A zero crossing discrimination technique for constant fraction timing

NUCLEAR

INSTRUMENTS

AND M E T H O D S

163 ( 1 9 7 9 ) 5 3 5 - 5 3 9 ,

(~) N O R T H - H O L L A N D

PUBLISHING

CO

A ZERO CROSSING DISCRIMINATION TECHNIQUE FOR CONSTANT FRACTION TIMING JA.NOS G,~L and GYORGY BIBOK

lnstttute of Nuclear Research of the Hungartan Academy of Sctences, H-4001, Debrecen, Pf 51, Hungary Received 17 August 1978 and m revtsed form 29 January 1979 A zero crossing d~scrlmmatton techmque ~s gtven for constant fraction timing A very s~mple method makes a walkmgless ttmmg possible, independently of delay and fractton settings

The simplest method of zero crossing detection applies an integral discriminator with a hysteresis equal to the threshold The integral dlscnmlnator is biased in such a way that it is triggered if the input signal overcomes the noise level and it reverts to the initial state at the instant of the input s~gnal zero crossing. The actual triggering level of such a system (fig 1) is determined by the bias and the hysteresis (i e the output level of the discriminator), so it changes after every triggering, and there is a finite time necessary for stabilization of the new actual tnggerlng level The cause of this is the finite propagation delay and nse time of the comparator signal. Therefore the triggering level of the comparator reaches the zero level with different accuracy for fast bipolar signals having different amplitudes This means, that the timing will not be walklngless The so called " p n m e d " technique is a twodtscnminator system 1) One of these discriminators acts on the input pulses and ~s biased above the noise level The second one acts on the bipolar signal and is biased to zero level, so it is triggered not only by the bipolar signal, but by the noise too. To ehmlnate spurious triggering the second dlscnmlnator is enabled by the first one The priming discriminator has to be biased to generate an output during the time interval between the beginning of the onglnal, and the zero crossing point of the bipolar signal If priming occurs too early, spurious noise triggering will result If it occurs after the zero crossing instant, the pnmlng discriminator enters on duty of the zero crossing one, causing substantml walk So this method has the limitation that the walk of the priming discriminator has to be smaller than the above mentioned time interval and this requirement is ddTicult to achieve over a wide dynamic range, particularly ff the delay used for shaping of the bipolar signal is short 2,3)

Although the former hmltation is overcome by the so-called dual-primed ARC (amphtude and rise time compensated) technique, in which an auxlhary ARC timer is used to prime the main ARC timer 3, it seems to be too complicated. The method described here is very easily reahsable It makes walklngless timing possible, independently of the delay and fraction settings The operating pnnclple of this technique utihses the fact that true signals and noise have a different duration distribution, so separation between them can be perfomed by duration discrimination The use of this principle in timing circuits was proposed and presented by Deroche 4 Other authors also published articles in this field 5-7) These circuits were used for low level leading edge timing, where the timing discriminator was biased below the noise level Complete separation between true signals and noises is not possrble on the basis of duration discrimination, so an adjustable amplitude discrimination (by the help of an adjustable finite threshold of the discriminator) is generally introduced in these systems, but this causes the Increase of walk In this work the duration discrimination is used for the zero crossing detection timing, combining It with an amplitude dlscnmlnatlon that does not cause any walk. The substance of the method is shown in fig 2. The bipolar signal is coupled to a comparator biased to zero level This comparator is triggered by true events and noise as well The output of the zero crossing comparator is t~d to an integrator. The amplitude of the output of this integrator is proportional to the zero crossing comparator output pulse w i d t h (Severe p r o p o r t i o n a l i t y is not required, It IS supposed only for the sake of slmplloty. Exponential charging up of the integrator capacitor is also good). In the case of a true signal the amphtude of

J GA L A N D G

536

BIBO K

ACTUAL TRIGGER LEVEL

INPUT

ACTUAL TRIGGER SMALL SIGNAL

LEVEL

FOR

THRESHOLD t

ACTUAL TRIGGER L E V E L F O R LARGE SIGNAL

i

CflMPARATOR OUTPUT FOR LARGE S I G N A L

\ i COMPARATOR~ OUTPUT FOR SMALL SIGNAL

/<

/

t TIMING

EDGES

t__

F~g 1 Zero crossing trigger with threshold equal to hysteresis, and its simphfied waveforms

the integrator output is h~gher than ff ~t were triggered by noise This integrator output signal is fed to a second comparator, which selects signals by thetr amplitude, i.e. their pulse duraUon It ~s seen m fig 2 that the duraUon d~scnmmauon ~s performed on the second lobe of the bipolar slgnal To avoid spurious output pulses due to pde-

up, the integrator output has to be restored to zero level by the negatwe edge of the first comparator A s~mple reahsaUon of this gated integrator ~s shown in fig 3. As was menUoned earher the durauon discrimination ~s combined w~th amphtude d~scnmmation In the c~rcmt the amphtude d~scnmmaUon ~s real-

ZERO C R O S S I N G D I S C R I M I N A T I O N T E C H N I Q U E

GATED

INTEGRATOR

q

F . . . . . . . . . . ZERO

CROSSING

COMPARATOR

537

I

+

I

AMPLITUDE C OMP ARAT OR

I'q

INPUT

.i.

O

+ [

,

OUTPUT O

2

Rf

DURATION

INPUT

TIME REFERENCE

POINT<

ZCC OUTPUT

k. t

GATED INTEGRATOr OUTPUT

AMPL. CO~. OUTPUT

TIMING EDGE

m

~

t

Fig 2 Block diagram and slmphfied waveforms of zero crossing dlscnmmatlon

lsed in such a way that a portion of the integrator output is fed back to the zero crossing comparator input by feedback resistor Rf (fig 2) The influence of this feeback on the wave forms is illustrated by a dotted hne in fig 2, and it can be seen that the d~scnmtnaUon level of the zero crossing comparator will rise after the zero crossing instant, allowing to detect only those signals of which the amplitude is greater than Ur measured after the zero crossing

point by z Since at the zero crossing instant the discrimination level is always zero, this kind of amplitude discrimination does not cause any walk. (We note that such an amplitude discnminauon for low level leading edge Umlng is also advantageous ) To adjust separation against noise there are two possibilities One of these is when the discrimination level of the amplitude comparator is adjusted.

538

A

B

C

J

:::2

l~.V (A) •

,,IL,It~

- ,gB -

i~

iV

,,,

GAL

G

A N D

instant has to be p~cked up. Trace B 1s the integrator output and trace C the fed back pomon of the integrator output The amphtude jmer of these signals is caused by nome Trace D presents the timing s~gnal, shaped by a monostable orcmt To justify the separation abd~ty of the orcmt the following experiments were made. The zero crossmg detector was driven from such a signal source where the s~gnal and no~se amphtude could be set independently of each other The feedback was adjusted m such a way that UF (shown m fig 2) was 25 mV The no~se was chosen such that the average rate of false triggering onglnatlng from no~se was about 1 Hz The duraUon threshold was set at about 50 ns, which corresponds to the Ume interval from the zero crossing point to the Ume, when the second lobe of the pulse falls to N 80% of its maximum amphtude The zero crossing detector was dnven by 1 kHz bipolar signals and the rate of Ummg pulses from true signals (they were saparated by the help of a comodence orcmt wtth a resolving ume of 25 ns) was measured as a function of the second lob amphtude of the b~polar s~gnal After th~s first measurement the noise amphtude was held constant and Uf was adjusted to such a value that the average rate of false triggering was 200 Hz, and the rate of the true ummg pulses as a funcUon of the second lobe amphtude also was measured This measurement was repeated m the cases when the average rate of the false triggering was 2 kHz,

53r8

.q~mV :(c)

•

II I I ' ) ; ' , ; ' : L , ~ ; : . . . . . . . . . . . . . . . . . . . .

~'~

~

. ij ~' "l

I

5

(B)

I

I

I

I

(D)

Fig 3 Wave forms m the clrcmt, A - the bipolar signal, B - the integrator output, C - the feedback voltage, D - the u m m g output

The d~sadvantage of this soluUon ~s that during the adjustment the delay r between the time reference point and the Ummg edge of the amplitude comparator output ~s vaned. The other posmbd~ty, which is free from the former &sadvantage, is adJusting the feedback, while the discnmmaUon level of the araphtude comparator is maintained on a constant value The feedback can be adjusted by the help of potentlometer P To demonstrate the operation of the c~rcu, an oscilloscope photograph is shown m fig. 3. Trace A is a notsy bipolar signal of which the zero crossing pulses second

BIBOK

2kHz

1000

--\

900 800

X

o....-----~= ~ ~ - - - ' ~ -

y ,+--/

200kHz ~ / ' ~

/

/ /

--

/

•

°

l"

700 600 500

~1Hz

/ ///

400 300

A oo.z

200 100

g

16

i~

2'0

2'5

a'o

3g

Lo

4's

s'o

Uin Ftg 4 Counting efficiency as a funcUon of the input pulse amphtude by different triggering from norse

~v

ZERO C R O S S I N G D I S C R I M I N A T I O N T E C H N I Q U E

+5 +4 + 3+ 2-

l\

\

+I0- I-

-3-

]/-/

-4-

-6-

2500

5000

m~

SECOND LOBE AMPLITUDE Fig 5 Time resolutton and walking m the dynamic range of 1 200

20 kHz, 200 kHz respectively The results of these measurements are shown in fig 4 In fig 5 the result of a time resolution measurement is shown The bipolar signal was generated by a periodic source. Uf was 25 mV The average rate of false triggering by noise was about 1 Hz. The shape of the bipolar stgnal was the same as ts shown m fig. 3 This measurement was performed by the help of a time-amplitude converter and a multlchannel analyser The start signal of the

539

time-amplitude converter was the prepulse of the penodlc source and the stop stgnal the timing output of the zero crossing dmcrlmlnator. In fig. 5 the full wtdth at half-maximum of the time spectrum and the peak postton ms shown as a function of the second lobe amplitude of the b~polar signal. For the zero crossing comparator a &gnetms NE521 integrated circuit was used Because of the charge sensltlWty of this clrcmt the minimum overall walk m a dynamic range of 200 was about 2 ns But in our arrangement this kind of walk can be compensated in a stmple way: a small portion of the analog bipolar signal has to be added with appropriate polarity either to the ramp or to the duration comparator biasing. If thts addltton IS performed with the appropriate polarity the delay r wdl vary to compensate the above menttoned walk. During the ttme resolution measurement thts compensation was used (We should like to note that when this compensatton is used, the saturation of the second lobe amphtude must be avoided )

References 1) D A Gedcke and W J McDonald, Nucl Instr and Meth 55 (1967) 377 2) R L Chase, Rev Scl Instr 39 (1968) 1318 3) Z H Cho and R L Chase, Nucl Instr and Meth 98 (1972) 335 4) j Deroche, Proc 6th Int Symp on Nuclear electromcs, Warsaw (1971) p 83 5) Z H Cho, S Beshal and J Becker, IEEE Trans Nucl Scl NS-20 (1973) 199 6) Yu K AklmOV, K Andert, A I Kalmln and H G Ortlepp, Nucl lnstr and Meth 104 (1972) 581 7) j Gal, Gy Btbok ATOMK1 Bulletin 18 (1976) 599