5-7
Figure 5-4.—Power meter block diagram.
The HP-431C power meter indirectly measures microwave frequency power by using two bridge
circuits—the detection bridge and the compensation and metering bridge. The detection bridge
incorporates a 10-kilohertz (kHz) oscillator in which the amplitude is determined by the amount of
heating of the thermistors in that bridge caused by microwave power. (Thermistors were covered in
chapter 2 of this module.) The compensation and metering bridge contains thermistors that are affected by
the same microwave power heating as those of the detection bridge.
An unbalance in the metering bridge produces a 10-kHz error signal. This error signal, plus the 10-
kHz bias that is taken directly from the 10-kHz OSCILLATOR-AMPLIFIER, is mixed in the
SYNCHRONOUS DETECTOR. The synchronous detector produces a dc current (Idc) that is proportional
to the 10-kHz error signal. The Idc error signal is fed back to the compensation and metering bridge, where
it substitutes for the 10-kHz power in heating the thermistor and drives the bridge toward a state of
balance. The dc output of the synchronous detector also operates the meter circuit for a visual indication
of power.
Q-2. What condition produces the 10-kHz error signal generated by the metering bridge in the HP-
431C power meter?
The HP-431C power meter measures rf power from 10 microwatts (-20 dBm) to 10 milliwatts (+10
dBm) full scale in the 10 MHz to 18 GHz for a 50-ohm coaxial system and 2.6 GHz to 40 GHz for a
waveguide system.
