Agilent Technologies e4419b, e4418b manual Agilent E4418B/E4419B Service Guide

Theory of Operation

A2 Processor Assembly

The LCD controller on the A3 front panel assembly is configured as a memory mapped peripheral, and as such requires only to be fed with the appropriate address, data and control lines from the microcontroller circuits. The bias voltage for the LCD is produced by a DC to DC converter that takes the +5 V (DIST) voltage and converts that to a nominal +21 V. The DC to DC converter is adjusted by a combination of the contrast control signal from the microcontroller and the temperature sense voltage that is generated on the A3 front panel assembly. The temperature sense voltage helps to compensate for the normal variation of LCD contrast with temperature. Current to the LCD’s LED backlight array is sourced from a constant current circuit that compensates for variation in LED knee voltage. A control input to this circuit from the front panel temperature sense voltage allows for de-rating of the maximum LED current at high operating temperatures. A control input from the microcontroller allows the optimum backlight brightness to be factory set. The temperature sense voltage, LED current and LCD bias voltage are all measured at self test.

An analog multiplexer provides means by which several diagnostic points can be switched to the A6 measurement assembly(s) for analogue to digital conversion. The microcontroller uses this function to perform self test.

A6 measurement assembly(s) to microcontroller communication is conducted on a serial interface bus, the Internal Serial Bus (ISB). The ISB is connected to the bi-directional serial port on the microcontroller via buffers. The system clock and some control signals are connected to the measurement modules. The measurement module interface is completed with the distribution of switched and direct power (+12 V, -12 V and +5 V).

The TTL outputs are connected to the A6 processor assembly via in line resistors located on the A4 interface assembly. A TTL logic level is output from the processor assembly when the user defined windows based limits are exceeded.

The TTL inputs are connected to the processor card via in line resistors located on the interface card and can be used to initiate zero and calibration cycles on the power meter.

Agilent E4418B/E4419B Service Guide 4-5