad
13. nterface The OmniSys interface to the ARB is provided through either the ARB1 or the ARB2 device located in the arb library see Figure 5 The ARB1 device with one input is the interface to a single ARB the ARB2 device with two inputs one for the inphase component and one for the quadrature component of the signal is the interface to two ARBs in the master slave mode The ARB 1 interface can be used to generate baseband signals or RF signals at a low carrier frequency the ARB2 interface can be used in conjunction with an HP ESG D series signal generator to generate RF signals at high carrier frequencies MaxAmp li tude 1 MaxAmp i tude 1 Filter None Filter None Impedance 50 Ohms Impedance 50 Ohms DataAvg None DataAvg None PrimAddr 9 MPr imAddr 9 SecAddr 10 MSecAddr 10 SPrimAddr 9 SSecAddr 11 Figure 5 OmniSys ARB Interface Because data is sent from the simulator to the ARB through an HPIB bus the workstation where the OmniSys simulator runs must be equipped with an HPIB card E2071C Plug amp Play HPIB Card for HP UX The user settable parameters for the ARB 1 device follow Start specifies the time index of the first sample that should be sent to the ARB If there is no sample corresponding to this time index then the sample with the smallest time index greater than Start is the first sample sent A Flexible Waveform Generator Using OmniSys and Instrumentation Stop specifies the time index of the last sample that should
14. o the ARB2 device The spectrum of the generated signal Figure 10 shows the two notches due to the multipath LOGICTONRZ C_OSamp le C_USomple C_FIRT L3 c4 c C2 PR ODQAM AntBase PropNADCcdmal AntMcbile DEMQAM ARB2 VOLTS 1 A2 x1 AS D1 A1 VOLTS1 TimeControl LFSR LOGICTONRZ C_DSample C_USomple C_FIRT CTRL L L4 c7 c5 cs LISTI LQ Figure 9 OmniSys Schematic of an IS 95 CDMA Pilot Transmitter with a 3 Path Rayleigh Fading Channel a a a E ae FREQ 0 5 MHz DIV Figure 10 Spectrum of a 3 Path Rayleigh Faded IS 95 CDMA Pilot Tone Signal A Flexible Waveform Generator Using OmniSys and Instrumentation 10 C Code Considerations The ARB1 and ARB2 devices in OmniSys were created as user defined devices This makes it possible for a user to modify the code if needed You must edit the file arbl1 c recompile any changes and rebuild the OmniSys executable follow theinstructions in the Series IV User Defined Elements manual on how to add your own elements to OmniSys You must also modify the makefile omnisys sr maketo link in the SICL library which is obtained along with the HPIB card for the workstation The basic flow of the C code for the ARB interface devices is shown in Figure 11 One problem that was encountered while developing the code was a memory problem with the SICL library function iprintf while sending the array of data points to the ARB To avoid this problem theinternal buffering by the SICL funct
15. ontrol LFSR LOGICTONRZ C_DSampleC_USomple C_FIRT Quadrature component CTRL L2 4 c7 3 TStop 26680 TStep 0 203 LISTI LQ DSP ______p gt ______ _ __ RF _ gt Figure 3 Extracting Inphase and Quadrature Components of an Omnisys RF Signal HP 1445A Arbitrary Function Generator Basics The HP 1445A is a programmable arbitrary function generator It contains a 13 bit D A with enough memory to store 256K samples which can be loaded through the VXI bus The samples can be clocked out at a clock rate that can be varied from 0 01 samples per second up to 21 47483648 Msamples per second with a resolution of less than 0 02 samples per second a 40 M Hz clock is also available but is not used in the OmniSys ARB interface This variable clock is generated by the reference oscillator DDS DAC filter combination shown in Figure 4 The ARB clocks data out in a cyclic manner that is after clocking out the last sample it starts with the first sample again A continuous waveform is generated in this manner 10 Mhz filter VXI bus Reference Oscillator Figure 4 ARB Block Diagram Certain parameters of the ARB can be controlled from OmniSys For example the maximum amplitude of the output can be set to be in the range 0 1 to 5 1V The output A Flexible Waveform Generator Using OmniSys and Instrumentation impedance can be set at either 50 or 75 ohms If required the ARB has two filters at its ou
16. tput either the 250 kHz or the 10 MHz filter can be selected For generating RF signals two ARBS are required one for the inphase component and one for the quadrature component The ARBs must be synchronized so that they clock out the data in step This is automatically done by OmniSys by configuring the ARBs ina master slave configuration where the clock of the master ARB is also used to clock out the data of the slave ARB The outputs of the ARBs must then be fed to an HP ESG D series signal generator which modulates the signals to the appropriate carrier frequency ESG D Series Digital Signal Generator Basics The ESG D series of digital and analog signal generators can accept inphase and quadrature inputs and quadrature modulate the signals on to a carrier whose frequency can be varied up to 4 GHz different ESG models have different specs so please refer to the data sheets for details about a particular model The full scale input 41 Q equals 0 5V rms for certain signals with a large peak to rms value such as IS 95 CDMA signals the peak to peak value of each of the and Q inputs should at most be 0 5V The 1dB bandwidth of the inphase and quadrature input signals can be up to 10 MHz The output magnitude accuracy rms is maintained within 0 75 percent over the 4 GHz range the phase error rms is maintained within 0 5 degrees the origin offset is less than 40 dBc and the EVM rms is less than 1 percent OmniSys ARB I
17. veforms then using the HP ESG D series signal generators to modulate the waveforms to the RF frequency see Figure 1 VXI card cage Inphase component Filters if needed HP Unix HPIB workstation card Modulator with OmniSys RF output HPIB controller Figure 1 Setup for Generating RF Signals With OmniSys samples of ideal waveforms can be generated or samples of waveforms resulting from impairments such as non linear amplifiers phase noise or fading channels Ez HEWLETT PACKARD A Flexible Waveform Generator Using OmniSys and Instrumentation can be generated This can reduce the instrumentation required for testing transmitters and receivers or individual components such as amplifiers and mixers OmniSys Basics Some basics about OmniSys that are relevant to the ARB interface are described in this section Figure 2 shows the schematic of an IS 95 CDMA transmitter for the pilot channel LOGICTONRZ C_DSample C_USamp leC_F IRT c4 c6 c2 GAINI SpecAnalyz G1 SpecA2 TimeControl LFSR LOGICTONRZ C_DSomple C_USomp 1 _FIRT CTRL L2 L4 c7 c5 c3 TStop 26680 TStep 0 203 LISTI LO DSP pe RF gt p Figure 2 Typical OmniSys Schematic IS 95 CDMA Pilot Channel Transmitter It is important to note that the sampling period of the samples at the output of the CLOCK source are specified by TStep and all other signals in this design except for the output of the downsampler have the same samplin
Download Pdf Manuals
Related Search