B. Performance Adjustments
This appendix describes how to make several performance adjustments: adjusting the
rise time and the flat top, matching the pole/zero manually, setting the baseline re-
storer, setting the fast discriminator threshold, and operating the 9660 with reset
preamps.
Rise Time and Flat Top Adjustments
The digital filter employed in the 9660 has a Triangular/Trapezoidal weighting or
shaping function. The processing time (Shaping) is set by the Rise Time and Flat Top
selections and is generally a compromise between optimizing throughput and resolu-
tion. Having the ability to independently set the Rise Time and Flat Top allows greater
flexibility when optimizing the processing time or shaping for a wide variety of detec-
tor applications. The Rise Time sets the noise filtering characteristics of the Digital
Filter while the Flat Top allows for the charge collection time of the particular detec-
tor. Independent adjustment of the flat top allows the shaping function to be optimized
for detectors with long charge collection time, without a large increase in the overall
processing time. For small detectors with minimal charge collection time variation or
ballistic deficit, the trapezoidal shape reduces to triangular shaping when the Flat Top
is set to minimum or zero. The triangular/trapezoidal shaping function is symmetrical.
The fall time cannot be set independently, it always equals the Rise Time selection.
Shaping is adjusted by selecting the Rise Time and Flat Top, which determine the
Trapezoid pulse shape and optimizes performance for the specific detector, spectral
energy range and count rate. As in any signal processing application, a performance
tradeoff exists between high resolution and high throughput. For example when using
a small Ge detector, 5.6
µ
s rise time and 0.8
µ
s flat top settings provide optimum reso-
lution over a wide range of count rates. However, a 2.8
µ
s rise time and 0.6
µ
s flat top
will degrade low count rate resolution performance slightly, but results in less resolu-
tion broadening and peak shift over a much wider count rate range.
For ultra high counting and throughput rates, rise time and flat top settings of less than
1
µ
s may be used. For this case, optimum resolution is traded off for increased count
rate performance. For high resolution detectors, longer rise time settings offer a better
signal to noise (S/N) ratio and longer flat top settings reduce the effects of ballistic
deficit. However, as the system count rate increases, resolution may degrade more rap-
idly due to increased processing time and the effects of pulse pile-up.
User’s Manual - ICN 9231014G
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Rise Time and Flat Top Adjustments
