Discriminator; Audio; Scale Ranging; Meter Drive - LUDLUM MODEL 3 Technical Manual

Model 3
Ludlum Measurements, Inc.
Technical Manual

Discriminator

Comparator U8 provides discrimination. The discriminator is set by a
voltage divider (R21 and R23), coupled to pin 3 of U8. As the amplified
pulses at pin 4 of U8 increase above the discriminator voltage, 5-volt
negative pulses are produced at pin 1 of U8. These pulses are coupled to pin
5 of U9 for meter drive and pin 12 of U9 for audio.

Audio

Discriminator pulses are coupled to univibrator pin 12 of U9. The front-
panel audio -
ON OFF
pulses from pin 10 of U9 turn on oscillator U12, which drives the housing-
mounted unimorph speaker. Speaker tone is set by R31 and C14. Tone
duration is controlled by R22 and C7.

Scale Ranging

Detector pulses from the discriminator are coupled to univibrator pin 5 of
U9. For each scale, the pulse width of pin 6 of U9 is changed by a factor of
10 with the actual pulse width being controlled by the front-panel switch, the
analog switches U1 and U2, and the related potentiometers. This
arrangement allows the same current to be delivered to C9 by 1 count on the
×0.1 range as 1000 counts on the ×100 range.

Meter Drive

Pulses from pin 6 of U9 charge capacitor C9. A constant current driver
(opamp U10 and transistor Q2) delivers proportional current to the meter.
For battery test (
switch U3 to the batteries through resistor R8.

Meter Reset

Ratemeter reset is initiated by changing the voltage differential at C9 to 0
when the button is depressed.
RESET

Fast/Slow Time Constant

For the slow time constant, C17 is switched from the output of the meter
drive to parallel C9.
selector controls the reset at pin 13 of U9. When
), the meter is directly coupled by the analog
BAT TEST
Page 8-2
Section 8
,
ON
April 2016