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voltage controlled variable oscillator with a center frequency of 22.75 MHz. The oscillator output
frequency is flltered and applied through a buffer amplifier to a divide-by-SO circuit. The output of
this divide-by-SO circuit provides the variable 447 to 463 kHz BFO signal for reinsertion at the product
detectors on circuit card assemblies A4 and AS.
A digital control circuit is used to vary the basic oscillator frequency of 22.75 MHz. The
output of this circuit is coupled to a phase comparator, along with a 500 Hz reference signal. This
500 Hz reference signal is derived by applying the 1 MHz reference sign.al, from the second local
oscillator, to a divide-by:2000 circuit. The phase comparator compares the output ·of the digital
cQntrolcircuit and the 500
ijz
reference signal and applies the difference signal through a digital to .
analog converter to the veo. The oscillator may be varied between 22.35 and 23.15 MHz in 560 Hz·
increments, which when applied to the divide-by-SO circuitry, provides a BFO frequency between 447
and 463 kHz variable in 10
Hz
increments. The BFO frequency is flltered before being routed to the
product detectors.
4.2.3
Automatic Gain Control (AGe) Section
The AGC circuits provide the Receiver with constant level AF output signal with large
variations in the incoming RF signal .. For example, the change in IF or AF output levels
is
less than
6 dB for a change in the input level of -100 dBm to -10 dBm. This automatic gain control is ac-
complished through AGe circuitry, located mainly on circuit card assembly A4. The optional ISB
circuit card assembly AS contains its own AGe for signal gain control in the ISB mode of operation.
The Receiver may be operated in anyone of three different gain control modes: manual.
automatic, and automatic with a manually set threshold. In the manual mode, the gain is set through
a front panel control. In the automatic mode, the AGC circuits
will
compensate for changes in the
receiver input signal level. In the automatic/manual mode, the front panel control is used to set the
operating threshold of the AGC circuits.
.
The AGC operates from a portion of the IF signal taken after the
gain
control IF amplifier
stage, thus maintaining a closed AGC loop. The AGC detects the IF signal, provides three different
decay times, and provides for automatic threshold control of the output signal or manual threshold
control. The output of the AGC circuit is routed to the second IF amplifier on circuit card assembly A4
to a current amplifier on circuit card assembly A3. The current amplifier controls the first IF signal
by controlling the gain of two IF amplifier stages; one located on circuit card assembly A3 and the
other located on circuit card assembly A2.
4.2.4
Receiver Control Section
Signals that control the receiver's operational parameters (such as operational mode, receiver
frequency, BFO frequency, bandwidth, AGC and BITE sequence) are produced by the microcomputer
(A6A2) and routed by the front panel receiver control circuit card assembly (A9). The microcorriputer
under program control follows instructions it receives from the front panel in LOCAL operation and
. from the remote controller in REMOTE operation. In both LOCAL and REMOTE, the microprocessor
functions essentially the same; however, when being operated from a remote location, the power must be
turned on at the front panel and the optional serial asynchronous interface assembly. (A6AI) must be
installed. The serial asynchronous interface circuit card assembly (A6Al) interfaces the external
remote controller (when used) with the microcomputer.
Figure 4-1 shows the receiver control circuits and shows the signal flow between the front
panel receiver control assembly (A9), the microcomputer circuit card assembly (A6A2) and the
serial asynchronous interface circuit card assembly (A6Al). The front panel receiver control assem-
bly (A9) contains the Liquid Crystal Displays (LCD), connects to both sets of keyboard switches
4-6
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