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from that circuit card is coupled to Ul7D through diode CR27. The two diodes biases the strongest
of the two signals to the input of UI7D. The output of this amplifier is coupled through J2 to AGC
circuits on circuit card assembly A3.
An AF /RF meter comparator circuit is contained on circuit card assembly A4. This circuit
monitors the main RF, the ISB RF and the AF that may be input from either the main or the ISB
signal. The circuits consist mainly of comparator amplifiers U24A (AF), U24B (ISB-RF) and U24C
(main-RF). All three amplifiers operate in the same manner with their negative inp'uts accepting the
AF or RF reference while the positive input is referenced from the digital to analog converter U21 ..
The output of each amplifier is output through connector 12 to Receiver control. From this infor-
. '.
mation the microprocessor adjusts the input to U21 which in turn adjusts the converter signal to' the
AGC in
all
modes except manual. The output of the three comparators are also processed to the
front panel meter readout where the RF or AF signal level can be
monito~d.
4.3.5
Independent Sideband (lSB), AS
The Independent Sideband (lSB) circuit card assembly AS contains IF circuits, AF cir-
cuits and AGC circuits. The description of these basic circuits are divided into those three basic
functions and shown in two separate functional block diagrams in Figures 4-7 and 4-8. Input signals
to the circuit card include; IF signal, BFO signal, AGe, and audio signals from A4. Output signals
include; AGC and audio to A4, and AGC and audio to AF OUT-J3 on the rear
pan~l,
but through
circuit card assembly A4. The ISB circuit card assembly schematic diagram is shown in Figure 7-5 .
4.3.5.1
IF Circuits
Figure 4-8 shows a functional block diagram of the IF circuitry as it functions on circuit
card assembly A4 along with the AF function. This circuitry consists of a four stage IF amplifier,
a BFO amplifier, a product detector, an audio amplifier, two audio line drive amplifiers, and AGC
circuits. The IF signal is routed from bandpass fUter FLI by the fUter selection switch, located on
circuit card assembly A4 to the IF amplifier on A5. This fIlter is selected in the ISB mode of opera-
tion, so that an IF signal is routed to A5 only in that mode. The IF amplifier is identical to the one
located on A4 and is described in Paragraph 4.3.4.1. The IF signal from the AGe controlled IF am-
plifier is routed both to the AGC circuits and to the signal input port of a product detector Ul1.
A BFO signal from A4 is applied to the base of transistor Q7 through capacitor C30, amplified and
applied to the carrier input port of the same detector. The detector removes the carrier and applies,
through its audio output port, the audio signal, through capacitor C43, to an audio amplifier.
4.3.5.2
AF Circuits
Figure 4-8 shows a functional block diagram of the AF circuitry, along with the IF cir-
cuits, as they function on circuit card assembly A5. This circuitry consists of an audio amplifier and
two audio line driver amplifiers. The audio signal, as received from the product detector, is con-
nected, through capacitor C43 and resistors R78 and R79, to the base of emitter follower amplifier
Q8. The output of this amplifier is then routed to circuit card assembly A4. Refer to 4.3.4.2 for a
description of the ISB audio on A4. The attenuated audio signal is routed back to A5 and applied
to the input oLone line driver amplifier integrated in U12. The output of this amplifier drives
transformer T2 with a center tapped 600 ohm output. This output is routed to AF OUT-J3 on the
rear panel. The main audio signal from the A4 audio is connected to the second amplifier U 12 and
processed in the same manner as the ISB audio except through transformer Tl.
4.3.5.3
AGe Circuits
Figure 4-7 shows a functional block diagram of the AGC circuits for circuit card assembly
A4. These circuits are identical to ISB AGC circuits and are described in Paragraph 4.3.4.3.
4-23
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