Cornelius DPC 230 Specifications download pdf (Page 15)

Relative Timing Modes 9

of the excitation source, and the distribution of the events within the excitation period is built

up. Thus, the multichannel scaler mode is a relative-timing mode.

Multichannel scaler operation is illustrated in Fig. 12. The hardware structure is the same as in

the ‘absolute time’ mode. However, one channel of the DPC is used as a reference channel.

The reference channel receives synchronisation pulses from the light source. The other chan-

nels receive the single-photon pulses from the detectors. The data stream of the DPC contains

the times of the excitation pulses, and the times of the detected photons. From these data, the

operating software extracts the relative times of the photons since the last excitation pulse.

These times are used to build up the distribution of the photon density over this time. Because

several detectors can be active, several waveforms for the individual detectors can be obtained

within the same measurement.

Reference,

from laser

Reference,

from laser

Photons Photons

Accumulation over many laser periods:

Pulse density versus time in signal period

Fig. 12: Multiscaler mode of the DPC. One channel records reference pulses from the excitation source, the other

channel records the photons. Relative times of the photons are determined, and curves of the photon density over

the time within the excitation period is built up.

The multichannel scaler mode is recommended for excitation rates up to about 1 MHz. For

higher excitation rates the high reference pulse rate can cause data transfer problems, and the

TCSPC mode should be used.

TCSPC Mode

The TCSPC mode was implemented for waveform measurements at high repetition rate. The

TCSPC mode measures the photon times with reference to the next excitation pulse. In classic

TCSPC the principle is known as ‘reversed start-stop’ technique [1, 2, 22]. The benefit of the

reversed start-stop principle is that reference pulses need to be recorded only for excitation

periods that contain valid photons. In classic TCSPC reversed start-stop avoids the problem of

excessive TAC start rates; in TDC-based instruments it avoids saturation of the TDC channel

of the reference. The configuration of the DPC-230 in the TCSPC mode is shown in Fig. 4,

page 3.

The build-up of the signal waveform in the TCSPC mode is illustrated in Fig. 13. In the typi-

cal TCSPC applications the count rate is considerably lower than the signal repetition rate.

Thus, there are a large number of signal periods in which no photons are detected. In these

periods the reference synchronisation circuit (see Fig. 4 and Fig. 5) suppresses the reference

pulses, and nothing is recorded. In signal periods that contain a photon both the photon and

the subsequent reference pulse are recorded. The software analyses the data stream, deter-

mines the relative times from the photons to the next reference pulse, and builds up the distri-

bution of the photon density over the relative time. Except for a reversal of the time scale, the

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 69 70

No comments

Cornelius DPC 230 Specifications Page 15