F-100Tv9 Manual - Advanced Research Instruments Corp.
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1. F 100T F 100TD Amplifier Discriminator Cable Model T P1 6 Leer H Analog NIM TTL ECL N U N U SUB MINI D 9 PINCONNECTOR SYSTEM PART NO PRM 400T F 100T F 100TD Ampli fier Discriminator Cable Model T P4 6 PRM 100 Counter Timer Co CI Co Co a i di SUB MINI D 9 PIN CONNECTOR CHA CHB PRM 400 Counter Timer CI CI Co A Figure 3a Cable Selection Guide Cable Model T P1 6 F 100TD to PRAT 100 ar to F 100FS Cable Model T CUS6 F 100TD OFEN ENDED CABLE RG174 U Connectors Spin Submin D Female Figure 3b Cable Diagrams Potential Problems Two potential problems may be associated with the 500 termination These are discussed as follows e Some rate meters or counters may have only a high impedance input available therefore an external terminator must be used If your rate meter does not have 500 input impedance you can easily modify it by attaching a metal film or carbon resistor across the input Do not use wire wound resistors because they tend to have high inductance If modifying the input is not feasibl2. the noise from the supply enters the preamplifier via R1 and C1 with almost the same amplitude as at the power supply Note that we are trying to detect pulses of amplitude of 0 2mV in the presence of almost 20mV noise Yet the detector preamplifier i e an F 100TD preamplifier system may be capable of detecting and processing signals as low as 100uV The 20mV noise degrades the detection level from 100uV to 20mV This problem can be remedied by obtaining an H V power supply with noise less than 100uVp p such as the Advanced Research Instruments model PMT2000 2k Series Noise Reduction by Negative H V Biasing Technique If a negative H V biasing technique see Figure 5b is used the 20 mV noise does not appear directly at the preamplifier s input The residual noise reaching the preamplifier is only 20pV well below the detection capability of the system The H V is applied to the front end of the detector while the output from the detector is at virtual ground When the preamplifier is electrically at the ground side of the biasing system the preamplifier will see only a small fraction of the power supply induced noise in other words a negligible noise level Even higher noise levels than this may be quite acceptable Additional benefits of negative H V biasing are The output of the detector is at virtual ground potential and can be DC coupled to the input of the preamplifier This direct coupling eliminates an H V blocking cap
3. be greater than 1 2 turns of the threshold dial If you cannot reduce the detector voltage as described you may operate at the higher voltage without any damage to the preamplifier Check the best setting at least once a year to assure that detector aging or changes in the high voltage bias do not shift the threshold out of the correct setting range 10 The Digital Threshold Display The digital threshold display represent a threshold value referred to the input of the amplifier The actual threshold is 350X greater than indicated since the signal that is applied to the analog comparator is also 350X greater than the input due to the linear amplifier voltage gain For example a one mV pulse applied to the input would appear as 350mV pulse on the output of the linear amplifier and at the entrance to the analog comparator To detect this pulse the analog comparator s threshold needs to be set to less than 350mV A good value may be about the half the signal 175mV To avoid the gain calculation when setting the threshold and understanding its function the display is already calibrated by the gain to display the threshold as it would appear at the input of the amplifier In this example the display would read 175mV 350 0 5mV which is half of the 1mV input signal exactly what we wanted without the gain calculation For most applications it is important only to remember that the display reading is referred to the input The discriminator with th
4. input connector This should produce plenty of a signal on the output If you suspect that the F 100TD may have insufficient sensitivity check the Sensitivity Measurement section If no signal appears at the output the preamplifier may need to be repaired However if the preamplifier appears to be functioning then you should consider the detector its power supply or even the signal source itself Use an oscilloscope or different preamplifier to make sure the detector is producing a useful signal If it is you may have to slightly increase the H V bias on your detector The reason for this is because some slower preamplifiers are even more sensitive then the F 100TD and it is possible that the signal produced by your detector at this bias setting is below the sensitivity level of the F 100TD Too Much Noise First determine where the noise originates whether in the amplifier itself or from another source To find out where the noise is coming from disconnect the input to the preamplifier and if the noise persists check the noise level on the positive and negative supply The noise level should be less than 10mVp p 120Hz anda high frequency noise even less Check the supply ground connection and if there is no obvious power supply problem then the preamplifier needs to be repaired If the noise disappears when you disconnect the input then the noise is originating at the detector or in the way the preamplifier is connected to th
5. to determine the required capacitor corresponding to the desired pulse width However it is important to use the minimum pulse width that can be tolerated by the rate meter This is because the minimum pulse pair spacing maximum pulse pair resolution is directly related to the output pulse width and pulse pair resolution is limited by the output pulse width Therefore the wider the output pulse width the worse will be the pulse pair resolution and the lower the maximum count rate 10000 1000 Capacitance pF gt o 10 100 1000 Pulse W idth ns Figure 1 Timing Capacitor Selection for Desired Output Pulse Width Pin Number O DO NDUIDWNA I Signal TTL Output Not Used V 8 to 24 VDC Remote Threshold Control Monitor Optional Not Used Signal Ground Power Supply Ground V 8 to 24VDC Threshold Monitor Optional Figure 2 Power connector pin Out F 100TD Amplifier Discriminator Cable Model T CUS 6 BNC TTL Output Optional remote threshold control monitor be SUB MINI D 9 PIN CONNECTOR GND V V SYSTEM PART NO F 100TPS F 100T F 100TD Amplifier Discriminator Cable Model T P1 6 F 100PS Power Supply Opt SUB MINI D 9 PINCONNECTOR SYSTEM PART NO PRM 100T
6. 18 KQ 50 Q 2uA 4 mA 100 uV to 200 mV 2 uA 160 uA 100 uV to 8mV lt 20 fC 50dB lt 0 4 uAp p 20 Vp p Negative lt 2 3ns Standard BNC 0 14 mV referred to input 0 5V 0 14mV ref to input gt 50MHz lt 20ns 10 ns standard ius 5V TTL 500 9 pin mini D type BNC 8V to 24VDC 50mA 2 5 x 5 x 2 USER MANUAL for the F 100TD TRANSIMPEDANCE PREAMPLIFIER DISCRIMINATOR V9 This manual is written to assist you in the installation and operation of your F 100TD transimpedance preamplifier discriminator INSTALLATION Refer to Figure 2 on the following page for the connector pin out The preamplifier requires an unregulated dual power supply of 8V to 24V typically 12V 15V or 24V at 50mA and 30mA This is the least critical connection since built in voltage regulators virtually eliminate any ripple instability or noise on the power lines Output Considerations Output pulses from pin 1 represent the standard TTL CMOS level 5 0V unterminated with respect to ground pin 6 The output impedance of the F 100TD is 50 ohms To avoid any potential problems related to an improperly terminated transmission line such as ringing or multiple pulsing use a 500 coaxial cable terminated by a 500 resistor or a ratemeter with 500 input impedance Pulse Width The minimum output pulse width is about 10ns You can extend the pulse width by adding a C8T capacitor inside the F 100TD The graph in Figure 1 enables you
7. Model F 100TD Amplifier Discriminator V9 USER MANUAL Revision August 30 2005 TABLE OF CONTENTS F 100TD VS SPEGIFICGATIONS ipa 2 INSTALLATION spola 3 Output Considerations iiiiiiiiiiiiiiiiniii inte 3 PuleWidilis e nepalesi 3 Figure 1 Timing Capacitor Selection for Desired Output Pulse Width 3 Figure 2 Power Connector Pin Out 111 e 4 Figure 3a Cable Selection Guide 1 2 e 4 Figure 3b Cable Diagrams aaa 5 Potential Problems g iiini1tiiiiiiii iiiiii1iiiiaii 6 Figure 4 Example of Ringing Reduction by 502 Termination 7 Input Gonsiderations iii 7 Figure 5 Electron Multiplier BiaS 1 ri 8 OPERATION uaar 9 Connecting and Disconnecting Your Preamplifier 1 rrrrrr reni 9 Discriminator Set Up 9 Figure 6 Typical Signal from the Detector and the Threshold setting 9 Figure 7 Relationship Between the Threshold Position and Resulting Count Rate 9 The Digital Threshold Display ri 11 TROUBLE SHOOTING GUIDE iii 12 No Quiput Pulses li iene rit 12 ToO Much N Oj e Eee ei 12 Figure 8 Missing Ground in the BNC Connector 13 Double Fulsng _ ea 14 Sensitivity Measurement 1 1 0r0e rece i
8. acitor and the necessary additional wiring so that a potential source of noise is eliminated 15 e DC current can be measured as the signal when the count rate exceeds the maximum count rate of the pulse processing channel e When pulses and a DC component both are present a special dual channel preamplifier such as a COMBO 100 can be used The COMBO 100 processes a low intensity signal as pulses and it processes the high intensity signal as a DC voltage proportional to the intensity A disadvantage of the negative biasing technique is the presence of the high negative potential at the detector s front end This negative potential prevents negatively charged particles from being detected and tends to collect any positive spurious ions thus increasing the background count Because of the higher background count the negative H V bias scheme is not recommended for applications with signal intensities count rates just slightly above the background level It should be used only for those applications in which the processing of higher count rates is more important than a low background Positive H V Biasing This technique can be used to advantage in some situations For example a photo multiplier tube can be operated at lower dark current or background level Also positive H V biasing can be used to bias a charged particle detector so the front end of the detector is at ground potential or at a slight positive or negative bias to attr
9. act charged particles of given polarity A disadvantage of this technique is the need for an H V supply with a noise level below the detection capability of the detector Summary As we can see both positive and negative biasing techniques present advantages and disadvantages So if you want to improve the sensitivity and detection limits of your detector you will want to consider the constraints of a given application plus the limitations described above Then you can better evaluate and choose a particular biasing arrangement and define the necessary specifications for the H V power supplies 16 PHOTON COUNTING APPLICATIONS Two applications are discussed how to optimize photon counting and how to automate your data acquisition How to Optimize Photon Counting The best way to perform very low light level measurements is by counting individual photons with a photo multiplier tube PMT and associated counting electronics When we use the PMT for counting individual photons we are usually concerned with the background level dark count as well as the accuracy at the maximum count rates The background count rate creates the low light level limit the point at which the signal becomes smaller than the background fluctuation We can use high pulse pair resolution to improve accuracy at high count rates thus minimizing the error caused by pulse pile up and the resulting loss of counts So the best PMT systems feature the lowest pos
10. d to virtually any spectrometer To control the spectrometer to change the wave length etc the parallel interface provides a driver for a low power stepping motor and input for a set of limit switches The computer can be programmed to provide flexible intelligence to the system to adapt specific requirements for a particular measurement to improve accuracy to store data for subsequent analyses or to simply archive many spectra on a mass storage media Since most spectrometers are driven by a synchronous motor which produces a constant speed movement of the wave length selector shaft a strip chart recorder can be driven at the same speed to produce a wave length vs intensity chart Although the PRM 100 produces an analog output compatible with most strip chart recorders there is an easier more accurate and less expensive way to produce a wavelength vs intensity chart Just use any personal computer to do the data acquisition and store and plot the charts on a dot matrix printer Also to automate the acquisition of X Ray diffraction patterns and have them readily available for further processing Advanced Research Instrumentation Inc provides software which is specifically designed for an X Ray diffractometer This software can be easily tailored to particular spectrometers typically without any additional expense 18
11. e a BNC tee adaptor may be used with a BNC 50 Q terminator The 50 terminators are available from Advanced Research Instruments Corp or any major electronic supplier e The 500 terminator reduces the pulse height to 3 0V which may not be sufficient for some rate meters to detect If the 3 0V pulse height is insufficient for your rate meter or counter you might consider an unterminated signal connection but only if the physical length of the coax cable is less than approximately one meter The shorter the connection the safer it is to use the unterminated cable The decision as to whether to use a terminated cable must be based on the input specifications of the your counter timer rate meter A fast sensitive rate meter gt 20 MHz with sensitivity lt 1V always requires proper termination A relatively slow rate meter usually has an input sensitivity control or a threshold adjustment that can be used for input signals of 1V to 25V and high input impedance In this case although external termination for a short cable may not be absolutely necessary it is nevertheless recommended The 3 0V signal level with a properly adjusted input threshold to about 1V is the best solution A long cable longer than 15 feet gt 5 meters must almost always be terminated The following examples Figures 4a and 4b on page 7 show significant improvement of reflectance for a terminated cable compared with an unterminated one The most significa
12. e detection system To find out the source of the noise in the detection system first turn off the detector s bias supply and the signal source and reconnect the preamplifier to the detector If the noise persists try to locate its level by turning the discriminator dial clockwise until the noise disappears If the noise is too high to be eliminated by the dial check the input connections for a missing ground or shielding connection The input signal conductor should not be exposed anywhere from the detector to the preamplifier Good R F rules must be observed so as not to turn the input connection into a receiving antenna One of the less obvious missing ground problems in the connection may be found in the BNC connector itself In some cases an intermittent noise is detected especially when the preamplifier is touched or moved Such a symptom usually indicates a poor ground connection at the input The most likely culprit is the BNC connector Check the ground connection at the BNC connector where the ground shroud of the male connector mates with the female The shroud may be collapsed and 12 failing to make solid ground connection Fix or replace the male connector Figure 8 Missing Ground in the BNC Connector The ground shroud in the BNC connector may be responsible for poor ground connection When you successfully locate the noise level with the discriminator dial the next step is to lower it to an acceptable level via impr
13. e digital threshold display also serves another useful function As mentioned in the previous section you can increase the threshold to the point of disappearance of the signal and read the signal strength or with other words the height of the incoming pulses directly from the display The threshold height at the point of loss of the signal is equal to the incoming pulse height Please note that the input pulse height measurement is useful only within the linear range of the linear amplifier section namely 2 160uA or 0 1 8mV range Higher level signals would reach the saturation level of the linear amplifier and will enter the analog comparator as fixed flat top pulses The linear portion of the amplifier limits the excessive pulses to 4V maximum at the output of the linear section to maintain the normal function of the unit even in the presence of some out of range pulses Although using the amplifier with signal greater than 160uA or 8mV is not recommended it is harmless to the unit up to and in an excess of 200mV at the input 11 TROUBLE SHOOTING GUIDE No Output Pulses First check the D C supply voltage Unplug the 9pin cable from the preamplifier and measure the voltage on the cable connector You should see 8V to 24V on pin 3 and 8V to 24V on pin 8 relative to pin 7 power ground Then disconnect the input connector and turn the discriminator control fully counterclockwise With a piece of wire touch the center pin of the
14. e polarity Reversed V with V may destroy the preamplifier Although the input to the F 100TD is protected against high voltage spikes do not connect or disconnect the input with high voltage bias applied to the detector Use the following procedure to ensure virtually trouble free performance in your preamplifier e Us connect the preamplifier to the detector first make the connection then turn on the high voltage e To disconnect the preamplifier from the detector first turn off the high voltage then wait 5 10 seconds Then you may safely disconnect the preamplifier Discriminator Set Up The discriminator is factory set to the position that works well for the Advanced Research Instruments PMT test system However this setting may need to be changed for different systems and different bias settings Consider Figures 6 and 7 1V CW E 3 Threshold too high region 5 Missing most of the signal n 2 ao A Top ofthe D signal pulses 2 yA 4 E zl i a H Threshold 1 Best Setting Peak to Sa peak noise Noise region level e I OV Ons Time 50ns Count rate True Count Output from the Comparator rate L_JIII ie Figure 6 Typical signal from the Figure 7 Relationship between the detector and the threshold setting The threshold pos
15. g system to use Use a 50 Q resistor for a 50 Q cable and 75 Q resistor for 75 Q cable If the 50 Q termination is reducing the pulse amplitude excessively you may have to resort to a 75 Q cable termination system Sensitivity Measurement The sensitivity measurement is relatively simple but requires some test equipment and an impedance matching network at the input of the amplifier The F 100TDV9 is transimpedance amplifier thus a current sensitive amplifier and requires a current input Since most of the pulse generators have only low impedance 500 output this has to be converted to a current signal via 2kO resistor located directly at the input of the amplifier To produce the minimum detectable current signal at the input apply a negative pulse of 4mV amplitude and less than 5ns rise time to the 2kO input resistor The resulting 4mV 2kOQ 2uA pulse will be converted into a TTL voltage pulse at the output of the F 100TD if the threshold is set to less than 0 1mV The threshold voltage is referred to the input and reflects the gain of the linear stage The 2uA input signal would appear as 100pV at the input due the 500 input impedance of the amplifier When you increase the threshold above the top of the signal 0 1mV the output should disappear In the same way 8mV should produce 0 2mV at the input and should be detectable with the discriminator setting around 0 2mV This way the discriminator is actually detecting the input signal level and the se
16. he F 100E preamplifier coupled with the PRM 100 precision rate meter is a system capable of processing pulses only 5ns wide which results in counting frequencies of up to 75 MHz Even higher count rate up to 400 MHz can be obtained from a F 400E and PRM 400 system The key word here is frequency rather than count rate Frequency implies a periodic wave form or pulses equally spaced which is not usually the case in which intensities are measured ion photon electron counting or X Ray intensity measurements So the pulse overlapping probability that results can be described statistically by the Poisson distribution function The measurement here consists of the number of counts during a fixed period of time regardless of spacing or distribution This gives us an average number of counts during that period This number is usually recalculated to give us a number of counts per second CPS For a given pulse pair resolution the closer we approach the 5ns limit the more likely we are to encounter pulses closer than our pulse pair resolution and the more often we lose them However the loss is statistically predictable and we can correct for it using the Poisson distribution function and push the maximum count rate a bit closer to the maximum frequency How to Automate Your Data Acquisition The PRM 100 coupled with a computer possessing a fast parallel interface becomes an intelligent easy to use fully automated system when attache
17. ii iii 14 Reducing Power Supply Induced Noise 15 Using a Suitable Power Supply to Reduce Noise 15 Noise Reduction by Negative H V Biasing Technique 15 Positive H V BiasiNgix ale ie alii RA Amii 16 PHOTON COUNTING APPLICATIONS 000ii 17 How to Optimize Photon Counting cece cece cece eee e eee e eee e eee e eee eeeeeeeeeeeeeeeeeeeees 17 How To Minimize the Background Count 17 Maximizing the Useful Count Rate erre 18 How to Automate Your Data Acquisition rreeii 18 F 100TD V9 SPECIFICATIONS Advanced Research Instruments Corporation Lic 5151 Ward Road Sulla 1 Wheat Ridge CO 66033 Made in USA wD 163 5500 F 300 3 A Transimpedance Input impedance Input range Input linear range Input charge sensitivity Linear section transimpedance gain Noise referred to input Input pulse polarity Input pulse rise time Input over voltage protection Input connector Threshold adjustment range Remote threshold input Maximum pulse repetition rate Pulse pair resolution Minimum output pulse width Maximum output pulse width Output signal Output impedance Power connector and signal Second Output Signal Power requirements Physical size DISCRIMINATOR TTL OUTPUT
18. ition and resulting count output from the comparator and the rate final TTL output from the F 100TD Figure 6 shows signal and noise coming from the linear preamplifier just before it enters the discriminator for thresholding When the threshold dial is turned fully counterclockwise the threshold is buried in noise input noise plus preamplifier noise and the F 100TD may produce high count rate pulses even if no signal is present see Figure 7 Note the count rate when the threshold is set close to zero Since this is an unacceptable situation the solution is to increase the threshold by turning the dial clockwise until the noise disappears e Perform this adjustment with the F 100TD connected to the detector and the rate meter e Be sure that when you perform the adjustment that power is applied to the F 100TD but without the high voltage applied to the detector Note If there are no pulses even with the discriminator fully counterclockwise the noise level might be below the minimum detectable threshold or additional problems may be present With the signal source photons electrons etc turned off turn on the high voltage and slowly increase to the approximate operating range If no H V adjustment is feasible then just turn it on Noise should increase because the bias background is significantly higher than the noise of the preamplifier Increase turning clockwise the threshold above the noise level A count rate of 1 t
19. nsitivity If a much larger signal is required for the detection that indicates lower than standard gain and the amplifier should be repaired 14 Reducing Power Supply Induced Noise Noise is one of the basic limitations of virtually any signal detection system this is true no matter how advanced the system is However excess noise defined as noise above the theoretical limits can often be virtually eliminated The most common sources of excess noise are poor shielding poor grounding and noisy power supplies Since the noise induced by power supplies is the easiest to spot and remedy the following paragraphs deal with reducing this type of noise Using a Suitable Power Supply to Reduce Noise Those who have system noise often comment that the power supply cannot be responsible They say they have checked the specifications and they are incredibly good However the facts are often otherwise For example a typical high quality high voltage power supply specification may indicate noise less than 001 of the output voltage Sounds good but is it Let s say the high voltage is 2000VDC The 001 represents a 20mV noise level Many detector preamplifier configurations use a positive H V biasing technique directly coupling this noise to the preamplifier and therefore directly determining the minimum detection limit see Figure 5a on page 8 Noise from the Positive H V Supply When a positive H V supply is used see Figure 5a
20. nt value is the positive excursion of the ringing value since the tripping threshold for the TTL signals is about 0 7V minimum The 1 0V glitch on the unterminated cable will very likely produce a double pulsing The problem is most severe when a high frequency and high sensitivity counter is used When selecting the counter timer rate meter make sure that only the signal pulses are being counted Measurements on a shorter cable indicate only minor ringing amplitude So it is possible to use even unterminated cables but only if an input threshold is or can be adjusted safely above the ringing level Or you could use high threshold integrated circuits such as 7414 74LS14 74514 74HC series etc on the input of your counter timer or rate meter Figure 4a 16 feet of unterminated 500 cable Figure 4b The same 16 foot 500 Note the 1 0V glitch that may be detected by terminated cable Note the absence of the counter as a second pulse glitches but the pulse height is reduced to Figure 4 Example of Ringing Reduction by 509 Termination Horizontal 25 ns Vertical 1V div To conclude most TTL signals do not need termination unless long coaxial cables are involved If you terminate the cable ringing is reduced significantly unfortunately the signal is thereby reduced to about half its original amplitude If this is a problem an intermediate solution may be the answer A 100Q terminator on a 500 cable produces only slightly larger
21. o 5 cps may be acceptable The background count rate is usually determined by the quality of the detector or the entire experimental set up Now apply the signal to your detector PMT or electron multiplier The count rate should increase to the correct level With the signal applied increase the threshold by turning the threshold dial further clockwise and note the change in the count rate There should be minimal change within certain ranges i e the best setting as shown in Figure 7 When you reach a threshold setting that is too high the count rate drastically drops off The best setting is approximately in the middle between the noise level and loss of the signal An ideal detector would produce signal pulses all of the same height and in that case there should be no change in the count rate when changing the threshold within the indicated range in fig 6 In addition one would observe an abrupt and complete loss of signal when the threshold passes the top of the uniform pulses This is easily demonstratable by replacing the detector with a pulse generator Any real detector however will produce non uniform signal pulses with approximately Gaussian pulse height distribution If you cannot locate the point of losing the signal this indicates unnecessarily high voltage to the detector Reduce this voltage to the level where you can observe the output change with respect to the threshold shown in Figure 7 The best setting range should not
22. oved grounding and shielding When the noise level is below the minimum threshold setting turn on the detector s bias and note any changes of the noise level A slight increase is normal A drastic increase of noise points to either a noisy bias supply or an H V insulations breakdown Check the noise level of the supply if your biasing is similar to Figure 5a and make sure there is a low pass filter installed in the high voltage line before it reaches the detector see Figure 5a capacitor CF Since even the best supplies may have a 2 50 mV high frequency ripple that may be directly coupled to the input of the preamplifier you may need to get a better H V supply such as ARI s model PMT 2000 If your biasing corresponds to Figure 5b you are using a negative high voltage supply and your preamplifier is connected to the ground side of the supply The ripple in this arrangement is not critical because only a small portion of it reaches the preamplifier Attention should instead be focused on the H V breakdown effect where the most likely cause is contamination or device break down Note In many experimental set ups the researcher forgets the high frequency grounding and shielding rules Then any high and unpredictable noise is usually blamed on the preamplifier since that is where the noise shows up Double Pulsing 13 The count rate is consistently about twice or even triple the rate it is supposed to be This condition is usually the
23. result of a poor connection between the preamplifier and the detector such as e Ground connection e Mismatched impedance of various connectors e Vacuum feedthrough devices e Exposed input signal connection This poor connection results in a standing wave condition on the input of the amplifier that may produce one or more extra pulses for each authentic pulse arriving from the detector Raising the threshold of the discriminator usually solves the problem However if good R F practice was not followed when the input connection was done standing waves may be so severe that the preamplifier will enter sustaining oscillations Avoid severe ringing Here is the primary rule that must never be compromised the connection from the detector to the input of the preamplifier must be done with a 50 Q impedance coaxial cable as short as possible without exposing the center conductor at feedthrough or anywhere else Any compromise at all of this primary rule results in the increase of the standing wave condition ringing and an unnecessary high threshold level of the discriminator to avoid the double pulsing Double pulsing can also occur on the output of the preamplifier if a long and unterminated cable connects the preamplifier to a counting system such as a rate meter This is perhaps the easiest problem to rectify Just terminate the connection on the rate meter side with a 50 Q or 75 Q resistor and make sure there is enough signal for the countin
24. ringing than a 50 Q terminator but the pulse amplitude is only slightly less than the original Input Considerations The input connection is far more critical than the output connection High sensitivity input requires the shortest possible connection to the PMT or to an electron multiplier Also the noise in the input signal or in the input ground connection may be eliminated by setting the discriminator to the lower sensitivity region However this will reduce the overall performance Bear in mind that the 100 uV input sensitivity of the F 100TD preamplifier is about the same as most radio and television receivers except with a much larger bandwidth making the situation much more critical Also the preamplifier must operate in an electrically hostile environment without responding to the severe interference of switching power supplies the R F part of mass spectrometers computers SCRs etc These interference sources are a serious threat to an interference free operation of the preamplifier especially if they are located in the vicinity of the detector preamplifier Therefore good R F grounding and shielding rules must be strictly observed Preamplifiers are normally set to accept negative pulses since all PMT s and electron multipliers produce such pulses Although the input of the F 100TD is compatible with either a positive or a negative biasing scheme of the PMT the details may vary substantially Therefore be sure to examine
25. sible background count coupled with the highest pulse pair resolution The following two sections explain how to minimize the background count and to maximize the useful count rate How To Minimize the Background Count Select a PMT with the certified lowest dark count rate usually specified at a given temperature and bias voltage Lower the operating temperature by cooling the PMT to the lowest level that is practical CAUTION Too low a temperature may result in moisture condensation on the tube and cause problems with the high voltage bias Lower the high voltage bias so that the signal incoming photons can be comfortably detected i e the threshold is safely above the noise level but not too high To do this you need a good preamplifier The F 100TD preamplifier is probably the best choice because it combines high sensitivity with high pulse pair resolution Minimize light leaks This suggestion sounds so obvious it seems ridiculous to even mention it But experience shows that a light leak is the most common culprit when it comes to minimizing background count For example just as a simple test if you think there is a light leak in the system try turning the room lights off and on and note the change of the background count rate 17 Maximizing the Useful Count Rate You ll need to get the minimum pulse width your system can handle since the maximum count rate is limited by the pulse pair resolution For example t
26. your particular bias diagram and compare it with Figure 5 F 100TD Amplifier TTL Out Disriminator PMT or Electron Multiplier H V Blocking BNC 9 Pin Mini D ane i rouni E A Comector Connections c1 N Threshold Adj N Noise 20mVp p Signal 0 2mVp p Output BNC TTL Out Optional 100 MQ Resistor 100kQ RF VV NNV NWI NN Positive H V Bias R3 R2 R1 100kQ CF Noise 20mVp p Figure 5a Electron Multiplier Bias with Positive Supply F 100TD Amplifier TTL Out Disriminator Power and BNC Or Combo 9Pin Mini D Ground 100 Connector Connections mit 7 PMT or Electron Multiplier Threshold Adj Noise 20uV sce Output BNC N TTL Out Optional 100 MQ Resistor A os Negative H V lt Bias 100kQ gt Noise 20mVp p 5 o Figure 5b Electron Multiplier Bias with Negative Supply Figure 5 Electron Multiplier Bias Note there is no high voltage blocking capacitor in the F 100TD If the detector biasing system requires such isolation the capacitor must be located outside the F 100TD preamplifier or a different preamplifier with built in blocking capacitor network may be used such as MTS 100 Finally for the input connection itself use a BNC to BNC adaptor or a very short cable Under some circumstances an additional ground strap may be necessary OPERATION Connecting and Disconnecting Your Preamplifier CAUTION Before applying power recheck the low voltag
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