Cornelius DPC 230 Dokumentacja Pobierz pdf (Strona 10)

4 Principle of Data Acquisition

Three of the CFD channels are used for the detector signals. The CFD output pulses of these

channels are fed directly into the ECL inputs of the TDC chip. The fourth CFD input is used

for the reference from the laser. Before the reference pulses enter the TDC chip they pass a

frequency divider, FD, and a synchronisation circuit, SYNC. The divider ratio of FD is select-

able and can be set to 1:1, 1:2, or 1:4. The subsequent synchronisation circuit transmits one -

and only one - synchronisation pulse if one of the first three CFDs has received a photon pulse

within the previous pulse period of the divided reference signal. Thus, TDC2 receives one

reference pulse when at least one photon was recorded in the previous 1, 2, or 4 excitation

periods. As a result, the reference pulse rate is reduced to less than the photon rate. The reduc-

tion is achieved without sacrificing any excitation periods or photons.

Pulse diagrams for FD = 1:1 and FD = 2:1 are shown in Fig. 5 and Fig. 6, respectively. With

FD = 1:1 the time of a photon is measured with respect to the next reference pulse. Thus, the

measurement time interval extends over one period of the light source.

With FD = 2:1 (Fig. 6) the photon time is measured with respect to the next or second next

excitation pulse, depending on whether the photon was detected in an even or uneven excita-

tion period. Thus, the measurement time interval extends over 2 laser periods. A frequency

divider ratio of four results in a recording over four signal periods.

Reference,

from laser

Photons

reference

Time of photon

Recorded

pulse

Interval

recorded

Fig. 5: Reference synchronisation for FD=1:1. After the detection of a photon the next reference pulse is re-

corded. The time of the photon is measured with respect to the selected pulse.

Reference,

from laser

Photons

Reference

after

SYNC

Time of photon

Reference,

after FD=2

Interval

recorded

Fig. 6: Reference synchronisation for FD=2:1. The reference pulses are divided by a ratio of 2:1. After the detec-

tion of a photon the next pulse from the frequency divider is recorded. The time of the photon is measured with

respect to this pulse. The measurement time interval extends over 2 reference periods.

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 69 70

Komentarze do niniejszej Instrukcji

Brak uwag

Cornelius DPC 230 Dokumentacja Strona 10

Komentarze do niniejszej Instrukcji

Powiązane produkty i podręczniki dla Kostkarki do lodu Cornelius DPC 230