b. The ac amplifier is a high impedance amplifier whose gain is
controlled by the resistance selected by the RANGE control. The amplified
ac signal is then detected by the synchronous demodulator.
c. The synchronous demodulator is driven by the 84 Hz reference signal
and detects the amplified ac signal.
The detected ac signal is then
amplified by an ac amplifier whose gain is controlled by fixed feedback.
The output signal of the ac amplifier is applied to the isolation converter
which drives the isolated recorder output and the meter which indicates the
fed back to the input of the ac amplifier to control overall amplifier gain.
The feedback ratio is determined by the setting of the RANGE control and
The Model 845AR High Impedance Voltmeter - Null Detector circuit
theory of operation description is contained in the following paragraphs.
understanding circuit theory, and in troubleshooting.