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A spectroscopy grade multiple pulse generator1
Nuclear Instruments and Methods in Physics Research A 416 (1998) 123—126
A spectroscopy grade multiple pulse generator1 Mikhail A. Mikhailov Institute of Nuclear Research and Nuclear Energy, 72 Tzarigradsko Chaussee blvd., 1784 Sofia, Bulgaria Received 27 May 1997; received in revised form 31 March 1998; accepted 8 April 1998
Abstract An inexpensive signal source combining most of the features of a double pulse generator and a research pulser is reported. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Pulse generators are used with pulse processing systems to simulate the detection of an event in the detector with an electronic pulse. Mercury-wetted relay pulsers are widely used. Their maximal repetition rate, however, is about 100 cps [1]. This limits their use in many applications. For example, oscilloscope observations of the signal at different points in a spectroscopy system is difficult. Noise performance measurements of charge-sensitive preamplifiers, spectroscopy amplifiers, etc. by means of such a pulser and an multichannel analyser are time consuming. Investigation of pile-up rejectors and the behaviour of baseline restorers is impossible. In Ref. [2] two relay activated pulsers have been successively triggered for pile-up effects simulation. Double-pulse generators are generally not suitable for spectroscopy applications. Therefore, it seems a good idea to combine the features of both instruments in one device. The purpose of the work
1 This work was supported by the International Atomic Energy Agency under technical contract No. 8684 RB/TC.
presented here is the design of an low-cost instrument of this type.
2. Description of the device The spectroscopy pulse generator consists of two main blocks — a digital pulse generator and an analogue signal shaper. The digital pulse generator schematic diagram is shown in Fig. 1. The signal from a 2 MHz crystal oscillator is divided by 2 and then fed to a synchronous 12-bit binary counter and to one of the inputs of a 74HC133 NAND gate. The remaining 12 inputs of the NAND gate are connected to the counter outputs through 74HCT125-type buffers. When the buffers outputs are disabled by sw.1 to sw.12 opening, the 12 inputs of the 74HC133 NAND gate are pulled high by the 6k8 resistors. This corresponds to the highest repetition rate of the pulser. If sw.1 to sw.12 are successively closed, the repetition rate will decrease by a factor of two with every switch closure from 106 cps down to 244 cps. At repetition rates, smaller than the highest possible rate, a double pulse can be produced if sw.1 is opened. The separation time
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 3 8 - X
124
M.A. Mikhailov/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 123—126
Fig. 1. Schematic diagram of the digital pulse generator.
between the two pulses will be 1 ls. If sw.2 is opened the separation time will double. If both sw.1 and sw.2 are opened a quadruple pulse will be produced, etc. The inverted 74HC133 signal readily provides the trigger output signal. It is also applied to the D-inputs of an 8-bit register (74HCT574) through 10k resistors. These inputs can be grounded (sw.13 to sw.20). The oscillator signal is used to clock the register whose Q outputs are permanently enabled. The digital pulse generator power supply is #5 V. The analogue shaper schematic diagram is shown in Fig. 2. The signal from the digital pulse generator (logic pulses with duration of 500 ns) is applied to the digital inputs of an 8-bit DAC. The switch bank (sw.13 to sw.20 in Fig. 1) allows 255 different amplitudes to be selected. The switches can be buffered by 74HCT125 buffers, if necessary. A precise reference voltage source is built using a 78L12 voltage regulator and an LM399 voltage reference. The reference voltage is applied to the
Fig. 2. The analogue signal shaper schematic diagram. The components marked with “*” must have low-temperature coefficients. RN55-type resistors and NPO dielectric capacitors are recommended.
DAC reference inputs through 4k99 low-temperature coefficient resistors. The DAC output current is integrated by a 330 pF (NPO) load capacitor. The jumper J1 connects one of two different resistors in parallel with the charge integrating capacitor. Therefore, the voltage signal across the load capacitor has rise time of about 500 ns and exponential decay with time constant of either 50 ls or 2.0 ms. The amplifier A1 is configured as a voltage follower. The output of A1 is applied to an inverting amplifier A2. The switch sw.21 is used to select the polarity of the output signal by connecting the output to either A1 or A2. The output pulse from A1 is also amplified and shaped using the amplifier A3 and the associated shaping network. The 34k8 resistor and the 10k potentiometer constitute a p/z cancellation network. The amplitude range of the shaped pulses is 0 to #10 V. The LM311 comparator is set to switch low if the voltage at the A2 output exceeds 8 V, generating an “overload” signal.
M.A. Mikhailov/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 123—126
125
Fig. 3. Charge terminator.
The amplitude range of the tail pulse output is divided into two subranges (0.2 and 2 V) that are selected by the jumper J2. The tail pulse output is intended to emulate charge sensitive preamplifier or semiconductor detector pulses. In the latter case a charge terminator is used. The schematic of the charge terminator is shown in Fig. 3. The terminator is connected between the pulser output and the detector input of the preamplifier. To avoid the influence of the charge injecting capacitor discharge current, a p/z cancellation network is introduced. This is accomplished by letting k.9 G).0.5 pF"2 ms, where k is the part of the terminator input voltage fed to the 0.5 pF capacitor (2 ms is the recommended decay time constant of the pulser signal for such measurements). The effective value of the charge injecting capacitor is k.0.5 pF"2 ms/ 9 G)"0.222 pF. Alternatively, the charge terminator provided in most preamplifiers can be used. The pulser output is then connected to the preamplifier test input. In this case, however, multiple pulses should not be applied. The analogue electronics are powered by $15 V.
3. Tests and results The pulser RMS noise voltage at the shaped pulse output (0—10 V range) was measured to be 0.32 mV. This results in pulser line FWHM of 753 lV. To estimate the multiple pulse generator noise performance at the tail pulse output, the device was connected to a CANBERRA 2022 spectroscopy amplifier. Pulses with 50 ls decay time constant, in the 0 to 0.2 V amplitude range at 1.95 kcps repetition rate were generated and applied to the spectroscopy amplifier set at gain 3 k. For all possible
Fig. 4. The multiple pulse generator noise performance at the tail pulse output (see text).
Fig. 5. Peak shift during the device warm up time.
amplitudes pulser peaks were recorded and their FWHM were measured. The results are summarized in Fig. 4 (lower curve). The inaccuracies due to the signal amplitude changing and polarity switching are included into the error bars. It should be noted that the results obtained comply very well with the amplifier noise specifications [3]. Therefore, it may be concluded that at this voltage subrange the pulser noise does not contribute significantly to the output noise line width. Similar tests have been conducted for the 0 —2 V subrange. The amplifier gain was 1 k. The results were corrected for the amplifier noise contribution. They are also shown in Fig. 4 (upper curve). The temperature behaviour of the pulser is illustrated in Fig. 5 where the peak shift for pulses of &5.4 V amplitude (244 cps) during the device warm up time is shown. In most cases the device operates at higher repetition rates and the measurements are performed in a short time, therefore the residual drift after the fifth minute has little or no influence on the final results.
126
M.A. Mikhailov/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 123—126
tions, some of which are:
Fig. 6. Peak position as a function of the repetition rate (0 to #10 V range). Pulses of &4.8 V amplitude fed into CANBERRA 8075 ADC, 4k MCA.
Fig. 6 shows the count-rate capabilities of the pulser. The slight peak shift at 31.25 kcps is due to pulses pileup. For proper p/z cancellation, the multi-turn trimmer must be used at the A3 input (Fig. 2). 4. Conclusion The multiple pulse generator presented is a simple, inexpensive device useful in many applica-
z ADC functional tests; z Count rate and noise measurements, in spectroscopy shaping amplifiers z Investigation of pile up effects; z Performance estimation of noise in charge preamplifiers; z spectrum stabilization by means of a reference signal; z signal source for nuclear electronic equipment design and maintenance. This inexpensive pulser has characteristics that are suitable for most practical tests of radiation spectrometers.
References [1] EG&G ORTEC. Model 448 Research Pulser operating and service manual. [2] T. Fazzini, G. Poggi, P. Sona, N. Taccetti, Nucl. Instr. and Meth. A 356 (1995) 319. [3] CANBERRA, Edition Nine Instruments Catalog, 113.
A spectroscopy grade multiple pulse generator1 Mikhail A. Mikhailov Institute of Nuclear Research and Nuclear Energy, 72 Tzarigradsko Chaussee blvd., 1784 Sofia, Bulgaria Received 27 May 1997; received in revised form 31 March 1998; accepted 8 April 1998
Abstract An inexpensive signal source combining most of the features of a double pulse generator and a research pulser is reported. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Pulse generators are used with pulse processing systems to simulate the detection of an event in the detector with an electronic pulse. Mercury-wetted relay pulsers are widely used. Their maximal repetition rate, however, is about 100 cps [1]. This limits their use in many applications. For example, oscilloscope observations of the signal at different points in a spectroscopy system is difficult. Noise performance measurements of charge-sensitive preamplifiers, spectroscopy amplifiers, etc. by means of such a pulser and an multichannel analyser are time consuming. Investigation of pile-up rejectors and the behaviour of baseline restorers is impossible. In Ref. [2] two relay activated pulsers have been successively triggered for pile-up effects simulation. Double-pulse generators are generally not suitable for spectroscopy applications. Therefore, it seems a good idea to combine the features of both instruments in one device. The purpose of the work
1 This work was supported by the International Atomic Energy Agency under technical contract No. 8684 RB/TC.
presented here is the design of an low-cost instrument of this type.
2. Description of the device The spectroscopy pulse generator consists of two main blocks — a digital pulse generator and an analogue signal shaper. The digital pulse generator schematic diagram is shown in Fig. 1. The signal from a 2 MHz crystal oscillator is divided by 2 and then fed to a synchronous 12-bit binary counter and to one of the inputs of a 74HC133 NAND gate. The remaining 12 inputs of the NAND gate are connected to the counter outputs through 74HCT125-type buffers. When the buffers outputs are disabled by sw.1 to sw.12 opening, the 12 inputs of the 74HC133 NAND gate are pulled high by the 6k8 resistors. This corresponds to the highest repetition rate of the pulser. If sw.1 to sw.12 are successively closed, the repetition rate will decrease by a factor of two with every switch closure from 106 cps down to 244 cps. At repetition rates, smaller than the highest possible rate, a double pulse can be produced if sw.1 is opened. The separation time
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 3 8 - X
124
M.A. Mikhailov/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 123—126
Fig. 1. Schematic diagram of the digital pulse generator.
between the two pulses will be 1 ls. If sw.2 is opened the separation time will double. If both sw.1 and sw.2 are opened a quadruple pulse will be produced, etc. The inverted 74HC133 signal readily provides the trigger output signal. It is also applied to the D-inputs of an 8-bit register (74HCT574) through 10k resistors. These inputs can be grounded (sw.13 to sw.20). The oscillator signal is used to clock the register whose Q outputs are permanently enabled. The digital pulse generator power supply is #5 V. The analogue shaper schematic diagram is shown in Fig. 2. The signal from the digital pulse generator (logic pulses with duration of 500 ns) is applied to the digital inputs of an 8-bit DAC. The switch bank (sw.13 to sw.20 in Fig. 1) allows 255 different amplitudes to be selected. The switches can be buffered by 74HCT125 buffers, if necessary. A precise reference voltage source is built using a 78L12 voltage regulator and an LM399 voltage reference. The reference voltage is applied to the
Fig. 2. The analogue signal shaper schematic diagram. The components marked with “*” must have low-temperature coefficients. RN55-type resistors and NPO dielectric capacitors are recommended.
DAC reference inputs through 4k99 low-temperature coefficient resistors. The DAC output current is integrated by a 330 pF (NPO) load capacitor. The jumper J1 connects one of two different resistors in parallel with the charge integrating capacitor. Therefore, the voltage signal across the load capacitor has rise time of about 500 ns and exponential decay with time constant of either 50 ls or 2.0 ms. The amplifier A1 is configured as a voltage follower. The output of A1 is applied to an inverting amplifier A2. The switch sw.21 is used to select the polarity of the output signal by connecting the output to either A1 or A2. The output pulse from A1 is also amplified and shaped using the amplifier A3 and the associated shaping network. The 34k8 resistor and the 10k potentiometer constitute a p/z cancellation network. The amplitude range of the shaped pulses is 0 to #10 V. The LM311 comparator is set to switch low if the voltage at the A2 output exceeds 8 V, generating an “overload” signal.
M.A. Mikhailov/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 123—126
125
Fig. 3. Charge terminator.
The amplitude range of the tail pulse output is divided into two subranges (0.2 and 2 V) that are selected by the jumper J2. The tail pulse output is intended to emulate charge sensitive preamplifier or semiconductor detector pulses. In the latter case a charge terminator is used. The schematic of the charge terminator is shown in Fig. 3. The terminator is connected between the pulser output and the detector input of the preamplifier. To avoid the influence of the charge injecting capacitor discharge current, a p/z cancellation network is introduced. This is accomplished by letting k.9 G).0.5 pF"2 ms, where k is the part of the terminator input voltage fed to the 0.5 pF capacitor (2 ms is the recommended decay time constant of the pulser signal for such measurements). The effective value of the charge injecting capacitor is k.0.5 pF"2 ms/ 9 G)"0.222 pF. Alternatively, the charge terminator provided in most preamplifiers can be used. The pulser output is then connected to the preamplifier test input. In this case, however, multiple pulses should not be applied. The analogue electronics are powered by $15 V.
3. Tests and results The pulser RMS noise voltage at the shaped pulse output (0—10 V range) was measured to be 0.32 mV. This results in pulser line FWHM of 753 lV. To estimate the multiple pulse generator noise performance at the tail pulse output, the device was connected to a CANBERRA 2022 spectroscopy amplifier. Pulses with 50 ls decay time constant, in the 0 to 0.2 V amplitude range at 1.95 kcps repetition rate were generated and applied to the spectroscopy amplifier set at gain 3 k. For all possible
Fig. 4. The multiple pulse generator noise performance at the tail pulse output (see text).
Fig. 5. Peak shift during the device warm up time.
amplitudes pulser peaks were recorded and their FWHM were measured. The results are summarized in Fig. 4 (lower curve). The inaccuracies due to the signal amplitude changing and polarity switching are included into the error bars. It should be noted that the results obtained comply very well with the amplifier noise specifications [3]. Therefore, it may be concluded that at this voltage subrange the pulser noise does not contribute significantly to the output noise line width. Similar tests have been conducted for the 0 —2 V subrange. The amplifier gain was 1 k. The results were corrected for the amplifier noise contribution. They are also shown in Fig. 4 (upper curve). The temperature behaviour of the pulser is illustrated in Fig. 5 where the peak shift for pulses of &5.4 V amplitude (244 cps) during the device warm up time is shown. In most cases the device operates at higher repetition rates and the measurements are performed in a short time, therefore the residual drift after the fifth minute has little or no influence on the final results.
126
M.A. Mikhailov/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 123—126
tions, some of which are:
Fig. 6. Peak position as a function of the repetition rate (0 to #10 V range). Pulses of &4.8 V amplitude fed into CANBERRA 8075 ADC, 4k MCA.
Fig. 6 shows the count-rate capabilities of the pulser. The slight peak shift at 31.25 kcps is due to pulses pileup. For proper p/z cancellation, the multi-turn trimmer must be used at the A3 input (Fig. 2). 4. Conclusion The multiple pulse generator presented is a simple, inexpensive device useful in many applica-
z ADC functional tests; z Count rate and noise measurements, in spectroscopy shaping amplifiers z Investigation of pile up effects; z Performance estimation of noise in charge preamplifiers; z spectrum stabilization by means of a reference signal; z signal source for nuclear electronic equipment design and maintenance. This inexpensive pulser has characteristics that are suitable for most practical tests of radiation spectrometers.
References [1] EG&G ORTEC. Model 448 Research Pulser operating and service manual. [2] T. Fazzini, G. Poggi, P. Sona, N. Taccetti, Nucl. Instr. and Meth. A 356 (1995) 319. [3] CANBERRA, Edition Nine Instruments Catalog, 113.