MM0474
(2) The same 10 kHz power change produced in the RF bridge by RF power
also affects the metering bridge through the series connection of A1T1 and A1T2
primaries. Although this change of 10 kHz power has equal effect on both the
RF and metering bridges, it is initiated by the RF bridge circuit alone. The
metering bridge cannot control 10 kHz bias power, but the 10 kHz bias power
does affect the metering circuit. Once a change in the 10 kHz bias power has
affected (unbalanced) the metering bridge, a separate, closed DC feedback loop
(metering loop) re-establishes equilibrium in the metering circuit.
(3) Variations in the 10 kHz bias level, initiated in the RF bridge
circuit, cause proportional unbalance of the metering bridge, and there is a
change in the 10 kHz error signal (I10kHz) applied to the 10 kHz tuned
amplifiers in the metering loop. These error signal variations are amplified
by three 10 kHz amplifiers, and rectified by the synchronous detector.
From
the synchronous detector the DC equivalent (IDC) of the 10 kHz signal is
returned to the metering bridge, and is monitored by the metering circuit to be
indicated by the meter. This DC feedback to the metering bridge acts to return
the bridge to its normal, near-balance condition.
(4) The reactive components of the metering bridge are balanced with
variable capacitor C1 and inductor A1Ll.
Null adjust, C1, is an operation
adjustment and L1 is a maintenance adjustment. Null adjust, C1, is adjusted
with the RANGE switch in the NULL position. The 10 kHz signal is taken at the
synchronous detector, rectified by A1CR8, and read on the meter. The rectified
signal contains both reactive and resistive voltage components of the bridge
unbalance.
k. Synchronous Detector.
(1) A simplified schematic of the synchronous detector is shown in
Figure 11. The synchronous detector converts the 10 kHz error signal from the
metering bridge to a varying DC signal.
The detector is a bridge rectifier
which has a rectifier in series with a linearizing resistance in each of its
arms. Two 10 kHz voltages, designated E3 and E4 in Figure 11, are applied to
the bridge; 1) voltage E3, induced in the secondary of A1T3, is proportional to
the metering bridge error signal and is incoming from 10 kHz tuned amplifier
Q3; 2) voltage E4, induced in the secondary A1T4, is proportional to a voltage
supplied by the 10 kHz oscillator-amplifier. Voltage E4 is much larger than
voltage E3 and switches appropriate diodes in and out of the circuit to rectify
voltage E3.
Section (a) of Figure 11 shows the current path through diodes
A1CR2 and A1CR3 for a negative-going signal. The rectified output is taken at
the center taps of transformers A1T3 and A1T4.
(2) The synchronous detector operates in the following manner. When the
left side of A1T4 is positive with respect to the right side, as in Figure
11(a), diodes A1CR4 and A1CR5 conduct while diodes A1CR2 and A1CR3 are biased
off. With the polarities reversed, as in
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