Model 78 and 78-1
Technical Manual
Section 7
Ludlum Measurements, Inc.
Page 7-1
March 2021
Technical Theory of Operation
Detector Input/Amplifier
Negative-going detector pulses are coupled from the detector through C021 to
Amplifier U021. R024 and CR021 protect the input of U021 from inadvertent shorts.
Self-biased amplifier U021 provides gain in proportion to R022, divided by R025.
Transistor pins 4, 5, and 6 of U021 provide amplification. Pins 10-15 of U021 are
coupled as a constant current source to pin 6 of U021. The output is self-biased to 2 Vbe
(approximately 1.4 volts) at pin 7 of U021. This provides just enough bias current
through pin 6 of U021 to conduct all of the current from the constant current source.
Positive pulses from pin 7 of U021 are coupled to the discriminator (U011) through
R031 and C012.
Discriminator
Positive pulses from amplifier U021 are coupled to pin 2 of U011 comparator. The
discrimination level is set by the
DISC
control connected to pin 3 of U011. As the
positive pulses at pin 2 of U011 increase above
DISC
reference at pin 3, pin 1 goes low,
producing a low pulse. Pin 1 of U011 is normally held high (+5 volts) by R014.
The low pulse from pin 1 of U021 is coupled to univibrator U001. U001 shapes and
fixes the pulse width to approximately 10 µs. The univibrator is configured in the
non-retriggerable mode. Negative pulses from pin 9 of U001 are coupled to the µP
(microprocessor) for counting.
Low Voltage Supply
Battery voltage is coupled to DC-DC converter U231. U231 and related components
p5 V to power the µP, op-amps, and logic circuitry. R135 and R136 provide
voltage division for low-battery detection. Pin 6 of U231 provides a low signal when the
battery voltage decreases to +2.2 ±0.1 Vdc. U121 provides the +2.5 Vdc reference for
the
HV
and
DISC
control references.
Section
7
Refer to the Main
Board schematic
for the following:
