A superhetrodyne receiver works on the principle the receiver has a local oscillator called a variable frequency oscillator or V.F.O. which maintains a constant difference between itself and the received frequency resulting in a constant intermediate frequency
This is a bit like having a little transmitter located within the receiver. Now if we still have our T.R.F. stages but then mix the received signal with our v.f.o. we get two other signals. (V.F.O. + R.F) and (V.F.O. - R.F).
In a traditional a.m. radio where the received signal is in the range 540 Khz to 1650 Khz the v.f.o. signal is always a constant 455 Khz higher or 995 Khz to 2105 Khz.
Several advantages arise from this and we will use our earlier example of the signal of 540 Khz:
(a) The input signal stages tune to 540 Khz. The adjacent channels do not matter so much now because the only signal to discriminate against is called the i.f. image. At 540 Khz the v.f.o. is at 995 Khz giving the constant difference of 455 Khz which is called the IF frequency. However a received frequency of v.f.o. + i.f. will also result in an i.f. frequency, i.e. 995 Khz + 455 Khz or 1450 Khz, which is called the i.f. image.
Put another way, if a signal exists at 1450 Khz and mixed with the vfo of 995 Khz we still get an i.f. of 1450 - 995 = 455 Khz. Double signal reception. Any reasonable tuned circuit designed for 540 Khz should be able to reject signals at 1450 Khz. And that is now the sole purpose of the r.f. input stage.
(b) At all times we will finish up with an i.f. signal of 455 Khz. It is relatively easy to design stages to give constant amplification, reasonable bandwidth and reasonable shape factor at this one constant frequency. Radio design became somewhat simplified but of course not without its associated problems.
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Updated 13th July, 2000