Note
Using a reset preamplifier the P/Z control is adjusted fully CCW. Some ampli-
fier shapings might exhibit small undershoots. These arise primarily from am-
plifier shaping component tolerances and secondary time constants associated
with the detector/preamp system. If an undershoot is present and is less than 20
mV, its impact on performance is insignificant. However, if small shaping
undershoots are present, they should not be confused with preamp matching
misadjustments undershoots, which exhibit a much longer time constant and
have a larger performance impact. At high count rates, P/Z matching
misadjustment will affect spectral peak shape and resolution.
Pole/Zero Matching Using a Square Wave Generator
1.
Driving a preamp test input with a square wave will allow a more precise
adjustment of the pol/zero matching. The square wave signal must be of good
quality with a flat top and bottom.
2.
The amplifier’s COARSE GAIN, SHAPING, and INPUT POLARITY
controls should be set for the intended application.
3.
Adjust the square wave generator for a frequency of approximately 1 kHz.
4.
Connect the square wave generator’s output to the Preamp’s TEST INPUT.
5.
Remove all radioactive sources from the vicinity if the Detector.
6.
Using an oscilloscope, monitor the Model 2016’s AMP OUT unipolar signal.
Set the scope’s vertical sensitivity to 5 V/div, and adjust the main time base
to 0.2 ms/div. Adjust the square wave generator’s amplitude control
(attenuator) for a AMP OUT signal of 8 V.
Note
Both positive and negative unipolar pulses will be observed at the output. Re-
duce the scope vertical sensitivity to 50 mV/div. To prevent scope overload,
use a clamp box, such as the Canberra Model LB1502 Schottky Clamp Box,
and set the CLAMP/DIRECT switch to CLAMP. With the unipolar signal
clamped, adjust the P/Z control as illustrated in Figure 25.
When adjusting the P/Z matching using the square wave technique, the calibra-
tion square wave generated by the oscilloscope can be used. Most scopes gen-
erate a 1 kHz square wave used to calibrate the vertical gain and probe
compensation. Connect the scope Calibration Output through an attenuator to
the preamp test input and perform the steps in this section again.
39
Pole/Zero Matching Using a Square Wave Generator
