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Time-difference measurement in the picosecond range
NUCLEAR
INSTRUMENTS
AND
METHODS
I27
(1975)
TIME-DIFFERENCE MEASUREMENT
557-559;
©
NORTH-HOLLAND
IN THE PICOSECOND
PUBLISHING
CO.
RANGE
WOLFGANG ROHRBECK and GISELA KITZE Akademie der Wissenschaften der DDR, Zentralinstitut fiir Elektronenphysik, 108 Berlin, Mohrenstrafle 40/41, D.D.R.
Received 6 May 1975 Working principles, circuitry, and results of a very simple device for measuring time differences in the picosecond range are given, The time resolution of the device, that was originally intended for
delay-time measurement of vacuum breakdown but may be of interest for many other applications, is in the order of 4-2 ps for a single measurement.
1. Working principle
and:
N o r m a l l y the measurement o f small time differences is done by time-to-pulse height converters o f the startstop type. In these converters the start pulse switches on a voltage which rises linearly with time, and the stop pulse switches off this voltage. The peak voltage is then a measure o f the time difference between the two pulses. The measurement o f a time difference in the picosecond range by this type o f converter is very difficult, because it is nearly impossible to generate voltages rising linearly with time in the first picoseconds. These difficulties could be overcome to some extent by differential converters3), but they use somewhat complex circuitry. The block diagram o f the new device we developed 1, 2) is given in fig. 1. The fundamental idea is the generation o f two fully identical r a m p voltages by arrival o f the start pulse and the stop pulse, respectively. These two r a m p voltages feed a differential stage, and it can easily be seen f r o m the pulse diagram in fig. 2 that the o u t p u t pulse of the differential stage has an amplitude which is exactly proportional to the time difference between the two input pulses. The length o f this output is of the order o f the rise time o f the r a m p voltages for small time differences. The two r a m p voltages must not be fully linear with time to get an output pulse with a peak voltage which is proportional to the time difference. Let: u l ( O = Uo [1 - e x p ( - t / z ) ] ,
ex ( for t >1 At,
(1)
be the two voltages. Neglecting quantities of higher than first order in At~z, we get for the differential
jl _ At ~
1 Uout - at
Fig. 2. Pulse diagram. Numbers of the curves refer to the points depicted in fig. 1. • 60V
1oo K
~ "lOOv
"L'.2n
400~c
16"n 68
Z ,tZ0
-41-4
trigger
"~
1' I----I ~
I '°i
romp volta
2'1
13'1
'
4K
--
r--" differential I stage
Fig. 3. Circuit diagram of one of the two fully identical input stages. (Collector voltages of both transistors are adjustable for alignment.)
Fig. 1. Block diagram of the device. 557
558
WOLFGANG
ROHRBECK
voltage: Au = u l ( t ) - u 2 ( t ) ,.~ Uo Atexp ( - t / z ) .
(2)
T
Hence the peak amplitude of this differential voltage is proportional to the time difference At, if the relation A t/z ,~ 1
(3)
holds. Because we are interested in measuring small time differences only, we can use voltages of the form given in formula (1). Voltages of this form can easily be generated by charging or discharging a capacitor via an ohmic resistor. In fig. 3 the circuit diagram of one of the two fully identical input stages is given. The input pulse fires an avalanche transistor which delivers a high current to the base of the second transistor, which goes into heavy saturation and discharges the capacitor C via the resistor R. The discharge develops a voltage of the form given in eq. (1) across the two leads of the symmetrical 120 f2 transmission line at the output. Fig. 4 shows the diagram of the differential stage and the peak-height detector. The differential stage is formed by three transmission-line transformers. These transformers consist of a 30 cm length of small symmetrical 120 f2 cable wound up to a ferrite core. Such transformers have been found to give excellent transmission characteristics as pulse reversing devices and baluntransformers4). The peak-height detector consists of a fast-switching diode, via which a 47 pF capacitor is charged to the
AND GISELA KITZE
peak voltage of the differential pulse. The capacitor is discharged by a constant current source, and the signal is picked off by a MOS-FET transistor and an emitter follower, thus forming a pulse stretcher which delivers a triangular voltage pulse with a slope of about 2/~s to the output, which can be fed into a multichannel analyzer. The amplitude of this pulse is 2.5 V per nanosecond time difference at the input. The first pulse stretcher is followed by a second one, which consists of a 33 nF capacitor charged by an emitter follower to the peak voltage of the tiangular pulse, and discharged by a constant current source delivering a pulse with a linearly descending slope. This pulse is fed to a comparator and a gate passing a 100kHz quartz frequency into a digital counter, thus forming a digital display of the time difference, as depicted in fig. 5. The slope is adjusted by the constant current source to deliver 1000 pulses to the counter for a time difference of 1 ns, that means a length of the slope of 10 ms. A 10 × amplifier can be put between the two pulse stretchers by a switch to give a range of 1000 pulses per 100 ps. For measuring of larger time differences than 1 ns the capacitors of the input stages can be switched.
2 nd from 1-st puls pulse stretcher comparator gate
counter
r.tcher t
I I
1 0 0 k H z clock
input =tageZ
.~
input stage]I
~
~
°
Fig. 5. Block diagram o f conversion from pulse height to digital display. N(At)
*q8V 1 38K
n~t)
%0 :-__
_ d'~
f ~/(
-lev
llllllllll4 175 RIO ps
Fig. 4. Circuit diagram o f differential stage and first pulse stretcher.
Fig.
6.
= 1~I
-- t61
=- i~ z
:- tb 2
Ops tOy
• t6 z
I IOns. /~er. ,
/ 2ns-Ber. -3 / Ng-:820 i~t m l l l l i i l i j i
t
119O
oops
INg--~.
liJlllll a d• ~00 ~Sns
Distribution o f measurements for range I . . . . , III.
TIME-DIFFERENCE MEASUREMENT IN THE PICOSECOND RANGE
2. Results The four m e a s u r i n g ranges o f the device are the following: 1 II III IV
0 . 0 . . . 200.0 ps, 0.0 ... 2.000ns, 0.0 ... 10.00 n s , 0.0 ... 50.00 ns.
In fig. 6 the results are given for three ranges. Time resolution for range I a n d II is ___1.6 ps a n d _+ 1.8 ps, respectively. N(At)/N9 means the relation o f the n u m b e r o f digital displays with time difference At to the t o t a l n u m b e r o f m e a s u r e m e n t s t a k e n in the range. T h e a d j u s t m e n t o f the device was carried o u t by feeding pulses (10 V; 10 ns half w i d t h ; 0.5 ns rise time) to the two inputs via coaxial cables with different length a n d k n o w n delay time.
559
The i n p u t stages are t e m p e r a t u r e - r e g u l a t e d by a small t h e r m o s t a t . It seems to be possible to i m p r o v e t i m e r e s o l u t i o n f u r t h e r by t e m p e r a t u r e regulation o f the o t h e r stages and, p e r h a p s , by m o r e careful design o f the i n p u t stages. M a y b e the use o f t u n n e l - d i o d e triggers will give some further i m p r o v e m e n t s .
References 1) W. Rohrbeck, DDR Patentanmeldung WF G01 r/175342. z) W. Rohrbeck and G. Kitze, Anordnung zur Messung von Zeitdifferenzen im Pikosekundenbereich, Zentralinstitut f. Elektronenphysik der AdW der DDR (Jahresbericht 1973) p. 133. a) F. Meiling, J. Schintlmeister and F. Stary, Nucl. Instr. and Meth. 21 (1963) 275. 4) W. Rohrbeck and G. Kitze, l~lbertrager ftir Nanosekundenimpulse, Zentralinstitut f. Elektronenphysik der AdW der DDR, Berlin, internal paper (unpublished).
INSTRUMENTS
AND
METHODS
I27
(1975)
TIME-DIFFERENCE MEASUREMENT
557-559;
©
NORTH-HOLLAND
IN THE PICOSECOND
PUBLISHING
CO.
RANGE
WOLFGANG ROHRBECK and GISELA KITZE Akademie der Wissenschaften der DDR, Zentralinstitut fiir Elektronenphysik, 108 Berlin, Mohrenstrafle 40/41, D.D.R.
Received 6 May 1975 Working principles, circuitry, and results of a very simple device for measuring time differences in the picosecond range are given, The time resolution of the device, that was originally intended for
delay-time measurement of vacuum breakdown but may be of interest for many other applications, is in the order of 4-2 ps for a single measurement.
1. Working principle
and:
N o r m a l l y the measurement o f small time differences is done by time-to-pulse height converters o f the startstop type. In these converters the start pulse switches on a voltage which rises linearly with time, and the stop pulse switches off this voltage. The peak voltage is then a measure o f the time difference between the two pulses. The measurement o f a time difference in the picosecond range by this type o f converter is very difficult, because it is nearly impossible to generate voltages rising linearly with time in the first picoseconds. These difficulties could be overcome to some extent by differential converters3), but they use somewhat complex circuitry. The block diagram o f the new device we developed 1, 2) is given in fig. 1. The fundamental idea is the generation o f two fully identical r a m p voltages by arrival o f the start pulse and the stop pulse, respectively. These two r a m p voltages feed a differential stage, and it can easily be seen f r o m the pulse diagram in fig. 2 that the o u t p u t pulse of the differential stage has an amplitude which is exactly proportional to the time difference between the two input pulses. The length o f this output is of the order o f the rise time o f the r a m p voltages for small time differences. The two r a m p voltages must not be fully linear with time to get an output pulse with a peak voltage which is proportional to the time difference. Let: u l ( O = Uo [1 - e x p ( - t / z ) ] ,
ex ( for t >1 At,
(1)
be the two voltages. Neglecting quantities of higher than first order in At~z, we get for the differential
jl _ At ~
1 Uout - at
Fig. 2. Pulse diagram. Numbers of the curves refer to the points depicted in fig. 1. • 60V
1oo K
~ "lOOv
"L'.2n
400~c
16"n 68
Z ,tZ0
-41-4
trigger
"~
1' I----I ~
I '°i
romp volta
2'1
13'1
'
4K
--
r--" differential I stage
Fig. 3. Circuit diagram of one of the two fully identical input stages. (Collector voltages of both transistors are adjustable for alignment.)
Fig. 1. Block diagram of the device. 557
558
WOLFGANG
ROHRBECK
voltage: Au = u l ( t ) - u 2 ( t ) ,.~ Uo Atexp ( - t / z ) .
(2)
T
Hence the peak amplitude of this differential voltage is proportional to the time difference At, if the relation A t/z ,~ 1
(3)
holds. Because we are interested in measuring small time differences only, we can use voltages of the form given in formula (1). Voltages of this form can easily be generated by charging or discharging a capacitor via an ohmic resistor. In fig. 3 the circuit diagram of one of the two fully identical input stages is given. The input pulse fires an avalanche transistor which delivers a high current to the base of the second transistor, which goes into heavy saturation and discharges the capacitor C via the resistor R. The discharge develops a voltage of the form given in eq. (1) across the two leads of the symmetrical 120 f2 transmission line at the output. Fig. 4 shows the diagram of the differential stage and the peak-height detector. The differential stage is formed by three transmission-line transformers. These transformers consist of a 30 cm length of small symmetrical 120 f2 cable wound up to a ferrite core. Such transformers have been found to give excellent transmission characteristics as pulse reversing devices and baluntransformers4). The peak-height detector consists of a fast-switching diode, via which a 47 pF capacitor is charged to the
AND GISELA KITZE
peak voltage of the differential pulse. The capacitor is discharged by a constant current source, and the signal is picked off by a MOS-FET transistor and an emitter follower, thus forming a pulse stretcher which delivers a triangular voltage pulse with a slope of about 2/~s to the output, which can be fed into a multichannel analyzer. The amplitude of this pulse is 2.5 V per nanosecond time difference at the input. The first pulse stretcher is followed by a second one, which consists of a 33 nF capacitor charged by an emitter follower to the peak voltage of the tiangular pulse, and discharged by a constant current source delivering a pulse with a linearly descending slope. This pulse is fed to a comparator and a gate passing a 100kHz quartz frequency into a digital counter, thus forming a digital display of the time difference, as depicted in fig. 5. The slope is adjusted by the constant current source to deliver 1000 pulses to the counter for a time difference of 1 ns, that means a length of the slope of 10 ms. A 10 × amplifier can be put between the two pulse stretchers by a switch to give a range of 1000 pulses per 100 ps. For measuring of larger time differences than 1 ns the capacitors of the input stages can be switched.
2 nd from 1-st puls pulse stretcher comparator gate
counter
r.tcher t
I I
1 0 0 k H z clock
input =tageZ
.~
input stage]I
~
~
°
Fig. 5. Block diagram o f conversion from pulse height to digital display. N(At)
*q8V 1 38K
n~t)
%0 :-__
_ d'~
f ~/(
-lev
llllllllll4 175 RIO ps
Fig. 4. Circuit diagram o f differential stage and first pulse stretcher.
Fig.
6.
= 1~I
-- t61
=- i~ z
:- tb 2
Ops tOy
• t6 z
I IOns. /~er. ,
/ 2ns-Ber. -3 / Ng-:820 i~t m l l l l i i l i j i
t
119O
oops
INg--~.
liJlllll a d• ~00 ~Sns
Distribution o f measurements for range I . . . . , III.
TIME-DIFFERENCE MEASUREMENT IN THE PICOSECOND RANGE
2. Results The four m e a s u r i n g ranges o f the device are the following: 1 II III IV
0 . 0 . . . 200.0 ps, 0.0 ... 2.000ns, 0.0 ... 10.00 n s , 0.0 ... 50.00 ns.
In fig. 6 the results are given for three ranges. Time resolution for range I a n d II is ___1.6 ps a n d _+ 1.8 ps, respectively. N(At)/N9 means the relation o f the n u m b e r o f digital displays with time difference At to the t o t a l n u m b e r o f m e a s u r e m e n t s t a k e n in the range. T h e a d j u s t m e n t o f the device was carried o u t by feeding pulses (10 V; 10 ns half w i d t h ; 0.5 ns rise time) to the two inputs via coaxial cables with different length a n d k n o w n delay time.
559
The i n p u t stages are t e m p e r a t u r e - r e g u l a t e d by a small t h e r m o s t a t . It seems to be possible to i m p r o v e t i m e r e s o l u t i o n f u r t h e r by t e m p e r a t u r e regulation o f the o t h e r stages and, p e r h a p s , by m o r e careful design o f the i n p u t stages. M a y b e the use o f t u n n e l - d i o d e triggers will give some further i m p r o v e m e n t s .
References 1) W. Rohrbeck, DDR Patentanmeldung WF G01 r/175342. z) W. Rohrbeck and G. Kitze, Anordnung zur Messung von Zeitdifferenzen im Pikosekundenbereich, Zentralinstitut f. Elektronenphysik der AdW der DDR (Jahresbericht 1973) p. 133. a) F. Meiling, J. Schintlmeister and F. Stary, Nucl. Instr. and Meth. 21 (1963) 275. 4) W. Rohrbeck and G. Kitze, l~lbertrager ftir Nanosekundenimpulse, Zentralinstitut f. Elektronenphysik der AdW der DDR, Berlin, internal paper (unpublished).