MM0474
Figure 8.
Block Diagram, Model 431C.
(3) Unbalance in the metering bridge produces 10 kHz error signal; this,
plus 10 kHz bias taken directly from the oscillator-amplifier, are mixed in the
synchronous detector to produce an error-proportional direct current. Fed back
to the metering bridge, DC power substitutes for the 10 kHz power in heating
the thermistors and drives the bridge toward balance.
(4) The DC output of the synchronous detector also operates the meter
circuit.
(1) RF Detection Bridge (Figure 9). The RF detection bridge and the 10
kHz oscillator-amplifier are connected in closed loop (the detection loop)
which provides positive feedback to cause oscillation. The RF bridge includes
thermistor element Rd, the secondaries of transformer A1T2, capacitance Ca and
Cb, and the resistive arm consisting of A1R10 and parallel resistors selected
by the MOUNT RES switch.
(2) When the power meter is off, thermistor Rd is at room temperature
and its resistance is about 1500 ohms.
The bridge is unbalanced.
When the
power meter is turned on, a large error signal is initially applied to the
bridge. As this signal heats Rd, its resistance decreases toward the operating
value of 100 or 200 ohms and the RF bridge approaches balance.
The 10 KHz
feedback diminishes until there is just sufficient power dissipated in the
thermistors to maintain them at the operating resistance.
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