The superheterodyne receiver takes advantage of two important facts of radio operation. First, amplification of a radio signal at a low frequency can be more successful than amplification at a high frequency. Second, amplification of a signal by fixed tuned circuits is more successful than amplification by variable tuned circuits.
The superheterodyne converts all input frequencies to a single, fixed, lower frequency which is amplified by fixed tuned circuits. This process distinguishes the superheterodyne from other receivers in performance, for it provides outstanding sensitivity and selectivity over the whole tuning range. The superheterodyne is distinguished from other receivers in construction, as shown on page 30.
Note the IF (intermediate frequency) amplifier stage. It is this stage which employs the fixed tuned circuits. The fixed frequency which it amplifies is called the intermediate frequency, because it is lower than any input frequency within the receiver's tuning range, but higher than audio frequencies. The receiver shown in the block diagram is a broadcast receiver with an IF of 455 kc. This fre quency is substantially lower than the lower limit of the broadcast band which is 550 kc.
Note the local oscillator stage. The output frequency of this stage is combined with the output frequency of the RF amplifier stage to produce a new frequency the intermediate frequency. This process is called heterodyning. The frequencies on the block diagram show the frequencies at which various stages operate when the receiver is tuned to receive a signal of 800 kc. Note that the oscillator frequency is given in the block diagram as 1255 kc, and the RF amplifier frequency as 800 kc. This means that the oscillator operates at a frequency 455 kc above the RF amplifier frequency.
The difference frequency, equal to the IF, appears in the output of the mixer. The oscillator frequency is heterodyned against the RF amplifier frequency in the mixer stage. The output of the mixer contains the oscillator frequency, the RF amplifier frequency, the sum of these two frequencies, and the difference between these two frequencies. The fixed tuned circuits of the IF amplifier are tuned to 455 kc. Therefore, they accept the difference frequency but reject the others.
The heterodyning process is illustrated by the waveforms shown in the block diagram. The RF modulated wave intercepted by the antenna is shown at A. At B, the waveform shows the same modulated carrier after it has been amplified by the RF amplifier stage. At C, the waveform shows the output of the local oscillator. It is unmodulated. Its amplitude is considerably higher than the amplitude of the waveform put out by the RF amplifier. At D, the waveform represents the amplified
KC RF AMPLIFIER 800 KC MIXER 455 KC IF AMPLIFIER
Typical Superheterodyne difference frequency. Note that it bears the same modulation pattern as the RF carrier. At E, the waveform represents the detected audio frequency. At F, the waveform represents the amplified audio frequency.
Of course, the superheterodyne receiver is not confined to an input frequency of 800 kc. The RF amplifier tuned circuit is variable and can select any frequency in the broadcast band. The tuned circuit of the oscillator is also variable. It is ganged with the RF amplifier tuned circuit so that it is always 455 kc above the frequency to which the RF amplifier is tuned. Thus, the difference frequency between oscillator frequency and the RF input frequency is always 455 kc. This arrangement for keeping frequencies separated by a fixed amount is called tracking. It means that the frequency presented to the IF amplifier is always the same, no matter what the RF input frequency may be.
For low, broadcast, and medium frequencies, the oscillator usually tracks above the signal. For VHF and UHF, the oscillator usually tracks below the signal.
There is one major disadvantage to super heterodyne operation. If a local oscillator frequency of 1255 kc can mix with an input frequency of 800 kc to produce a difference frequency of 455 kc, this same local oscillator frequency of 1255 kc can also mix with an input frequency of 1710 kc to produce a difference frequency of 455 kc. Thus, the mixer section might present to the IF amplifier the signal from two different stations at the same time, both converted to the same IF. The IF amplifier would accept and amplify both at the same time. The demodulator would detect the signal of both at the same time. The intelligence of both would be present in the speaker at the same time. Such a mixture of signals would be confusing, if not unintelligible.
The second signal which might interfere with the desired signal is called the image frequency. Image frequencies can best be prevented by selective tuning of the RF amplifier section. Highly selective RF amplifier tuned circuits, when tuned 455 kc below the oscillator frequency, will reject a frequency 455 kc above the oscillator frequency. In other words, the RF stage tuned to a frequency of 800 kc rejects the image frequency of 1710 kc.
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