Ludlum Measurements 2241-2, Manual

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Model 2241-2RK Technical Manual
Section 7
Ludlum Measurements, Inc. Page 7-1 July 2012
Technical Principle of Operation
Detector Input/Amplifier
Negative-going detector pulses are coupled from the detector through C021
to Amplifier U021. R024 and CR021 protect the input of U021 from
inadvertent shorts. Self-biased amplifier U021 provides gain in proportion to
R022, divided by R025. Transistor pins 4, 5, and 6 of U021 provide
amplification. Pins 10-15 of U021 are coupled as a constant current source
to pin 6 of U021. The output is self-bias to 2 Vbe (approximately 1.4 volts)
at pin 7 of U021. This provides just enough bias current through pin 6 of
U021 to conduct all of the current from the constant current source.
Positive pulses from pin 7 of U021 are coupled to the discriminator (U011)
through R031 and C012.
Discriminator
Positive pulses from amplifier U021 are coupled to pin 2 of U011
comparator. The discrimination level is set by the
DISC
control connected to
pin 3 of U011. As the positive pulses at pin 2 of U011 increase above
DISC
reference at pin 3, pin 1 goes low, producing a low pulse. Pin 1 of U011 is
normally held high (+5 volts) by R014.
The low pulse from pin 1 of U021 is coupled to univibrator U001. U001
shapes and fixes the pulse-width to approximately 10 µs. The univibrator is
configured in the non-retriggerable mode. Negative pulses from pin 9 of
U001 are coupled to the µP for counting.
Low Voltage Supply
Battery voltage is coupled to DC-DC converter U231. U231 and related
components p5 V to power the µP, op-amps, and logic circuitry.
R135 and R136 provide voltage division for low-battery detection. Pin 6 of
U231 provides a low signal when the battery voltage decreases to +2.2 ±0.1
Vdc. U121 provides the +2.5 Vdc reference for the
HV
and
DISC
control
references.
Section
7
Refer to the Main
Board schematic
for the following: