Model 177
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-1
October 2018
Technical Theory of Operation
Amplifier
Negative detector pulses are coupled through C124 to emitter follower Pin
U121. R127 protects the input from inadvertent high-voltage shorts. R129
couples the detector to the high-voltage supply.
Negative pulses from emitter, Pin 2 of U121, are coupled through C121 to
amplifier Pin 5 through Pin 7 of U121. This amplifier is self-biased and
provides gain in proportion to R029 divided by R0210. Transistor (pins 4, 5,
6, U121) provides amplification. Pins 12 and 15 of U121 are coupled as a
current mirror to provide a load for Pin 6 of U121. The output self-biases to
2 Vbe (approximately 1.4 volts) at Pin 7 of U121. This provides just enough
bias current through Pin 6 of U121 to conduct all of the current from the
current mirror.
Positive pulses from Pin 7 of U121 are coupled to the discriminator.
Discriminator
Comparator U021 provides discrimination. The discriminator is set by the
DIS (Discriminator) control located on the rear panel, coupled to Pin 5 of
U021. Negative pulses (approximately 5 volts) at Pin 7 of U021 are coupled
to Pin 5 of U011 for meter drive and Pin 11 of U011 for audio.
Digital Analog Conversion
Pin 7 of U021 is connected to the dual univibrator, U011. For each low
pulse for Pin 7 of U021, Pin 6 of U011 goes high. The pulse of Pin 6 of
U011 is typically 5.0 volts for 6 milliseconds on X1 to 6 microseconds on
X1K. This pulse is connected to the constant current drive U012. The pulse
width control (R3-C2 on calibration board) is utilized for calibration
adjustment. Controls R4 through R6 allow calibration on other scales.
For each positive pulse connected to Pin 8 of U012, a constant current pulse
is sourced at Pin 15 of U012. This current pulse charges C122, which is
Section
8
