Appendix C PUR/LTC Operation
170 Lynx II DSA User's Manual - 7096089
PUR Guard
The PUR Guard Time (GT) function is provided to optimize the performance of the Pileup
Rejector. The pileup reject interval is defined as GT x T
R
+T
Flat Top
where:
GT = PUR Guard Time selection; 8 selections ranging from 1.1 to 2.5 are provided
T
R
= Filter Rise Time selection
T
Flat Top
= Filter Flat Top selection
With the default (minimum) PUR GT setting (1.1x) the pileup reject interval and the
Peaking Time are the same; as shown in the figure below.
Figure 76: PUR Reject Interval
Subsequent events arriving within the PUR reject interval are rejected and events
occurring afterward are accepted. Increasing the Guard Time extends the pileup rejection
interval to protect subsequent events from being corrupted by anomalies associated with
the tail of the previous event. As expected, throughput is reduced as the Guard Time and
pileup rejection interval are increased. The maximum Guard Time setting (2.5x) requires
the previous event to fully return to the baseline before subsequent events are accepted.
The default Guard Time (1.1x) is minimum and provides optimum performance and
maximum throughput for most detector applications.
For the example shown above, the second event begins before the first returns to the
baseline. This is not normally a problem, and the second event should be accepted for
maximum throughput. However, if the tail of the first event exhibited detector-induced
anomalies, the second event would be corrupted and should not be accepted. To prevent
acceptance of this corrupted event, the PUR Guard should be increased as shown.
Some detectors with RC preamps may exhibit secondary time constants which is evident
by a short-lived undershoot or ring on the trailing edge of the shaped signal (see the
following figure).
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