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A flexible baseline restorer
NUCLEAR
INSTRUMENTS
AND METHODS
IOO (1972)
349-353; © N O R T H - H O L L A N D
PUBLISHING
CO.
A FLEXIBLE BASELINE R E S T O R E R * M. B E R T O L A C C I N I and C. BUSSOLATI
Istituto di Fisiea del Politecnieo di Milano, Milano, Italy Received 29 November 1971 A baseline restorer has been developed in which both the restorer time constant and the positive and negative voltage thresholds can be varied independently. This makes it possible to cope better with different experimental situations, thus reaching higher resolving power in nuclear spectroscopy.
1. Introduction
The present state of the art of semiconductor detectors and of the associated charge preamplifiers enables very high resolution in nuclear spectroscopy to be reached. Often, a fairly high resolution is needed in connection with rather high repetition rates for the events to be detected. These facts have led to an increased sophistication of nuclear spectrometry systems1'2), which are now generally made up, apart from the detector and the preamplifier, of a main shaping amplifier, a baseline restorer, a pile-up rejector and a multi-channel analyzer. Pole-zero cancellation 3) is generally used in the main amplifier. The baseline restorer makes possible better performances for two main reasons: (1) it reduces the baseline shift due to pile-up of long-lasting pulse tails, (2) it reduces the effect of low frequency disturbances, like hum and microphonism, and noise components, which could, in a pole-zero cancelled system, cause serious broadening of linewidth. The purpose of the present work is to present a baseline restorer, whose concept is different from that used in existing ones, allowing more flexibility to match different experimental situations and therefore better resolution. 2. Method
The diagram in fig. 1 is fairly representative of the * Work supported by C.N.R. Ricerche).
(Consiglio Nazionale delle
FROM
z~o
INTO
I
IIC
VIN~i~IR
z~¢~
/
VOUT
Fig. 1. Equivalent circuit for a baseline restorer.
working principle of a baseline restorer, as already observed by Radeka4). Different types of restorer have been developed, the main difference in the philosophy of the various types being the instant at which the switch opens and the value of the resistance R, which determines, together with the coupling capacitance C, the restorer time constant. In the Robinson restorerS), sketched in fig. 2a, the switch opens at a threshold voltage of a few hundred millivolts, while the value of R is that of the series resistance of the diodes. The I - V characteristic as seen at point A of fig. 2a is shown by the dashed line in fig. 3. The amplified-diode restorer 6) represented in fig. 2b has the I - V characteristics shown by the solid line in fig. 3. One disadvantage that could limit the high-rate performance of these restorers is due to the current/flowing when the switch is open, which causes both a drop in the head of the pulses and an unwanted undershoot, which can be recovered only in a time as long as the original pulse. This disadvantage was eliminated in the active restorer designed by Gere and Miller7), fig. 4, which acts as a nearly perfect clamp, while the gate current is flowing, and a very high impedance when the gate current is absent. Its I - V characteristic has the shape shown in fig. 5. The rigid clamp, while having good high-count-rate behavior [since it performs, essentially, the difference between the zero and the peak value, for unipolar pulses, and the peak to peak difference for bipolar ones8)] increases the low-rate linewidth. A restorer in which the long recovery time disadvantage of the simple diode or amplified-diode restorers is eliminated by performing an active restoration, but avoiding the complexity of gating, so that at low count rates the simple passive method can be accomodated by a switch, has been suggested by Williams9). The I - V characteristic for this type of restorer is shown in fig. 6.
349
350
M. BERTOLACCINI AND C. BUSSOLATI
c
INPUT
II
X
® o Cll INPUT
OUTPUT O°UTPUT
I
GATE
EIRRENT .~ lg
(o)
I
INPU~--tI I
0
,o,
i ...... ::
OUTPUT
i (b) Fig. 2. a) Passive (Robinson) restorer, b) Amplified-diode (ChasePoulo) restorer. A characteristic c o m m o n to the passive, the amplifier-diode and the William's restorer is that the restorerresistance values are set by the restorer current. This is a drawback as the restorer-current values cannot be chosen freely, owing to the drop that this current causes when the "switch" is open. The two thresholds, one positive and one negative, have fixed values for the passive (a few hundred millivolts) and amplified-diode (a few millivolts) restorers. In some cases, this is a
Ca) Fig. 4. Active (Gere-Miller) restorer.
far more serious drawback than the first one mentioned. Pulses followed by large, rapid undershoots (see fig.7) are better dealt with if the undershoot can recover freely to the baseline. The basic principle on which we have developed a new type of restorer is shown in fig. 8. Both the lower threshold, V1, and the higher one, V2, and the switch resistance, R, can be set independently of each other. The absence of restorer current beyond the thresholds, when the switch is open, eliminates the drawback connected with the undershoot. The possibility of setting V1, //2 and R independently should be useful for the optimum fitting of the restorer to different experimental situations, in respect both of the counting rate and of the shape of the spectra to be detected.
V Fig. 3. I-V characteristic of passive (dashed line) and amplifieddiode (solid line) restorer.
Fig. 5. I-V characteristic of the active restorer.
351
A FLEXIBLE BASELINE RESTORER
In the design of the experimental version that has been developed (fig. 10), the switch is open when the signal passes over two (one positive and one negative) thresholds set independently. In principle, the thresholds should be considered with reference to the undisturbed signal (i.e. the output signal). To avoid oscillation due to positive feedback, the thresholds have been considered with reference to the input signal, via a passive Robinson restorer.
/ 12
Fig. 6. I - V characteristic of Wifliams' restorer.
4. Performance The settings of the restorer's operating conditions, that is, the optimum values of the time constant and of the upper and lower threshold depends closely on the characteristics of the spectrum to be analyzed. The optimum value for the time constant depends chiefly on the shape of the noise spectrum (i.e. low
3. Circuit details
A block diagram of the restorer is shown in fig. 9. The input resistance of the output buffer is fairly high (gate-source resistance of a F.E.T.), so that with the switch open the time constant of the restorer approaches infinity. With the switch closed, the resstorer time constant is given by CR. Some discussion may be devoted to the logic which is followed in the opening of the switch. In our system, this logic can be chosen completely independently of the setting of the restorer time constant. In some cases, it may be found useful to open the switch for a given time after the beginning of the pulse, while in others the switch will be always closed when the output voltage lies within two given thresholds, and is open when the voltage lies outside this interval. Perhaps a safer choice in order to recover from long, large tails given by overloading pulses would be that of combining the two above mentioned criteria.
Fig. 7. The effect of the drop caused by restorer current oll pulses followed by a large undershoot.
I
I v2
Fig. 8. Our restorer's I - V characteristic.
frequency components vs high frequency components), on other low frequency disturbances like hum or microphonism, on the amplitude and shape of pulsetails and on pulse-rate. The optimum setting of the threshold is connected with pulse-shape, pulse-amplitude distribution (relative rates of small and large pulses), amplitude of pulse-tails, and amplitude of noise-pulses. As is quite obvious, no general recipe can be given, while a complete set of experiments to classify the exact performance of the restorer would be to lengthy. We shall summarize the results of tests made with artificial pulses in some experimental situations that we thought to be the most interesting ones. In the course
M. BERTOLACCINI AND C. BUSSOLATI
352 FROM.A.N.
AMPLIFIER
"
e
TO PULSE o HEIGHT ANALYZER
R
II
,~,
SWITCH ~ _ - - ~ - ~ 1 LCONTROL
, -,
Fig. 9. Block diagram~of our restorer. of experiments, an inspector circuit was used to avoid direct pile-up of pulses. Fig. 11 shows the spectra of monochromatic artificial pulses at a frequency of 5 x 104 pulses/s, recorded with our restorer, with a commercial "active" restorer, with a commercial passive restorer and without a restorer. The main amplifier was set in good conditions of pole-zero cancellation. (In real experimental situations pole-zero cancellation is never complete.) The optimum operating conditions for the restorer were: C R = I #s, V 1 = - 1 0 0 mV, V 2 = + 1 0 0 mV, while the pulse peak amplitude was V = 5 V. While in the experimental situation of fig. 11 the setting of the threshold voltage was not of paramount importance, with the pulse followed by a large tail the situation may be quite different, as shown in fig. 12.
Noise performance of the restorer has been tested too, showing an obvious dependence on the restoring time constant and threshold settings; in this case, too, its greater adaptability often enables this restorer to give a better performance. In conclusion we may say that, at the expense of a somewhat greater circuit complexity, this type of restorer is particularly flexible in the different experimental situations that may arise, although the setting of the instrument is obviously more cumbersome than with less flexible instruments. Its main advantage may be summarized by saying that in every experimental situation its operating conditions can be set so as to obtain the best performance of the commercial restorer best suited to that particular application, and a performance better than those obtained with other restorers. ~--2o./J 2~j~T._[ Hi I
+12
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Fig. 10. Circuit layout of the experimental version of our restorer.
+t2
U
~q-;
It°~ 3311 ~-o5÷-,
A FLEXIBLE
BASELINE
104
353
RESTORER
tb
lO3
102
C HANN E LS
Fig. 12. Effect of the threshold setting on the pulse height distribution of artificial monochromatic pulses at 5 x 104 pulses/s. Nearly gaussian pulses, 5 V peak-voltage, followed by an exponential tail ( r = 500 #s) 400 mV high. The time constant of the restorer was ~ - - 1 0 #s. Negative threshold: V1=450 mV. Positive threshold: a) V2=450 mV; b) Vz= I00 mV. Curve c) is taken without restorer. Fig. 11. Pulse height distribution of artificial monochromatic pulses at 5 x 104 pulses/s, a) with our restorer, b) with a commercial "active" restorer, c) with a passive restorer, d) without restorer.
References 1) M. G. Strauss et al., Rev. Sci. Instr. 38 (1967) 725. ~) H. Meyer and H. Verelst, Trans. Intern Symp. Nucl. Electron. (Versailles, 10-13 Sept. 1968)p. 512. s) j. L. Blankenship and C,H. Nowlin, Rev. Sci. Instr. 36 (1965) 1830.
4) V. Radeka, Rev. Sci. Instr. 38 (1967) 1397. 5) L. B. Robinson, Rev. Sci. Instr. 32 (1961) 1057. 6) R. L. Chase and L. R. Poulo, IEEE Trans. NucI. Sci. NS-t4, no, 1 (1967) 83. 7) E. A. Gere and G, L° Miller, IEEE Trans. Nucl. Sci. NS-14, no. 1 (1967) 89. 8) M. Bertolaccini, C. Bussolati and E. Gatti, Nucl, Instr. and Meth. 42 (1966) 286. 9) C. W. Williams, IEEE Trans. Nucl. Sci. NS-15, no. 1 (1968) 297.
INSTRUMENTS
AND METHODS
IOO (1972)
349-353; © N O R T H - H O L L A N D
PUBLISHING
CO.
A FLEXIBLE BASELINE R E S T O R E R * M. B E R T O L A C C I N I and C. BUSSOLATI
Istituto di Fisiea del Politecnieo di Milano, Milano, Italy Received 29 November 1971 A baseline restorer has been developed in which both the restorer time constant and the positive and negative voltage thresholds can be varied independently. This makes it possible to cope better with different experimental situations, thus reaching higher resolving power in nuclear spectroscopy.
1. Introduction
The present state of the art of semiconductor detectors and of the associated charge preamplifiers enables very high resolution in nuclear spectroscopy to be reached. Often, a fairly high resolution is needed in connection with rather high repetition rates for the events to be detected. These facts have led to an increased sophistication of nuclear spectrometry systems1'2), which are now generally made up, apart from the detector and the preamplifier, of a main shaping amplifier, a baseline restorer, a pile-up rejector and a multi-channel analyzer. Pole-zero cancellation 3) is generally used in the main amplifier. The baseline restorer makes possible better performances for two main reasons: (1) it reduces the baseline shift due to pile-up of long-lasting pulse tails, (2) it reduces the effect of low frequency disturbances, like hum and microphonism, and noise components, which could, in a pole-zero cancelled system, cause serious broadening of linewidth. The purpose of the present work is to present a baseline restorer, whose concept is different from that used in existing ones, allowing more flexibility to match different experimental situations and therefore better resolution. 2. Method
The diagram in fig. 1 is fairly representative of the * Work supported by C.N.R. Ricerche).
(Consiglio Nazionale delle
FROM
z~o
INTO
I
IIC
VIN~i~IR
z~¢~
/
VOUT
Fig. 1. Equivalent circuit for a baseline restorer.
working principle of a baseline restorer, as already observed by Radeka4). Different types of restorer have been developed, the main difference in the philosophy of the various types being the instant at which the switch opens and the value of the resistance R, which determines, together with the coupling capacitance C, the restorer time constant. In the Robinson restorerS), sketched in fig. 2a, the switch opens at a threshold voltage of a few hundred millivolts, while the value of R is that of the series resistance of the diodes. The I - V characteristic as seen at point A of fig. 2a is shown by the dashed line in fig. 3. The amplified-diode restorer 6) represented in fig. 2b has the I - V characteristics shown by the solid line in fig. 3. One disadvantage that could limit the high-rate performance of these restorers is due to the current/flowing when the switch is open, which causes both a drop in the head of the pulses and an unwanted undershoot, which can be recovered only in a time as long as the original pulse. This disadvantage was eliminated in the active restorer designed by Gere and Miller7), fig. 4, which acts as a nearly perfect clamp, while the gate current is flowing, and a very high impedance when the gate current is absent. Its I - V characteristic has the shape shown in fig. 5. The rigid clamp, while having good high-count-rate behavior [since it performs, essentially, the difference between the zero and the peak value, for unipolar pulses, and the peak to peak difference for bipolar ones8)] increases the low-rate linewidth. A restorer in which the long recovery time disadvantage of the simple diode or amplified-diode restorers is eliminated by performing an active restoration, but avoiding the complexity of gating, so that at low count rates the simple passive method can be accomodated by a switch, has been suggested by Williams9). The I - V characteristic for this type of restorer is shown in fig. 6.
349
350
M. BERTOLACCINI AND C. BUSSOLATI
c
INPUT
II
X
® o Cll INPUT
OUTPUT O°UTPUT
I
GATE
EIRRENT .~ lg
(o)
I
INPU~--tI I
0
,o,
i ...... ::
OUTPUT
i (b) Fig. 2. a) Passive (Robinson) restorer, b) Amplified-diode (ChasePoulo) restorer. A characteristic c o m m o n to the passive, the amplifier-diode and the William's restorer is that the restorerresistance values are set by the restorer current. This is a drawback as the restorer-current values cannot be chosen freely, owing to the drop that this current causes when the "switch" is open. The two thresholds, one positive and one negative, have fixed values for the passive (a few hundred millivolts) and amplified-diode (a few millivolts) restorers. In some cases, this is a
Ca) Fig. 4. Active (Gere-Miller) restorer.
far more serious drawback than the first one mentioned. Pulses followed by large, rapid undershoots (see fig.7) are better dealt with if the undershoot can recover freely to the baseline. The basic principle on which we have developed a new type of restorer is shown in fig. 8. Both the lower threshold, V1, and the higher one, V2, and the switch resistance, R, can be set independently of each other. The absence of restorer current beyond the thresholds, when the switch is open, eliminates the drawback connected with the undershoot. The possibility of setting V1, //2 and R independently should be useful for the optimum fitting of the restorer to different experimental situations, in respect both of the counting rate and of the shape of the spectra to be detected.
V Fig. 3. I-V characteristic of passive (dashed line) and amplifieddiode (solid line) restorer.
Fig. 5. I-V characteristic of the active restorer.
351
A FLEXIBLE BASELINE RESTORER
In the design of the experimental version that has been developed (fig. 10), the switch is open when the signal passes over two (one positive and one negative) thresholds set independently. In principle, the thresholds should be considered with reference to the undisturbed signal (i.e. the output signal). To avoid oscillation due to positive feedback, the thresholds have been considered with reference to the input signal, via a passive Robinson restorer.
/ 12
Fig. 6. I - V characteristic of Wifliams' restorer.
4. Performance The settings of the restorer's operating conditions, that is, the optimum values of the time constant and of the upper and lower threshold depends closely on the characteristics of the spectrum to be analyzed. The optimum value for the time constant depends chiefly on the shape of the noise spectrum (i.e. low
3. Circuit details
A block diagram of the restorer is shown in fig. 9. The input resistance of the output buffer is fairly high (gate-source resistance of a F.E.T.), so that with the switch open the time constant of the restorer approaches infinity. With the switch closed, the resstorer time constant is given by CR. Some discussion may be devoted to the logic which is followed in the opening of the switch. In our system, this logic can be chosen completely independently of the setting of the restorer time constant. In some cases, it may be found useful to open the switch for a given time after the beginning of the pulse, while in others the switch will be always closed when the output voltage lies within two given thresholds, and is open when the voltage lies outside this interval. Perhaps a safer choice in order to recover from long, large tails given by overloading pulses would be that of combining the two above mentioned criteria.
Fig. 7. The effect of the drop caused by restorer current oll pulses followed by a large undershoot.
I
I v2
Fig. 8. Our restorer's I - V characteristic.
frequency components vs high frequency components), on other low frequency disturbances like hum or microphonism, on the amplitude and shape of pulsetails and on pulse-rate. The optimum setting of the threshold is connected with pulse-shape, pulse-amplitude distribution (relative rates of small and large pulses), amplitude of pulse-tails, and amplitude of noise-pulses. As is quite obvious, no general recipe can be given, while a complete set of experiments to classify the exact performance of the restorer would be to lengthy. We shall summarize the results of tests made with artificial pulses in some experimental situations that we thought to be the most interesting ones. In the course
M. BERTOLACCINI AND C. BUSSOLATI
352 FROM.A.N.
AMPLIFIER
"
e
TO PULSE o HEIGHT ANALYZER
R
II
,~,
SWITCH ~ _ - - ~ - ~ 1 LCONTROL
, -,
Fig. 9. Block diagram~of our restorer. of experiments, an inspector circuit was used to avoid direct pile-up of pulses. Fig. 11 shows the spectra of monochromatic artificial pulses at a frequency of 5 x 104 pulses/s, recorded with our restorer, with a commercial "active" restorer, with a commercial passive restorer and without a restorer. The main amplifier was set in good conditions of pole-zero cancellation. (In real experimental situations pole-zero cancellation is never complete.) The optimum operating conditions for the restorer were: C R = I #s, V 1 = - 1 0 0 mV, V 2 = + 1 0 0 mV, while the pulse peak amplitude was V = 5 V. While in the experimental situation of fig. 11 the setting of the threshold voltage was not of paramount importance, with the pulse followed by a large tail the situation may be quite different, as shown in fig. 12.
Noise performance of the restorer has been tested too, showing an obvious dependence on the restoring time constant and threshold settings; in this case, too, its greater adaptability often enables this restorer to give a better performance. In conclusion we may say that, at the expense of a somewhat greater circuit complexity, this type of restorer is particularly flexible in the different experimental situations that may arise, although the setting of the instrument is obviously more cumbersome than with less flexible instruments. Its main advantage may be summarized by saying that in every experimental situation its operating conditions can be set so as to obtain the best performance of the commercial restorer best suited to that particular application, and a performance better than those obtained with other restorers. ~--2o./J 2~j~T._[ Hi I
+12
+12 IN
o,, ;oo
~20AI
.j
,IH o
5,3K
2N3906 TOO
O4
[-~
220
lOOp 3.3K
I00
+12
-12
+24
+6
Z 20'/J
+24
*24
!-~i~l~J,-
*12
•
J
IOC
,K_! "HoL
- 0.7
f
tFtl, 32~
5,6K
z21t
-6 -11.2
i.SF
oqA-
- :~
a3K
2N390~ -I0.5
"~O.IZJ ~_2ju _4~K
-6 . -~-12
-12
-6 -24
Fig. 10. Circuit layout of the experimental version of our restorer.
+t2
U
~q-;
It°~ 3311 ~-o5÷-,
A FLEXIBLE
BASELINE
104
353
RESTORER
tb
lO3
102
C HANN E LS
Fig. 12. Effect of the threshold setting on the pulse height distribution of artificial monochromatic pulses at 5 x 104 pulses/s. Nearly gaussian pulses, 5 V peak-voltage, followed by an exponential tail ( r = 500 #s) 400 mV high. The time constant of the restorer was ~ - - 1 0 #s. Negative threshold: V1=450 mV. Positive threshold: a) V2=450 mV; b) Vz= I00 mV. Curve c) is taken without restorer. Fig. 11. Pulse height distribution of artificial monochromatic pulses at 5 x 104 pulses/s, a) with our restorer, b) with a commercial "active" restorer, c) with a passive restorer, d) without restorer.
References 1) M. G. Strauss et al., Rev. Sci. Instr. 38 (1967) 725. ~) H. Meyer and H. Verelst, Trans. Intern Symp. Nucl. Electron. (Versailles, 10-13 Sept. 1968)p. 512. s) j. L. Blankenship and C,H. Nowlin, Rev. Sci. Instr. 36 (1965) 1830.
4) V. Radeka, Rev. Sci. Instr. 38 (1967) 1397. 5) L. B. Robinson, Rev. Sci. Instr. 32 (1961) 1057. 6) R. L. Chase and L. R. Poulo, IEEE Trans. NucI. Sci. NS-t4, no, 1 (1967) 83. 7) E. A. Gere and G, L° Miller, IEEE Trans. Nucl. Sci. NS-14, no. 1 (1967) 89. 8) M. Bertolaccini, C. Bussolati and E. Gatti, Nucl, Instr. and Meth. 42 (1966) 286. 9) C. W. Williams, IEEE Trans. Nucl. Sci. NS-15, no. 1 (1968) 297.