Model 15
Technical Manual
Section 8
Ludlum Measurements, Inc.
Page 8-2
June 2020
Digital Analog Converter
Pin 12, 15 of U4 are coupled as a current mirror. For each pulse of current
through R72, and equal current is delivered to C105. This charge is drained
off by R74. The voltage across C105 is proportional to the incoming count
rate.
Scale Ranging
Detector pulses from the discriminator are coupled to univibrator pin 5 of
U3. For each scale, the pulse width of pin 6 of U3 is increased by a factor of
10 with the actual pulse width being controlled by the front-panel calibration
controls and their related capacitors. This arrangement allows the same
current to be delivered to C105 by one count on the X1 range as 1000
counts on X1K range.
Meter Drive
The meter is driven by the emitter to Q6, coupled as a voltage follower in
conjunction with pin 1 of U6. For ratemeter drive, the meter is coupled to
C105 at P1 15.
For high voltage, the meter is coupled to R132 at P1 11.
For battery test, the voltage follower is bypassed and the meter movement is
directly coupled to the battery through R150.
Meter Compensation
When the unit is provided with a high torque meter movement, with 1.2 V
drive, a temperature compensation package may be located at the meter,
internal or external.
Fast / Slow Time Constant
For slow time constant, C104 is switched from the output of the meter drive
to parallel C105.
