Choosing the TAC Parameters

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. 

At high pulse repetition rates the recording-time interval of the TAC can be longer than the pulse period. Of course, there are no photons with TAC times longer than one stop period. Consequently, the reeorded photon distribution drops sharply down to zero left of the TAC time corresponding to the stop pulse period. An example is shown in Fig. 7.72. 

The resulting signal shape is a frequent souree of eonfusion. Sometimes the step at time A is even mistaken for the rise of a fluoreseenee signal. Of course, the reeorded curve is absolutely correct. The photons left of the eutoff point, A, are not lost. They were recorded shortly before the previous reference pulse and appear where they should be, i.e. in the late part of the period, B, at the right end of the photon distribution. The curve can be eentred in the reeorded time interval by adjusting the signal delay in the detector or reference channel. 

As in any electronic circuitry, there is some unavoidable electronie noise in the TAC. To get the best time resolution, the TAC should be operated in the shortest TAC range possible. The TAC core then delivers maximum voltage and the gain of the TAC amplifier can be kept low, which results in a correspondingly low noise at the output. 

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