Synchronisation signal delay

Although the diode array detects the same material a few seconds after the mass spectrometer, the software may be programmed with this time delay so that the two signals appear to be synchronised. Also, a second synchronisation is needed for the difference in time between the component being detected by the mass spectrometer (from the split flow) and the component arriving at the fraction collector (main flow). This timing is needed to ensure that the trigger from the mass spectrometer to start... 

Time-tag recording means that the TCSPC channels do not build up a photon distribution but store each individual photon with its TAC time ( micro time ) and its time from the start of the experiment ( macro time ). The computer calculates the photon distribution at each location along the delay line and the time in the signal period. Time-tag recording of delay line data requires that the macro time clocks of all TCSPC channels be synchronised. Even then it is difficult to assign the data in the position channel to the correct data in the time channel. Due to slightly different CFD thresholds and different dead times, a photon recorded in the position channel need not necessarily be recorded in the time channel, and vice versa. To avoid misinterpretation of the data, a macro time resolution of 50 ns or finer is required. 

The lasers must then be multiplexed at a rate faster than the changes expected in the sample. One way to multiplex lasers is to synchronise their pulse periods and delay the pulses of different lasers by different fractions of the pulse period. The fluorescence signals are recorded simultaneously in the same TAC range of a TCSPC device. The principle is shown in Fig. 5.26. 

The TCSPC-FCS technique can also be used in conjunction with a continuous laser. Of course, in this case the measurement does not deliver a meaningful miero time, and no lifetime data are obtained. Because the TCSPC module needs a synchronisation pulse to finish the time measurement for a recorded photon, an artificial stop pulse must be provided. This can be the delayed detector pulse itself or a signal from a pulse generator see Fig. 5.116. 

This is because there always exists a delay between the acquisition of data on the situation by means of the interface with the environment (sensors) and their understanding as a result of suitable signal processing. In addition, there is also a delay between the moment of a decision, togheter with a command initiating action, and the carrying out of that action. The delay is caused both by physical limitations (inertia, actuation times etc.) and by causes related to the necessity to inform and synchronise all the parts of the system which work togheter to carry out the action decided upon. 

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