Pole/Zero Compensation
Lynx II DSA User's Manual – 7096089 173
D. Methods of Detector Matching
This appendix discusses detector matching using pole/zero, manual fast discriminator
adjustment, and baseline restorer adjustments.
Pole/Zero Compensation
Pole/zero compensation is extremely critical for achieving good performance at high count
rates when using detectors with RC preamps. It is equally important for good overload
recovery due to high energy and cosmic events. The Pole/Zero adjustment range
accommodates RC preamp fall times of 40 µs to 1.7 ms. When a reset preamp is used,
Pole/Zero compensation is not required, and the Pole/Zero value must be set to zero (0). If
the Lynx II is connected to a detector with a reset preamp such as the Mirion Model 2101
or 2008, use the Genie Gamma Acquisition and Analysis to change the preamp type from
RC to TRP.
With TRP selected, the pole/zero compensation is automatically set to a value of zero,
representing the matching required for a fall time of infinite duration, which requires no
compensation. If the preamplifier type selected is RC and the Lynx II is connected to a
reset preamp, the P/Z setting must be manually set to a value of zero (0).
Note: Once the Pole/Zero is optimized for the intended detector, the digital filter shaping
parameters (Rise Time and Flat Top) can be changed as required without the need
to make further Pole/Zero adjustment. However, the Pole/Zero compensation must
be readjusted if the detector is changed.
Manual Fast Discriminator Threshold
In some cases, you may want to set the Fast Discriminator threshold manually. For the
best performance, set the threshold just above the system noise level.
1. Set the Amplifier Gain and shaping as required.
2. Set the FDisc Mode in the Gain Device Adjust screen to “Manual”.
3. Remove all excitation sources from the vicinity of the detector.
4. Use the FDisc setting slider bar in the Gain Device Adjust screen to set the fast
discriminator threshold just above the system noise as indicated in step 5.
5. The following steps optimize the discriminator sensitivity to ensure the threshold is
at its lowest setting, just above the noise level:
− Adjust the FDisc Setting to 0%. The ICR LED indicator continuously glows
green.
− Next, increase the FDisc Setting level until the ICR LED indicator is no longer
on continuously but shows low activity by blinking green occasionally. The fast
discriminator threshold is properly set.
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