(5) The required processing is derived as follows: P0 is absorbed power

needed by the RF thermistor to bring its resistance to R ohms (100 or 200

ohms). P0 consists of two components: RF power and DC power supplied by the

432A.

The self-balancing action of the bridge circuit automatically adjusts

the DC power so that the total power in the thermistor is P0. This DC power is

related to the voltage VRF at the top of the bridge by (VRF/2)2/R. Thus

(6) RF power can be determined by measuring VRF with and without applied

RF power and then doing some arithmetic. But this power measuring scheme is

neither convenient nor temperature compensated (since P0 changes with

temperature). The 432A introduces another thermistor bridge circuit exposed to

the same ambient temperature but not RF power.

This circuit includes

adjustments (COARSE and FINE ZERO) so that the DC voltage VCOMP at the top of

its bridge can be set equal to VRF.

Assuming matched RF and compensation

thermistors, VRFO (with no RF power) and VCOMP remain equal with ambient

temperature fluctuation. They differ only when the RF power to be measured is

applied to the RF thermistor. Thus, we have