A multistop time-to-digital converter

Nuclear Instruments and Methods 188 (1981) 99-104 North-Holland Publishing Company

99

A MULTISTOP TIME-TO-DIGITAL CONVERTER E. FESTA and R. SELLEM Insntut de Phystque Nucldalre, B P no. 1, 91406-Orsay, France

Recewed 16 February 1981

The article describes a nmltlstop TDC with a resolutmn of 1 ns designed for time-of-flight mass spectrometry (maximum 128 ~s) in heavy ion induced desorptlon experiments The dead time of the device is 80 ns. An optional circuit for random rejection of stop signals allows observatmn of the second of two masses desorbed simultaneously, whose time of fhght differs from the first by less than 80 ns

1. Introduction The time-to-digital converter we have developed is designed to measure times of flight of 16, 32, 64 and 128/is with an accuracy of 1 ns. This converter will allow studies of desorption phenomena. As is known [1], desorption consists of the ejection of one or several lonlsed masses from a thin Ni foil to which they are weakly attached, at an initial velocity near to zero. In the particular case of interest to the users of this TDC (fig. 1), the precise mstant of desorption is detected by an appropriate channel plate giving a start signal. The ejected masses are accelerated in an electric field whose value IS known. After a time of fhght, these masses are detected by another channel plate which gives a stop signal corresponding to the arrival time of each mass. Several stop signals can therefore correspond to a given start signal. The cases under study concern, on the one hand, the simultaneous desorptlon of several molecules, and on the other hand, the fragmentation of certain molecules in the

acceleration space. We need to detect the arrival time of each of these masses. These arrival times may be a few tens of nanoseconds apart, whence the need for the shortest possible dead time after the arrival of each stop signal. The multlstop converter is enabled by the start signal. Its arrival time, as well as the arrival time of each stop signal, causes the memorisatlon of numerical values, as we shall see later. The accuracy of 1 ns in the measurement of the time of flight is compatible with the accuracy on the other parameters, in particular on the time ptck-off (approximately 0.6 ns) and on the electric field high voltage supply (approximately 10-4).

2. Principle of the method used The dynamic of 100 000 makes the use of an analog method difficult. We have therefore chosen a numertcal method roughly identical to that used for drift time measurement in multlwire chambers [2]. We will briefly indicate the principle of the method

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before describing the techniques and measurement procedures used in its reahsatlon. From a quartz clock of a frequency of 125 MHz, we obtain a symmetrical signal o f half the frequency, 1.e. a 16 ns period (fig. 2). This signal increments a 13-bit counter. The content o f the counter gives the value of 16 ns, as well as multiples of this value up to 128 ~s. When a start or stop signal arrives, the content of this counter is transferred to a memory latch synchronously with the leading edge of the clock signal. The fast code giving values from " 0 " to " 1 5 " ns, is also generated, and memorlsed with maxamum accuracy, when a start or stop signal arrives. To generate the fast code, the 16 ns' period symmetrical signal is delayed by one nanosecond, then two, e t c , up to 8 ns (fig. 3). The signals 4o to 47 generate, in Gray's code, 16 different configurations (fig. 4), whose corresponding values " 0 " to " 1 5 " remain stable, except for one bit, when one value changes to the following value. The error in the fast code memorlsation is at most one nanosecond in our case. 47 is also used to increment the counter, which minlmlses the errors caused by shift relative to the clock signal, if an 1den-

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tlcal shift in time IS supposed for the e i g h t circuits that generate the signals 40 to 47. This condition is roughly verified m the c i r c u i t s whose description follows.

3. Realisation of the fast code The delays are created by mlcrostrlp lines. On a classical printed circuit of thickness 1.6 mm and dlalectrIc constant 5, we have laid mlcrostrlps 35 ~m thick and 800 /lm wide. The typical impedance of this mIcrostrlp line IS about 90 ~ . Compared with the delays obtained with ECL gates, the mIcrostrlps show a greater stability in propagation times with variations of temperature. In fact, we have used two lines (fig. 5), each connected to one of the two outputs of the symmetrlslng circuit. The first line creates the first four delays, and the second line the remaining four. The eight outputs

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o f the two lines are connected to eight inputs of ECL gates 100 102. These ECL gates separate the two lines of eight RC circuits used for the fine adjustment of delays. A second gate, identical to the previous one, is placed between the RC and the input of an ECL latch 100 150. The data present on the input is latched when a start or stop signal arrives (fig. 6).

4. Delays and latch m e m o r y threshold

adjustment

A capacitor between the second gate and the latch input allows adjustment o f the threshold of each latch. We found, in fact, that the probability o f a latch recording " 0 " or " 1 " was not the same. We

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measured this probability by computing in an "add one" memory core, the number o f "0"s and " l " s recorded by the latch memory when the symmetrical 62.5 MHz signal was fed into it, and a random signal latched it. For a given latch we found a discrepancy of up to 30% between the probabilities of recording a " 1 " or a " 0 " . We attribute this discrepancy to the slight differences o f output level between gates and to the differences between latch thresholds. In addition, as we shall see later, the leading edge and the trailing edge o f the 62.5 MHz signal are not exactly symmetrical due to the RCs used for fine delay adjustment. The threshold adjustment of the latch allows rapid reduction to less than 5% of the difference between the probabilities of recording " 0 " or " 1 " . We used an Identical method to adjust delays. In

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addresses "'0", " 1 " , "2"' and " Y ' . The fine delay adjustment is carried out by tile RCs, which causes a slight variation in the p r o b a b d m e s o f recording " 0 " or " 1 " in tile latch. In practice, the m e t h o d converges q m t e rapidly.

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U n f o r t u n a t e l y , if a start or stop signal arrives when tile fast code value changes from " 1 5 " to "0", an error o f one period can occur, 1.e. -+16 ns in our case (fig 8). To correct this error, an additional bit is used, of twice the clock period In our case, this bit (B.S. m fig. 9) is m fact the first bit o f the counter. Its value does not change at the b o u n d a r y between two periods, and is memorlsed as soon as a start or stop signal arrives. If B.S. = 1, an odd counter value must correspond to a fast code value o f " 1 5 " and an even counter value must correspond to a last code value of " 0 " If B.S. = 0, an even counter value must correspond to a fast code value o f " 1 5 " , and an odd counter value must correspond to a fast code value of " 0 " . In fact, to be sure of detecting all errors o f this type, these c o n d m o n s are checked for fast code values of " l Y ' , "14"', "15", and " 0 " , "1 ", " 2 " . A 2-bit code shows if the result o f the test is correct, or ff 1

Fig 9 An add]tmnal bit B S is generated to correct the one period error the example shown in fig. 7, the latch outputs which m e m o n s e ~o and ~ are c o n n e c t e d to the 2 0 and 2 l bits o f an " a d d o n e " m e m o r y core. As before, a rand o m s~gnal latches the m e m o r y . According to whether the signal arrives m the first, second, third or fourth time interval, the addresses " 1 " , " Y ' , "'2" or " 0 " respectively are i n c r e m e n t e d in the "add o n e " m e m o r y core. In this example, the delays are correctly adjusted when N1 = N 2 = ~ Ntotal , No = N 3 = ~ Ntotal • where No, N1, N2, N3' are

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6. Differential nonJinearity of the TDC The adjustment methods described above result easily in a maximum discrepancy of 30% between two extreme values o f At. Thus it can be said that dlfferenual nonqmearlty is -+15%, i.e. that the width of any channel can be at most 15% different from the average width. In fact, this error IS considerably reduced in our case, since when a start signal arrives the fast code is not reset (and nor, of course, is the counter). Thus the same stop-start difference is obtained from different start values and different stop values This sliding scale effect [3] contributes greatly to averaging differential non-linearlty errors. This effect IS shown In fig. 10, where a spectrum has been obtained with random start and stop signals. The count per channel mean ( A / = 1275.7) is roughly identical to the variance (o~ = 1347.6). This result indicates that the error of differential non-linearity is negligible with respect to statistical fluctuations.

7. Random

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After a start or stop signal, the TDC has a dead time o f about 80 ns, largely due to the 15 MHz frequency of the F 1 - F 0 74S225 memories used to record the values concerning the start and the stops of the same event. The addition of a fast register to the F 1 - F 0 , or the use o f a fast ECL m em o r y , would reduce this dead time. Because of the synchronisation method used, the average dead time cannot be less than 8 ns. We have added a random stop rejection, to eliminate 1 stop in 2 or 1 stop in 4 at random. Thus if two masses are less than 80 ns apart, the second can be observed. For random rejection we use a 7-bit shift register which generates a 127-step random sequence. This register is shifted by each stop signal that arrives at the TDC input. One stop in two is rejected by enabling the stops with one bit value o f the shift register. In this case, for any two masses arriving less

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than 80 ns apart, there is a 25% probabllaty of observIng the second one. One stop in four is rejected by enabling the stops with the "0.1" combination of two successive bits of the shift register. In this case, for any two masses arriving less than 80 ns apart, the probability of observing the first is the same as that of observing the second (figs. 11 a and 11 b). Simulation with a computer has shown that this rejectaon method does not introduce any distortion an the spectrum, af there is a slight probability of observing 3 masses during 80 ns [4].

tape, and displays them in reduced format on an "add one" memory core The hastogram an fig. 13 was obtained by measurang the tlme of flight of masses whose desorption is obtained by the fission fragments of a radioactive source, ~s2cf. We wish to thank Miss M.T. Coanmault who helped with program simulation, Mr. D. Desveaux whose technical collaboration was essential for the design of the printed circuits and Mr. J.W. Reed who kindly helped with the English text.

8. Presentation of results References In ItS present version the multi-stop can record, after a start, 31 stops. Fig. 12 shows the efficiency of the TDC at a given time t (for a Poisson process) as a functaon of product Xt, where )t is the average number of stops per tame unat After each event, the data present in the F 1 - F 0 memories are processed by a specaallsed microprocessor which records the values calculated on magnetic

[1] Y. Le Beyec, S Della Negra, C Deprun, P Vlgny and Y M Gmot, Rev. Phys Appl (Dec 1980) [2] Write up and vlewgraphs of talk given by W Slppach (Nevas Laboratory) at CERN 15 May 1975. [3] C. Cottma, E. GatU and V Svelto, Nucl Instr and Metb. 24 (1963) 241 [4] S. Della Negra, E Festa, D Jacquet and R. Sellem, to be pubhshed