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GDT (GATE dead time)
The dead time from GATE or VETO/GFLT is counted separately from SFDT for each channel.
As mentioned above and further described in section 7.4, the use of these signals may depend on
the application.
In the current firmware, a rising (or optionally falling) edge at the Gate input creates a GATE
PULSE with length GATE WINDOW (see section 7.4). Both this GATE PULSE and the
(optionally inverted) signal on the VETO input are combined into GDT_ON. While GDT_ON is
high, GDT is incremented every clock cycle and thus counts the time pulses can be rejected from
acquisition. (The rejection has to be enabled independently). It is possible to instead count the
time during which pulses are allowed by inverting the combined signal. In the “standard
statistics” mode, the GDT counter is additionally subject to the channel being live, i.e. GDT is
only counted if a run is in progress, signal in range, etc.
Fig. 6.12 GDT counting logic shown for standard statistics mode. In Gate statistics mode, the
channel LIVE signal is ignored for counting GDT and in turn LIVE is subject to GDT_ON.
To support the third case of gating mentioned above, where the acquisition is only of interest
when GATE or VETO are off, there is an alternate “GATE statistics” mode to count GDT and
livetimes. In Gate statistics mode, all time and rate counters except RUN TIME and TOTAL
TIME are only active if GDT_ON is high. This means GATE or VETO must be present for the
channel to be live. In turn, GDT is counted independently from whether the module is live or not.
If the VETO input is unused but defaults to high so that GDT is equal to the live time, invert the
VETO polarity in the CHANNEL REGISTER Panel to only count the GATE PULSE time. The VETO
input can be used instead of the GATE input for the gating circuitry, and if this option is used
only the GATE PULSE derived from the VETO input is counted, not the original VETO.
For the case that the Veto input is used for a GFLT-type validation pulse, it may be more useful
to work with the number of pulses issued. They can be counted by using the VETO input as the
source for GATE PULSEs, which are counted in the variable GCOUNT.
6.6.3 Count rates
Besides the live and dead times, the Pixie-4 counts the numbers of triggers in each channel,
FASTPEAKS, the number of valid events with one or more channels, NUMEVENTS, and the
number of valid pulses stored for each channel, NOUT. In addition, it counts the number of gate
pulses for each channel, GCOUNT. FASTPEAKS and GCOUNT are inhibited when the live
time is not counted. NUMEVENTS and NOUT by nature only count events captured when the
live time is counted.
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PIXIE-4 User’s Manual V2.54
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