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Principle of Data Acquisition 3

When a measurement is started the TDCs in all active channels simultaneously start running.

Any event detected at one of the 16 LVTTL inputs or at one of the four CFD inputs triggers a

time readout from the TDC of the corresponding channel. The time is first buffered in an in-

ternal first-in-first-out (FIFO) memory of the TDC chip. The data from the internal FIFO are

then written into an external FIFO that buffers up to 4 million events. The output of the exter-

nal FIFO is read by the computer, and the data are transferred into the main memory or to the

hard disc. The double-buffered structure avoids loss of photons by read-write collisions.

Moreover, the external FIFO is large enough to buffer the photon data during possible back-

ground operations of the computer.

In the configuration shown in Fig. 3 the DPC delivers the times of the photons from the start

of the measurement. Such data are often called ‘time -tagged photon data’. The interpretation

of the data is merely a matter of the software. Thus, intensity traces, photon counting histo-

grams, and fluorescence correlation and cross-correlation curves can be obtained (see

‘Operation Modes’ page 5).

Relative Timing

Relative timing is required if the waveform of an optical signal is to be recorded. The times of

the photons have then to be measured with reference to the pulses of the excitation source.

Relative timing can be achieved by recording reference pulses from the excitation source in

one of the TDC channels. The times of the photons in the excitation period are then obtained

by determining the differences of the photon times to the previous excitation pulse. This kind

of relative timing is used in the multichannel-scaler mode of the DPC (see ‘Multichannel Sca-

ler Mode’, page 8).

Excitation pulse rates in the MHz range, as they are typical of TCSPC measurements, do,

however, cause a problem if direct relative timing is used: The high repetition rate of the ref-

erence pulses would saturate the TDC channel that is used for the reference. Therefore, the

DPC has a TCSPC configuration implemented that records only reference pulses of excitation

periods that contain valid photons. The DPC-230 architecture in the TCSPC mode is shown in

Fig. 4.

CFD

TDC Channel1

TDC Channel2

TDC Channel9

TDC Channel10

CFD

In 1

In 2

FIFO

Bus

Interface

In 9

In 10

Reference

FIFO

2 million events

TDC Chip 1

TDC Chip 2

Clock Generator

SYNC

REF

Com-

puter

Fig. 4: DPC-230 architecture in the TCSPC mode

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