TCSPC reversed start-stop

The TAC parameters determine the time scale and the part of the signal that is reeorded. The available parameters may differ for different TCSPC deviees. Espe-eially deviees based on direct time-to-digital conversion (TDC) or sine-wave eon-version may differ eonsiderably from devices using the TAC/ADC prineiple and reversed start-stop, which will be considered below. 

The time measurement block of a TCSPC device working in the reversed start-stop mode is shown in Fig. 7.69. 

For TCSPC with nonreversed start-stop, the end of the dead-time interval is automatically synchronised with the laser pulses. For reversed start-stop this in not the case. The situation is shown in Fig. 7.87. 

The simplest and most accurate way to calibrate a TCSPC system is to use the pulse period of a high repetition rate laser as a time standard. The pulse period of Ti Sapphire lasers is between 78 and 90 MHz and accurately known. Diode lasers are usually controlled by a quartz oscillator and have an absolute frequency accuracy of the order of several tens of ppm. The signal is recorded in the reversed start-stop mode with a frequency divider in the reference path. The recorded waveform covers several laser periods, and the time between the pulses can be measured and compared with the known pulse period. 

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