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CIRCUIT DESCRIPTION:
Referring to the schematic, on the receiver side, the design is of a conventional
Superhet with an RF stage, designed for one frequency reception.
The RF input from the Antenna is passed, after appropriate impedance matching, to
Q1, the RF amplifier. The Crystal controlled local oscillator, called an Autodyne
Converter, or Mixer-oscillator Q2, runs above the received frequency.
The oscillator stage receives the signal from the RF amplifier and the mixing
products appear in Q2's collector circuit. Sum & difference frequencies of the
incoming carrier wave and the oscillator wave appear because the non linear mixing
results in products of these two waves. The first IF transformer T1 effectively filters
off the difference frequency of 455kHz and feeds this to transistor Q3, the first
intermediate frequency (IF) transistor.
Typically, in most Superhet radios, with a 455kHz IF channel, the receive oscillator
frequency runs 455kHz higher than the incoming carrier wave. In my GW-21A
radios, the transmit crystal and transmit frequency is 27.085MHz (Channel 11 of the
Citizen's band) and the receive oscillator crystal in the converter stage is 27.540
MHz.
From Q3 the IF signal passes via T2, Q4, then T3 in the IF amplifier to the detector
diode D1, where the amplitude modulation is recovered. In addition an AGC voltage
is generated by the detector, filtered and fed back to Q3 and Q1.
Of note, in a set with PNP transistors, the AGC voltage is positive going, with
increasing signal strength. This tends to take the transistors which the AGC is
applied to, out of conduction, with a shift towards a lower gain condition, with
increasing received signal strength. Essentially the AGC system is a long time
constant negative feedback loop.
The AGC's time constant, or frequency response to abrupt changes in received
signal level, along with circuit resistances, is set by the value of the 10uF electrolytic
capacitor C2. (With very high signal levels, the voltage on a transistor radio's AGC
capacitor can reverse polarity, so generally I replace the AGC capacitor with a
Bipolar or Film type).
The recovered modulation (audio signal) then passes via the "squelch diode" D2, to
the volume control. D2 is set up with a variable DC voltage applied to its cathode
from the squelch control. This allows the diode to be cut off, progressively
uncoupling the audio feed to the volume control, unless the dynamic signal peaks
are large enough to overcome the diode's voltage drop.
Testing shows that in the "un-squelched" condition, the diode has a 0.43V forward
bias, this is more than enough to have the germanium diode in solid forward
conduction. With the knob in the full "squelched" condition, the applied forward bias
is very close to zero volts, so the recovered audio signal from the detector has to
overcome the diode's forward drop to pass through to the audio amplifier.
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