PCM-2 Manual.pdf - Voss Associates

eplacing a board, since that information is wired into the chassis harness
connector.
A four-pin header, 13, is used only when the modular detector board is part of the
hand probe option. This connector contains power, ground and signals for a
"clicker" board used with the hand probe. J2 is connected to the switch which
enables clicking when the hand probe is removed from its holder.
Circuit Description
Amplifier and
Discriminator Circuits
Pulses from the detector are coupled into a low-impedance amplifier through a
DC-blocking capacitor. Two stages of amplification are used; the first stage
inverts negative-going detector pulses and feeds the alpha pulse threshold
comparator. The second stage, which is non-inverting, provides sufficient gain
for detection of beta pulses.
Each amplifier output is AC-coupled into its respective threshold comparator.
The thresholds voltages are obtained by filtering two pulse-width modulated
outputs from the microprcessor. One-shot multivibrators condition the comparator
outputs into logic-level signals with constant pulse widths. Anti-coincidence logic
may be enabled under software control to prevent alpha pulses from also being
included in the beta count, or disabled for test purposes. When a modular
detector board is used in the hand probe option, alpha and beta outputs are ORed
together to produce click pulses.
CAUTION:
THE HIGH VOLTAGE SECTION OF THIS BOARD PRODUCES
POTENTIALS OF UP TO 2500 VOLTS, WHICH MAY REMAIN PRESENT
FOR SEVERAL MINUTES AFTER POWER IS REMOVED! EXERCISE
CARE WHEN WORKING ON THIS MODULE! DISCHARGE THE INPUT
JACK TO CHASSIS GROUND WITH AN INSULATED SCREWDRIVER
BEFORE REMOVING THE CIRCUIT BOARD FROM ITS HOUSINGl
High Voltage
Supply
Accelerating potential for the detector is produced by an inverter and
voltage-multiplying ladder circuit. The output voltage is divided by a 1000: 1
resistor network and compared to a reference voltage generated by filtering a
pulse-width modulated output from the microprocessor. This feedback loop
controls the frequency of the inverter oscillator. Two additional comparators are
used which interrupt the processor chip if the high voltage output goes out of
tolerance.
13-2
PCM2.MAN/ Rev A/April 1995