Restorer Gate, Auto Threshold and Auto Rate
Baseline correction (schematic sheet 6) is prevented during unipolar output signal in-
tervals that exceed the baseline restorer threshold. Comparators A11A and A11B mon-
itor the unipolar output signal and disable or gate off the baseline restorer for signals
that exceed the automatic positive or negative threshold.
The auto baseline restorer threshold circuit, comprised of A9 and A24, peak detects
the negative noise excursions of the unipolar output signal and generates a positive dc
reference voltage equal to the average value of the unipolar output noise level. This
voltage serves as the baseline restorer gating reference that maintains precision for a
wide range of amplifier conditions and applications.
Transistors Q14, Q16, capacitor C92 and the baseline restorer gate signal (PBLR) pro-
duce a voltage reflecting the count rate (schematic sheet 5). This voltage in turn deter-
mines the correction rate of the baseline restorer.
Fast Discriminator
A Fast Discriminator circuit (sheet 6) is included to monitor the input events. This pro-
vides an output (ICR) to ratemeters as well as a visual indication of count rate given by
a front panel LED. The SCA uses the output from the Fast Discriminator (FDO) to de-
termine if multiple inputs are received during a pulse processing sequence. If pileup
reject (PUR) is enabled, the SCA outputs are inhibited when pileup occurs.
The signal from AMP 4 is differentiated and pole/zero compensated before being am-
plified by A34. Amplifier/limiter A34 is baseline restored by transconductance ampli-
fier and gating transistors Q36/Q37. The discriminator A21 generates a short timing
pulse whenever the fast signal from A34 exceeds the threshold level.
This threshold level can be set automatically or manually by using a front panel adjust-
ment. The AUTO threshold circuit (A20 and A22) peak detects the negative-going
noise level and uses this absolute value for the threshold. Its operation is similar to the
BLR threshold circuit described in “Restorer Gate, Auto Threshold and Auto Rate” on
page 27.
Bipolar Amplifier and Crossover Timing
The signal from integrator stage 3 is differentiated, producing a bipolar signal at A7
(sheet 8). The shaping time constant is selected by a section of the shaping switch.
The crossover time when this bipolar signal goes negative provides a consistent refer-
ence point after the peak of the unipolar output. Comparator A37 turns this into a neg-
ative-going logic pulse for the SCA logic.
27
Amplifier
