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In the AM mode, the active envelope detector converts the
9
MHz IF to a 300 to 3000 Hz audio frequency.
This audio is
normally routed by the bandwidth switch directly to the audio
amplifier (3 kHz bandwidth).
The 1 kHz band-pass filter has a center frequency of 2.5 kHz,
and provides sharp skirt selectivity to produce an ideal band-pass
for FSK signals.
A low level audio amplifier provides the signal
from the filter or directly from the AM envelope detector to an
audio distribution amplifier.
This latter amplifier distributes
the audio signal to an output amplifier, a monitor amplifier, and
an AGC detector.
The audio output amplifier is a push-pull circuit with 600-
ohm terminals for matching the input of an external Demodulator.
The OUTPUT LEVEL control permits the audio signal to be varied up
to a nominal 0 dbm.
The Monitor output amplifier is also a push-pull circuit with
16-ohm terminals for driving an external speaker or headphones.
The MONITOR LEVEL control permits the audio signal to be adjusted
to a suitable listening level.
The FSK (SSB mode) AGC circuits include a detector, fast and
slow release circuits, several de amplifiers, and an external input
circuit.
The AGC attempts to hold the Receiver output level constant
despite changes in input signal strength.
The AGC switch permits
selection of either an internally generated control voltage with
slow or fast release times, or an externally generated signal voltage
(with slow release time) from an associated Demodulator.
The in
ternal AGC voltage is derived from the 2.5 kHz audio signal; the
signal is successively detected, filtered, and amplified to produce
an average de level that reflects the audio amplitude.
The re
sultant average de level provides negative feedback to the cathode
circuits of the beam deflection mixer stages and to both IF ampli
fier stages.
If the signal received at the antenna begins to fade,
the generated AGC voltage tends to increase the mixer and detector
gain and, thus, a constant output from the Receiver is maintained.
Similarly, increases in signal strength reduce the gain of both
stages to produce the same effect.
The external circuits used to derive the AGC control voltage
are designed for operation with an FSK Demodulator such as the
Frederick Electronics Model 1200.
Mating of the Receiver and the
Model 1200 produces an ideal environment for the Detectors and
patented Decision Threshold Computer (DTC) in the FSK Demodulator.
Mark an� space tones from the Demodulator are separately amplified,
and the resultant outputs are combined in a summing amplifier.
An
AGC detector then extracts the de signal strength variations in
the same manner as described for the internal AGC detector.
The
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