4 Theory of Operation
88 Keysight E4416A/E4417A Power Meters Service Guide
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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