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Model 3-IS & 3-IS-1
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-1
October 2010
Technical Theory of Operation
Low Voltage Supply
Battery voltage is coupled to U11 and associated components (a switching
regulator) to provide 5 volts at pin 8 to power all logic circuits. A voltage
divider (R27 and R32) located at pin 1 of U11 sets the end-of-battery–life
squeal at 2.0 Vdc. Components R12 and C30 provide filtering to create
+5 VA used by the amplifier and discriminator circuits.
High Voltage Supply
High voltage is developed by pulses from the switching regulator U13 to
transformer T1. High voltage is multiplied by the ladder network of diodes
CR3 through CR7 and capacitors C18 through C27. High voltage is coupled
back through R39 to pin 8 of U13. High voltage output is set by front panel
potentiometer R42, which sets the voltage feedback of 1.31 Vdc to pin 8 of
U13. R38 and C28 provide filtering.
Detector Input
Detector pulses are coupled from the detector through C6 to amplifier input
pin 2 of U4. CR1 protects U4 from input shorts. R37 couples the detector to
the high voltage supply.
Amplifier
A self-biased amplifier provides gain in proportion to R15 divided by R14,
with some gain loss due to feedback capacitor C4. A transistor (pin 3 of U4)
provides amplification. U6 is configures as a constant current source to pin 3
of U4. The output self-biases to 2 Vbe (approximately 1.4 volts) at the
emitter of Q1. This provides just enough bias current through pin 3 of U4 to
conduct all of the current from the current source. Positive pulses from the
emitter of Q1 are coupled to the discriminator.
Section
8
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