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internal clock). Also, it does not take into account the time required to send commands from the
PC to the module.
RUN TIME
The RUN TIME variable tracks the time during which the DSP on the Pixie module was
“switched on” for data acquisition. The usefulness of this variable is limited. It may be less than
the TOTAL TIME because it omits the time the SDRAM is full and waiting for readout (during
which the data acquisition is paused in all channels). It is larger than the time an individual
channel is ready to take data because it does not account for the dead time from the pileup
inspection, out-of-range condition and energy filter in each individual channel. Thus its main
uses are to compute an average event rate (total output counts of all channels / RUN TIME), and
to compute the fraction of time the SDRAM was not full and acquisition not paused, which is the
“DAQ Fraction” displayed in the Pixie Viewer: RUN TIME / TOTAL TIME.
LIVE TIME
The LIVE TIME is counted in the FPGA independently for each channel and measures the time
the channel is ready for acquisition. The LIVE TIME counter starts when the DSP finished all
setup routines at the beginning of a run, omits the times the ADC signal is out of range, each
channels local 16-event buffer is full, or the SDRAM memory is full and ends when the DSP
encounters an end run condition (e.g. host stop). It is thus the time during which triggers are
counted and can cause recording (or pile up) of data, the best available measurement of the time
the channel was active. The difference between LiveTime and RunTime can be used to
determine how long the local 16-event buffers were full and waiting for readout or other events
prevented the channel from data taking (e.g. out of range).
FTDT (fast trigger dead time)
The fast trigger dead time counts the time the trigger filter is unable to issue triggers because the
trigger filter output is already above threshold (and can not recognize a second pulse). It does not
include the time triggers have been “paused” for a short time after a first trigger (an advanced
user option to suppress double triggering), because the concept is that all triggers occurring
during the pause are counted as only one trigger. When computing the input count rate, one
should divide the number of triggers counted (FASTPEAKS) by the difference (LIVE TIME –
FTDT) since triggers are not counted during FTDT.
SFDT (slow filter dead time)
The slow filter dead time counts the time new triggers will not lead to the recording of new data.
This includes effect 1 listed in section 6.6.1.1 as dead time associated with a pulse. In detail,
•
it includes the time the pileup inspection is taking place and the summation of energy
filter sums is in progress,
•
in case pileup inspection is inverted or disabled, it is not contributing to SFDT
For the “DAQ Fraction” displayed in the Pixie Viewer, the PC's time is used.
GDT (GATE dead time)
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PIXIE-500 Express User’s Manual V3.20
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