A2 Processor Assembly - Agilent Technologies E4416A Service Manual

4 Theory of Operation

A2 Processor Assembly

The processor assembly contains the following:
• Microcontroller and associated circuits.
• Power- on/ standby control and switching.
• 1 mW reference calibrator.
• Recorder outputs TTL input/output.
• 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 recharges and the fan is 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.
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E4416A/E4417A Power Meters Service Guide