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SECTION 5
In a superheterodyne type receiver the radio wave at
the antenna is amplified and then mixed with the local
oscillator to produce the intermediate frequency (IF).
Transistor Q1 not only amplifies the RF signal but
also simultaneously oscillates at a frequency 455kHz
above the desired radio station frequency. Positive
feedback from the collector to the emitter of Q1 is
provided by coil L2 and capacitor C3. During the
heterodyne process, the following four frequencies
are present at the collector of Q1.
1. The local oscillator frequency, LO.
2. The RF carrier or radio station frequency.
3. The sum of these two frequencies, LO + RF.
4. The difference of these two frequencies, LO - RF.
The “difference frequency” is used as the
intermediate frequency in AM radios. The collector of
Q1 also contains an IF transformer (T1) tuned only to
the difference frequency. This transformer rejects all
frequencies except those near 455kHz. T1 also
couples the 455kHz signal to the base of Q2 to be
processed by the IF amplifiers.
The antenna and the oscillator coils are the only two
resonant circuits that change when the radio is tuned
for different stations. Since a radio station may exist
455kHz above the oscillator frequency, it is important
that the antenna rejects this station and selects only
the station 455kHz below the oscillator frequency.
The frequency of the undesired station 455kHz
above the oscillator is called the image frequency. If
the selectivity of the antenna (Q factor) is high, the
image will be reduced sufficiently.
The oscillator circuit must also change when the
radio is tuned in order to remain 455kHz above the
tuning of the desired radio station. The degree of
accuracy in keeping the oscillator frequency exactly
455kHz above the tuning of the antenna is called
tracking accuracy.
MIXER AND OSCILLATOR
THEORY OF OPERATION
