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Walk reduction in timing circuits through amplitude information
NUCLEAR
INSTRUMENTS
AND
METHODS
95
(I97~) 327-33I; ©
NORTH-HOLLAND
PUBLISHING
CO.
WALK R E D U C T I O N IN T I M I N G C I R C U I T S T H R O U G H A M P L I T U D E I N F O R M A T I O N * N. ABBATT1STA, B. MARANGELLI and V. L. PLANTAMURA lstituto di Fisica, Bari, Italy
Received 3 February 1971 A method is presented for the time variance reduction, in high speed timing circuits, that exploits the amplitude information of the individual signals. An experimental circuit to test the method effectiveness is also shown. 1. Introduction An important problem of nuclear electronics is that of determining the instant of arrival z of an impulsive signal. The theoretical aspect is connected to such a definition of r as to imply its lowest variance as regards the variations of the other parameters which characterize the impulse we are dealing with. The experimental aspect is bound to the design of timing circuits capable of determining z without contributing in a significant way to its variance. Some authors have proposed different definitions of r such as: "the instant of crossing of the signal Si through a predetermined level" (level crossing), "the instant of crossing of the signal Sj - k . S ~ ( t - t j) through zero level" (zero crossing); "the instant of occurrence of the signal centroid" and so on 1-3). T]he experimental measurement of z, however defined, is generally made by threshold circuits which conl:ribute in a significant way to the overall walk owing to the non-zero value of the threshold. So as to minimize such a contribution, attempts to design circuits presenting a dynamic threshold as low as possible 4-6) have been made, compatible with the circuit reliability as regards noise. In this work we present a method which allows the determination of the value z to a higher accuracy, exploiting the amplitude information contained in the individual signals in order to compensate the errors introduced by the non-zero value of the threshold.
measured directly and which is represented in fig. 1 by the interval Az. In order to make Az null, we can consider compressing the input amplitudes to a unique value, for instance the lowest one (e .... -= ~min). In this case, it is necessary to place at the input a filter having a variable gain of: G =- Go/o~. This normally is realized only approximately by a limiting diode. Still observing fig. 1, the same result (AT = 0) can be obtained varying the threshold level of the circuit proportionally with the value of c~; this is what we have thought of realizing, varying the tunnel diode bias Jo as a function of the input signal amplitude. Actually the relationship between the t.d. bias and the signal amplitude necessary to make AT zero, is not one of a simple proportionality; this is due to the following causes: for a predetermined bias value the threshold Jin ~k
2. Description of the method Let us consider a threshold circuit realized with a tunnel diode (t.d.) monostable stage feed by input signals that reach their highest value in a time T, which we ,:an consider as ramp signals expressed by: .]in = s t ,
Omax
~min ~ 0~ ~ 0~max.
Let us use, for example, the definition of r as the instant of crossing a specific level set by the input signal. Then, the uncertainty in z is identical with that * Work supported by C.N.R.
t-- At
I
-4
t
Fig. 1. Diagram of walk Ar with regard to the level crossing definition. 327
328
N. A B B A T T I S T A et al. time [RpC
24
20
/
~6
t+E
INO'~OUT
I T
.01
_.I
! RpC
,1
_
R =
P
P
fP
_-=-
1
II
10
C( RpC
,
,
i!,
A
i
>
i
i
/
.01
.1
1
Fig. 2. a) Delay time for a ramp input, b) Bias variation. 1i, vs slope ~¢.
o i'~cl
329
W A L K R E D U C T I O N IN T I M I N G C I R C U I T S
T1j2
BFY 84
3[IH 7
T
2 Rp ~
T
T3t4j 5 2N 918 D1
BAY 71
BD
BD4
A
variable attenuator
.0~
T
I
T O O OUT
I
,~
:
~.,_~,o.,
lo
.o
/
)1o~
-t~v
~---I I-I,
|
1"
..
/~
too~.,
I
0
-24~'
°'
rj
24v
BD 100
4.7 kpf I INC
~ 15 nsec ~ , ~ 4 7
4.7 kpf
T5
St0kpf
.
~ 1 1
IF--~
-
= "--
470
"T 6v
-24v Fig. 3. Circuit diagram.
~oo
I I
oo
-I-
100
•
"----"~ ~
0
-
100
T
~
4db
~"--~
~ ~ ~ " ' ~ " ~ ' ~ ,
.
m
! ]
2oo
12 db
without comp.I 300
L
!
T__
\
i
400 1
10
20
30
Fig. 4. zlr vs input dynamics for the experimental circuit. Dynamics D t n =
40
~max/~mln.
D in
330
y. ABBATTISTA et al.
/". r - - - - -
-
:.....i[
!
--
I
) TD
IN
[.......
• 1715
1
4
! l l
- 200 0 db
i
!
J
I
Ji
~k
l
J 400
i 6db
1 600 [
1
20
10
30
40
50
D n
(a)
i0A,;Ff
T TD
200
-
-
IN 3858 I
-
0
--I
200
•
300
~ I
10
.
.
.
.
.
. 20
Din
(b) Fig. 5. a)/IT vs input dynamics for the 1N 3715 t.d. b) Ar vs input dynamics for the 1N 3858 t.d. is a function of the c~ valuer); the switching characteristic of the t.d. is not linear near the peak. In o r d e r to find the actual relationship existing be-
tween the p a r a m e t e r ~ and Jo, such that Ar = 0 , we proceeded as follows: let us consider T as the sum of two terms r = "cl(e ) + "c2(~) = const., where z~(c~) is the switching delay as a function o f e for a t.d. having a fixed bias Jo = 0 . 9 ; r2(cQ is the additional delay so that r is a constant, and is o b t a i n e d by v a r y i n g the bias for each ~. Such a p r o c e d u r e is p a r t i c u l a r l y useful in as m u c h as r~(~) can be deduced f r o m the results reported in figs. 7b, 8a o f ref. 7 and so r can easily be evaluated because below J0 = 0.9 the t.d. can be considered an RC system with c o n s t a n t parameters. In fig. 2a we have shown rl(c Q for the t.d. m o n o s t a b l e circuit, inserted in the figure; in fig. 2b we show the relationship between the bias variation Ajo and ~, o b t a i n e d in solving the e q u a t i o n o f an RC circuit, fed by a r a m p current o f slope c~, when p u t t i n g t = T 2 ( o ( ) . As one w o u l d expect the relationship offers a nonlinear interval. 3. Experimental realization and results
Fig. 3 shows the circuit d i a g r a m used for testing the m e t h o d p r o p o s e d . The input signal, suitably delayed, reaches the t.d. timer, just when its bias has been
WALK
REDUCTION
varied, proportionally to the input amplitude, by means of the variable attenuator A and the differential amplifier T12. In such a way, for the sake of a simple realization, we approximate the curve of fig. 2b by a variable slope straight line; t h o u g h this fact implies A r ~ 0. In fig. 4 the experimental behaviour of AT versus the input dynamics is shown. The lowest amplitude is 0.5I,, which gives the zero of the time axis. The curves are expressed as a function of the signal attenuation accomplished only by unit A in fig. 3. Also in fig. 4 the behaviour of A z without any compensation has been reported. In figs. 5a and 5b we show the same curves concerning t.d. with different values o f the peak current. We can then see h o w the utilization o f the signal amplitude information has not only notably reduced the timing walk in a circuit which originally did not
IN T1MING
CIRCUITS
331
present high timing performances, but has given us results which are valid in an absolute sense. To conclude we think that study o f compensation methods similar to the one we have described in this work, can help to improve the performances o f other devices such as amplitude discriminators etc. References
1) j. Grunber and L. Tepper, Trans. Nucl. Sci. NS-13 (1966) 389. 2) A. Bosire, B. De Cosnac and J. Lobb6, Proc. Syrup. Nuclear electronics (Paris, 1953) p. 325. a) C. Cottini, E. Gatti and G. Giannelli, Nuovo Cimento 4 (1956) 1550. 4) A. E. Bjerke, Q. A. Kerns and I. A. Nunamaker, Nucl. Instr. and Meth. 15 (1962) 249. ~) P. R. Orman, Nucl. Instr. and Meth. 21 (1963) 121. 6) D. L. Wieber and H. W. Lefevre, Trans. Nucl. Sci. NS-13 (1966) 406. 7) N. Abbattista et al., Nucl. Instr. and Meth. 45 (1966) 157.
INSTRUMENTS
AND
METHODS
95
(I97~) 327-33I; ©
NORTH-HOLLAND
PUBLISHING
CO.
WALK R E D U C T I O N IN T I M I N G C I R C U I T S T H R O U G H A M P L I T U D E I N F O R M A T I O N * N. ABBATT1STA, B. MARANGELLI and V. L. PLANTAMURA lstituto di Fisica, Bari, Italy
Received 3 February 1971 A method is presented for the time variance reduction, in high speed timing circuits, that exploits the amplitude information of the individual signals. An experimental circuit to test the method effectiveness is also shown. 1. Introduction An important problem of nuclear electronics is that of determining the instant of arrival z of an impulsive signal. The theoretical aspect is connected to such a definition of r as to imply its lowest variance as regards the variations of the other parameters which characterize the impulse we are dealing with. The experimental aspect is bound to the design of timing circuits capable of determining z without contributing in a significant way to its variance. Some authors have proposed different definitions of r such as: "the instant of crossing of the signal Si through a predetermined level" (level crossing), "the instant of crossing of the signal Sj - k . S ~ ( t - t j) through zero level" (zero crossing); "the instant of occurrence of the signal centroid" and so on 1-3). T]he experimental measurement of z, however defined, is generally made by threshold circuits which conl:ribute in a significant way to the overall walk owing to the non-zero value of the threshold. So as to minimize such a contribution, attempts to design circuits presenting a dynamic threshold as low as possible 4-6) have been made, compatible with the circuit reliability as regards noise. In this work we present a method which allows the determination of the value z to a higher accuracy, exploiting the amplitude information contained in the individual signals in order to compensate the errors introduced by the non-zero value of the threshold.
measured directly and which is represented in fig. 1 by the interval Az. In order to make Az null, we can consider compressing the input amplitudes to a unique value, for instance the lowest one (e .... -= ~min). In this case, it is necessary to place at the input a filter having a variable gain of: G =- Go/o~. This normally is realized only approximately by a limiting diode. Still observing fig. 1, the same result (AT = 0) can be obtained varying the threshold level of the circuit proportionally with the value of c~; this is what we have thought of realizing, varying the tunnel diode bias Jo as a function of the input signal amplitude. Actually the relationship between the t.d. bias and the signal amplitude necessary to make AT zero, is not one of a simple proportionality; this is due to the following causes: for a predetermined bias value the threshold Jin ~k
2. Description of the method Let us consider a threshold circuit realized with a tunnel diode (t.d.) monostable stage feed by input signals that reach their highest value in a time T, which we ,:an consider as ramp signals expressed by: .]in = s t ,
Omax
~min ~ 0~ ~ 0~max.
Let us use, for example, the definition of r as the instant of crossing a specific level set by the input signal. Then, the uncertainty in z is identical with that * Work supported by C.N.R.
t-- At
I
-4
t
Fig. 1. Diagram of walk Ar with regard to the level crossing definition. 327
328
N. A B B A T T I S T A et al. time [RpC
24
20
/
~6
t+E
INO'~OUT
I T
.01
_.I
! RpC
,1
_
R =
P
P
fP
_-=-
1
II
10
C( RpC
,
,
i!,
A
i
>
i
i
/
.01
.1
1
Fig. 2. a) Delay time for a ramp input, b) Bias variation. 1i, vs slope ~¢.
o i'~cl
329
W A L K R E D U C T I O N IN T I M I N G C I R C U I T S
T1j2
BFY 84
3[IH 7
T
2 Rp ~
T
T3t4j 5 2N 918 D1
BAY 71
BD
BD4
A
variable attenuator
.0~
T
I
T O O OUT
I
,~
:
~.,_~,o.,
lo
.o
/
)1o~
-t~v
~---I I-I,
|
1"
..
/~
too~.,
I
0
-24~'
°'
rj
24v
BD 100
4.7 kpf I INC
~ 15 nsec ~ , ~ 4 7
4.7 kpf
T5
St0kpf
.
~ 1 1
IF--~
-
= "--
470
"T 6v
-24v Fig. 3. Circuit diagram.
~oo
I I
oo
-I-
100
•
"----"~ ~
0
-
100
T
~
4db
~"--~
~ ~ ~ " ' ~ " ~ ' ~ ,
.
m
! ]
2oo
12 db
without comp.I 300
L
!
T__
\
i
400 1
10
20
30
Fig. 4. zlr vs input dynamics for the experimental circuit. Dynamics D t n =
40
~max/~mln.
D in
330
y. ABBATTISTA et al.
/". r - - - - -
-
:.....i[
!
--
I
) TD
IN
[.......
• 1715
1
4
! l l
- 200 0 db
i
!
J
I
Ji
~k
l
J 400
i 6db
1 600 [
1
20
10
30
40
50
D n
(a)
i0A,;Ff
T TD
200
-
-
IN 3858 I
-
0
--I
200
•
300
~ I
10
.
.
.
.
.
. 20
Din
(b) Fig. 5. a)/IT vs input dynamics for the 1N 3715 t.d. b) Ar vs input dynamics for the 1N 3858 t.d. is a function of the c~ valuer); the switching characteristic of the t.d. is not linear near the peak. In o r d e r to find the actual relationship existing be-
tween the p a r a m e t e r ~ and Jo, such that Ar = 0 , we proceeded as follows: let us consider T as the sum of two terms r = "cl(e ) + "c2(~) = const., where z~(c~) is the switching delay as a function o f e for a t.d. having a fixed bias Jo = 0 . 9 ; r2(cQ is the additional delay so that r is a constant, and is o b t a i n e d by v a r y i n g the bias for each ~. Such a p r o c e d u r e is p a r t i c u l a r l y useful in as m u c h as r~(~) can be deduced f r o m the results reported in figs. 7b, 8a o f ref. 7 and so r can easily be evaluated because below J0 = 0.9 the t.d. can be considered an RC system with c o n s t a n t parameters. In fig. 2a we have shown rl(c Q for the t.d. m o n o s t a b l e circuit, inserted in the figure; in fig. 2b we show the relationship between the bias variation Ajo and ~, o b t a i n e d in solving the e q u a t i o n o f an RC circuit, fed by a r a m p current o f slope c~, when p u t t i n g t = T 2 ( o ( ) . As one w o u l d expect the relationship offers a nonlinear interval. 3. Experimental realization and results
Fig. 3 shows the circuit d i a g r a m used for testing the m e t h o d p r o p o s e d . The input signal, suitably delayed, reaches the t.d. timer, just when its bias has been
WALK
REDUCTION
varied, proportionally to the input amplitude, by means of the variable attenuator A and the differential amplifier T12. In such a way, for the sake of a simple realization, we approximate the curve of fig. 2b by a variable slope straight line; t h o u g h this fact implies A r ~ 0. In fig. 4 the experimental behaviour of AT versus the input dynamics is shown. The lowest amplitude is 0.5I,, which gives the zero of the time axis. The curves are expressed as a function of the signal attenuation accomplished only by unit A in fig. 3. Also in fig. 4 the behaviour of A z without any compensation has been reported. In figs. 5a and 5b we show the same curves concerning t.d. with different values o f the peak current. We can then see h o w the utilization o f the signal amplitude information has not only notably reduced the timing walk in a circuit which originally did not
IN T1MING
CIRCUITS
331
present high timing performances, but has given us results which are valid in an absolute sense. To conclude we think that study o f compensation methods similar to the one we have described in this work, can help to improve the performances o f other devices such as amplitude discriminators etc. References
1) j. Grunber and L. Tepper, Trans. Nucl. Sci. NS-13 (1966) 389. 2) A. Bosire, B. De Cosnac and J. Lobb6, Proc. Syrup. Nuclear electronics (Paris, 1953) p. 325. a) C. Cottini, E. Gatti and G. Giannelli, Nuovo Cimento 4 (1956) 1550. 4) A. E. Bjerke, Q. A. Kerns and I. A. Nunamaker, Nucl. Instr. and Meth. 15 (1962) 249. ~) P. R. Orman, Nucl. Instr. and Meth. 21 (1963) 121. 6) D. L. Wieber and H. W. Lefevre, Trans. Nucl. Sci. NS-13 (1966) 406. 7) N. Abbattista et al., Nucl. Instr. and Meth. 45 (1966) 157.