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Theory of Operation
A2 Processor Assembly
4-4 Agilent E4418B/E4419B Service Guide
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 outputs, TTL input/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 AC power is connected to the rear panel or
the optional rechargeable battery is fitted. 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.
When the optional rechargeable battery is fitted and the meter is in
standby and connected to the ac power supply, the rechargeable battery
will recharge and the fan will be powered on.
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 outputs are
driven from a dual 12 bit DAC which is driven by the microcontroller. The
dual DAC outputs are buffered, filtered and scaled to give a 1 V full scale
output nominal.
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 kΩ 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.