Elenco Electronics AM-550TK Instruction Manual download pdf (Page 24)

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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

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Elenco Electronics AM-550TK Instruction Manual Page 24