Superheterodyne Receiver Theory & Principles

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The basic principles and theory behind the superheterodyne radio are relatively straightforward and can be understood quite easily.

The key technique that is employed in the development of the superheterodyne receiver theory is that of mixing. This is not the analogue mixing used in audio additive mixers, but non-linear mixing or frequency multiplication that enables frequencies to be changed or translated.

With many radios used for broadcast reception as well as two way radio communication using this principle, it helps to have a basic understanding so these radios can be used to their best, or the RF circuit design can be accomplished to give the optimum performance.

Basic superheterodyne receiver theory

The superheterodyne receiver operates by taking the signal on the incoming frequency, mixing it with a variable frequency locally generated signal to convert it down to a frequency where it can pass through a high performance fixed frequency filter before being demodulated to extract the required modulation or signal.

It is obviously necessary to look ay this in more detail to understand the principle behind what goes on, but the main process in the superheterodyne radio is that of mixing.

How the superheterodyne receiver works

In order to look at how a superhet or superheterodyne radio works and the RF circuit design, it is necessary to follow the signal through it. In this way the processes it undergoes can be viewed more closely.

The signal that is picked up by the antenna passes into the receiver and enters a mixer. There are three signal ports on the mixer: signal, local oscillator and IF. The signal is obviously applied to the signal port which is designed to accept lower level signals than the LO port.

Another locally generated signal, often called the local oscillator, or LO, is fed into the other port on the mixer and the two signals are mixed.

The mixer action is to multiply the instantaneous levels of the two signals together. The non-linear action of the mixer generates signals at frequencies equal to the sum and difference of the incoming signals.

Many mixers are what is termed balanced, and this means that the two incoming signals are not present, or at least greatly reduced at the output.

The output from the mixer is passed into what is termed the intermediate frequency or IF stages where the signal is amplified and filtered. Any of the converted signals that fall within the passband of the IF filter will be able to pass through the filter and they will also be amplified by the amplifier stages. Any signals that fall outside the passband of the filter will be rejected.

Tuning the receiver is simply accomplished by changing the frequency of the local oscillator. This changes the incoming signal frequency for which signals are be converted down and able to pass through the filter.

It is often helpful to look at a real example to illustrate how the process works. To see how this operates in reality take the example of two signals, one at 1.0 MHz and another at 1.1 MHz.

If the IF filter is centred at 0.25 MHz, and the local oscillator is set to 0.75 MHz, then the two signals generated by the mixer as a result of the 1.0 MHz signal fall at 0.25 MHz and 1.75 MHz. Naturally the 1.75 MHz signal is rejected, but the one at 0.25 MHz passes through the IF stages. The signal at 1.1 MHz produces a signal at 0.35 MHz and another at 1.85 MHz. Both of these fall outside bandwidth of the IF filter so the only signal to pass through the IF is that from the signal on 1.0 MHz.

If the local oscillator frequency is moved up by 0.1 MHz to 0.85 MHz, then the signal at 1.1 MHz will give rise to a signal at 0.25 MHz and another at 1.95 MHz. As a result the signal at 1.1 MHz giving rise to the 0.25 MHz signal after mixing will pass through the filter. The signal at 1.0 MHz will give rise to a signal of 0.15 MHz at the IF and another at 1.85 MHz and both will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished by varying the frequency of the local oscillator within the superhet or superheterodyne receiver.

The advantage of the superheterodyne radio process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

Additional superhet capabilities & principles

Whilst the basic superheterodyne radio receiver theory centres around the mixing process with a variable local oscillator, the radio also contains a number of other circuit blocks. They provide additional functions that are needed within the overall radio, whether for broadcast reception, two way radio communications or whatever.

An RF amplifier and tuning is added to select the right input signal and reject the image, and demodulators are added according to what signals the receiver needs to detect.

The superheterodyne radio theory and concept centre around the idea of mixing a signal within a non-linear multiplier or mixer to change the frequency of the incoming frequency down to a lower intermediate frequency, where there is a fixed frequency amplifier and filter. By changing the local oscillator frequency, so the frequency of received signals is changed.

The concept and theory is straightforward, although the actual RF circuit design can be exacting if the top performance is to be obtained.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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