Agilent Technologies e4418b, e4419b manual A2 Processor Assembly

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 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.

4-4 Agilent E4418B/E4419B Service Guide