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Theory of Operation
A2 Processor Assembly
A2 Processor Assembly
The processor assembly contains the microcontroller and associated
circuits, the power-on/ standby control and switching, the 1 mW reference
calibrator, the recorder output, and the front panel drivers. It provides
that platform on which the power meter can run, facilitating the system
inputs and outputs.
Regulated DC voltages at +12 V, -12 V and +5 V are converted by the A1
power supply assembly when the AC mains is attached to the rear. The
DC voltages are connected and distributed to the rest of the system by
circuits on the processor assembly. When the
key on the front panel
is pressed a bistable latch changes state. This, in turn, applies the correct
gate voltages to turn on three MOSFET switches, which connects the
power supply unit voltages to the distributed power buses. The bistable
latch is connected to a permanent power-supply which has the backup of
battery power when AC mains is removed from the power meter. The
+12 V supply is used to power the fan. This is filtered to decouple it from
the rest of the system.
The reference oscillator has a 50 MHz oscillator circuit with automatic
level control (ALC). The oscillator output is level detected and that level is
compared to a temperature stable precision reference voltage. This
comparison produces an error signal that gives negative feedback control
of the oscillator output power. The frequency and power level of the
calibrator are factory set to provide a 50 MHz 1 mW transfer standard.
The precision reference voltage and the ALC control signal are both
measured in the calibrator self test. The calibrator is switched on or off
using a signal from the microcontroller. The front panel LED indicator is
switched with a separate microcontroller signal.
The recorder is a 12 bit DAC driven by the microcontroller. The DAC
output is buffered, filtered and scaled to give a 1 V full scale output with a
nominal 1 k Ω output impedance.
Circuitry for the keyboard driver includes some damage protection, but it
is basically a direct connection from the keypad row and column matrix to
the microcontroller's control lines. Three LEDs on the front panel are
illuminated under the following conditions:
4-4
HP EPM-441A/442A Service Guide
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