MM0474
(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
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