What is heterodying?

Answer 1

Hetrodyning means to mix the incoming signal, with a locally produced signal, to produce a third.

When two frequencies are mixed, you get four. There are the two original frequencies, the sum of the two frequencies and the difference of the two frequencies.

By using a filter you can separate out one of the mixes.

Most modern radios are Superhetrodyne types.

Developed to stop 'howl round' feedback, when amplifying a radio signal.

To get better selection you need to amplify the incoming signal. When you do this, the receiver will then detect this amplified signal and reinforce it, causing unwanted feedback, unless extreme measures of screening are involved.

A superhet, mixes the incoming signal with a local oscillator. This is then passed to a filter which selects one of the mixes. This intermediate frequency can now be amplified without the front end 'seeing it'. The IF remains at a constant frequency and can be amplified many times, before detection to remove the carrier. Making for a very sensitive, selective and stable radio receiver. (AM radios typically have an IF of 470Khz)

Tuning is achieved by varying the local oscillator, a different incoming signal will then match with the filtered signal, after mixing.

Dual gang, variable capacitors, are used for tuning. One capacitor selects broadly from the aerial, the second controls the frequency of the local oscillator.