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readout, which means that pulses validated after the end of the coincidence window (and thus not
contributing to the hit pattern) are lost. The readout therefore causes dead time in all other
channels in the module as well.
5. Fast trigger dead time (FTDT)
Fig. 6.9. Fast Trigger Dead Time (FTDT). A second pulse is not detected if the trigger filter output
is still above threshold.
An additional type of dead time only affects the trigger filter. Triggers are issued when the
trigger filter output goes above the trigger threshold set by the user. However, depending on the
length of the trigger filter and the rise time of the input signal, the trigger filter output will
remain above threshold for a finite amount of time. During this time, no second trigger can be
issued8. Therefore triggers are not counted during this time, and when computing the input count
rate, the time lost has to be taken into account. FTDT is thus purely a correction for the
computation of the input count rate.
6. Summary
As listed in Table 6.2, the dead time associated with each pulse is essentially in the order of the
filter dead time, unless waveforms are requested by the user. Even with waveforms, few counts
are lost at low count rates for short waveforms. Losses are significant with longer waveforms or
at higher count rates. To some extend, this is due to the general purpose FIFO logic which allows
waveforms up to the maximum available memory in the FPGA. For application where only short
traces are required, lower dead times are possibly by reorganizing memory to buffer multiple
events in the FPGA. Please contact XIA for details.
8
The MAXWIDTH parameter can be used to define a maximum acceptable time over threshold and thus to reject
events piled up “on the rising edge”.
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PIXIE-4 User’s Manual V2.50
 XIA 2012. All rights reserved.