Model 15
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-1
June 2020
Technical Theory of Operation
Input
Detector pulses are coupled from the detector through C57 to emitter
follower Q96. R83, 89 provide bias. R137 protects Q96 from input shorts.
R27 couples the detector to the high-voltage supply.
Amplifier
A self-biased amplifier provides gain in proportion to R63 divided by R70.
Transistor U1 provides amplification. U8 functions as a current mirror to
provide a load to U10. The output self-biases to 2 Vbe (approximately 1.4
V), providing just enough bias current to conduct all of the current from the
current mirror.
Positive pulses from U10 are coupled to the discriminator.
Discriminator
Comparator U2 provides discrimination. The discriminator is set by the DIS
(discriminator) control located on the front panel, coupled to pin 3 of U2.
These pulses are coupled to pin 5 of U3 for meter drive and pin 12 of U3
for audio.
Audio
Discriminator pulses are coupled to univibrator pin 12 of U3. Front-panel
audio ON-OFF selector controls the reset at pin 13 of U4. When ON,
pulses from pin 10 of U3 turns on oscillator U5, which drives the can
mounted unimorph through Q149 and T136. Speaker tone is set by R84,
C112; duration by R86.
Section
8
