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5.2.11.6. Setting the Rise Time in Digital MCBs
To achieve the best results for your application, when using a digital spectrometer, such as a
DSPEC-series instrument, we recommend that you set the rise time of the pulses being processed by the digital filter to the minimum value for the resolution needed.
The pulse rise time (and also fall time) is based on the time required for each pulse to reach its peak value. This “peaking time” is about twice that indicated by the conventional time constants displayed on the front panel of commercial analog amplifiers. For example, germanium detectors are often specified at a 6-
μs time constant; this setting is equivalent to 12-μs peaking (rise) time in our digital spectrometers.
Up to some value of rise time, one can expect improved resolution with increasing rise time; there will, however, be a tradeoff in maximum throughput to memory. Figure 90 illustrates an example of this tradeoff.
ORTEC digital spectrometers operate well above the peak of the throughput curve. Operating there allows these instruments to handle an even higher rate of incoming counts, but with less data into memory and, therefore, a
Fig. 90. An Example of the Tradeoff Between
Throughput and Count Rate.
longer count time to the same detection limit. It is possible to move the peak of the curve to the right (more counts to memory with higher input count rate) by reducing the pulse rise (and fall) time, thereby trading off resolution for maximum count rate.
Table 2. Rise Time Selection Guide.
Table 2 is a guide to choosing a count rate that will ensure that the most efficient operation of your digital spectrometer over the range of anticipated input count rates for your application — that is, at or below the throughput peak — while achieving the best resolution obtainable from the detector consistent with that requirement. Enter the rise time that best matches your dynamic range of count rate (note that the available rise-time settings will vary by instrument; this chart is a general guide only).
Input Count Rate
Dynamic Range
0--->20000
0--->50000
0--->75000
0--->100000
0--->150000
0--->200k
0--->220k
0--->250k
0--->300k
Maximum
Throughput
9000
12500
23500
37000
50000
70000
85000
100000
120000
Rise Time
(
µs)
12
8
4
2.4
1.6
0.8
0.6
0.4
0.2
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