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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
Содержание SUPERHET AM-550TK
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