d. The basic full-scale sensitivity of the Model 845AR is limited to a
maximum of 1 millivolt. Therefore, input signals above this value must be
reduced. The input divider consists of R101 through R109 and RANGE switch
On ranges being a multiple of 1, input voltages above 1 millivolt
are divided down to 1 millivolt or less, upon selection of the proper range.
On ranges being a multiple of 3, input voltages above 1 millivolt are
divided down to 300 microvolts or less, upon selection of the proper range.
On ranges of 1 millivolt and below, a 10-megohm resistor, R104, is connected
across the input to provide a fixed value of input impedance.
e. The input signal from the input divider is filtered by a three
stage, low-pass filter composed of R110, C101, R111, C102, R112, and C103.
This filter reduces any ac voltage having a frequency above 1 Hz.
V101 and V102, which are driven by DS101 and DS102. The resulting square-
wave signal is coupled through C104 and amplified by Q101, Q102, and Q103
which form a three-stage amplifier having a high input impedance. The gain
of the ac amplifier is controlled by the common emitter resistance selected
by the RANGE switch S101B.
Maximum gain is used on the 1, 3, 10, and 30
microvolt ranges and is gradually reduced by the selection of R124 through
R126 as the range is increased. The output of Q103 is capacitively coupled
to a two-stage current amplifier composed of Q104 and Q105.
amplifiers have a constant gain controlled by fixed negative feedback
through R130 and C111.
f. The synchronous demodulator detects the magnitude and phase of the
The 84 Hz drive signal is applied to the base of
transistor Q106 which references the synchronous demodulator to the same
phase as the photocell modulator.
The demodulated signal is filtered by
R134 and C114 before being applied to the dc amplifier.
g. The dc amplifier amplifies the detected dc signal from the
synchronous demodulator. Transistors Q107 through Q112 comprise a two-stage
differential amplifier with a complementary emitter-follower output.
controls the dc amplifier gain. The output from the common emitter of Q111
and Q112 is one volt dc for a full range input on any range, which drives
the isolation converter.
Overall negative feedback through the resistive
network of R138 through R142 and R114 is controlled by the position of the
RANGE switch S101C.
h. The isolation converter drives the recorder output and meter while
providing isolation from the Model 845AR amplifier circuitry.
signal from the amplifier is applied to the transistors Q113 and Q114. An 84
Hz reference drive signal is applied to the bases of transistors Q113 and Q114
which causes modulation of the dc input signal to occur.
modulated signal is coupled to the secondary of T203 where transistors Q203 and
Q204 demodulate secondary signals occurring at their 84 Hz base signal rate.
Capacitor C204 charges to the peak of the demodulated signal and discharges
through the OUTPUT LEVEL control R1, R211 through R213, and the meter M1. The