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remaining operation is identical to that of the internal AGC
circuit.
The AGC switch on the Receiver has positions designated SLOW,
FAST, and RMT.
The first two positions are used with the internal
AGC circuit and function as follows:
SLOW AGC provides a fast
attack and a slow release.
This mode provides ideal conditions
for FSK operation by providing sufficient delay in release to
suppress noise during momentary absences of the mark or space sig
nal.
FAST AGC provides both fast attack and fast recovery times.
This mode of operation is advantageous only during the reception
of rapidly fading signals.
The RMT (remote) position of the AGC switch selects the ex
ternal Demodulator signal previously described.
This mode of
operation uses only the slow internal AGC time constant.
In AM operation, the switch must be in RMT with no signal
connections so that AGC can be generated directly from the AM
detector circuits.
Carrier derived AGC voltage is taken from the
envelope detector circuit, amplified, and then applied to the AGC
de amplifier.
The S-METER provides visual indication of both Receiver tuning
and relative signal strength.
The S-METER is connected in the AGC
de amplifier circuit.
4.2
CIRCUIT DESCRIPTION
4.2.1
PRESELECTOR
(Refer to Figure 6-1.)
The preselector comprises six switch-selectable RF filters,
a wave trap, and a noise generator.
Five of these circuits, cov
ering the 1.7 to 29 MHz range, consist of a 4-pole high-Q toroidal
filter and a tunable RF network.
The remaining circuit, covering
the 10 kHz to 550 kHz range, consists of a 4-pole high-Q toroidal
filter without the tunable RF network.
In operation, the preselec
tor circuits accept RF signals thru the ANT connector from any un
balanced antenna having a transmission line impedance of SO ohms.
The signals are directed to the proper preselector circuit by
means of BAND switch Sl.
For example, signals in the range of
1.7 to 3.5 MHz are directed to the band A preselector.
The tunable portion (1.7 to 29 MHz) of the preselector con
sists of the front panel MHz control (Cl) and inductors Ll through
LS.
Each inductor is associated with a different preselector
section.
When a specific frequency range is selected by the BAND
switch, the proper inductor is connected to Cl.
Manual adjustment
of Cl will then peak the preselector to the desired frequency.
Cl
is switched out of the fixed-tuned portion (10 kHz to 550 kllz) of
the preselector circuit.
4-4
