MM0474
(2) During operation, sufficient amounts of DC and 10 kHz bias currents
are supplied from the model 431 or 432 power meter to heat the thermistors
until their resistances are reduced to approximately 200 ohms per series pair.
Capacitor C1 offers high impedance to 10 kHz but is practically a short to RF.
This causes D to appear series connected to 10 kHz, but parallel connected to
RF.
In this manner, D appears to the audio bridge of the model 431 or 432
power meter as a 200 ohm resistance but terminates the coaxial cable in 50
ohms. Capacitor C2 blocks any DC and audio power that may be present in the
incoming signal, and passes only RF power. See Figure 30.
Figure 30. Schematic diagram of a model 478A thermistor
mount when connected to a 432 power meter.
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