form the collector tank circuit which must be tuned whenever the crystal frequency
is changed.
The output signal is extracted from the collector of Q4 and
capacitively coupled to the first mixer.
The crystal operates on its third
overtone mode. Tuning of the oscillator can be checked by monitoring the emitter
current at test point TP1.
4-7.
FIRST MIXER
The output of the second RF stage and the local oscillator are fed into the
first mixer stage (Q3).
This stage operates in the common-emitter configuration.
Miniature slug-tuned coil T4 serves as the collector load for Q3.
4-8.
FIRST IF AMPLIFIER
The first mixer frequency of 10.7 MHz is fed into the 10.7-MHz IF amplifier
(Q5), which is operated in the common-emitter configuration. A miniature shielded
coil (T6) with a powdered-iron core is used with capacitor C8 to form the tank
circuit.
The tuning range of the amplifier is restricted to prevent accidental
tuning to the second oscillator frequency. The output from the collector circuit
is capacitively coupled to tank circuit T7. A low-impedance tap on T7 couples the
signal to the base of the second mixer stage.
4-9.
SECOND OSCILLATOR
The second oscillator, operating at 10.245 MHz, employs a modified Colpitts
circuit with the crystal operating at its fundamental mode. Capacitor C12 provides
series-oscillator injection to transistor Q6, the second mixer stage.
4-10.
SECOND MIXER
The second mixer operates in the common-emitter configuration with base
injection of the second oscillator signal. The output of the 10.7-MHz amplifier is
mixed with the output of the 10.245-MHz second oscillator to produce a 455-kHz
second IF signal.
4-11.
FILTER AND AMPLIFIERS
a. 455-kHz Filter.
The major bandwidth characteristics of the receiver are
provided by this sealed ceramic filter.
The filter has a 6-db bandwidth of
approximately 40 kHz and a 60-db bandwidth of approximately 70 kHz.
b. IF Amplifiers.
The first and second 455-kHz amplifiers, Q8 and Q9, both
operate in the common-emitter configurations.
These two circuits require no
tuning.
4-12.
LIMITER AND DISCRIMINATOR STAGES
Transistor Q10 acts as a limiter stage with increasing signal level; limiting
action occurs also at Q9.
R7 couples the signal to the primary of discriminator
transformer T8. A modified Foster-Seeley type of discriminator is
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