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In radio reception using heterodyning in the tuning process, the image frequency is an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces. It is a potential source of interference to proper reception.

In a heterodyne receiver, a mixer fed by a local oscillator whose frequency f_{o}\! is tunable converts the desired input frequency f_{s}\! to a fixed IF f_{i}\! that then passes through selective filter(s), amplification and detection. The output of a simple mixer contains the sum and difference of its two input frequencies. Therefore both the input frequencies f_{o}\pm f_{s} are converted to f_{i}\!. Normally one wants to receive only one of these input frequencies. The unwanted frequency is called the image of the wanted frequency, alternatively the mirror frequency, because of the mirror-like symmetry of the detectable frequencies about f_{o}\!. Sensitivity to the image frequency can be minimised only by (a) a tunable filter that precedes the mixer or (b) a much more complex mixer circuit[1].

Choosing a high IF allows a simple filter for A above. The fixed IF filters cannot contribute to image rejection but they can be designed to pass a range of frequencies, denoted the bandwidth. This defines the overall bandwidth centered on f_{s}\! of the receiver.

References

PD-icon.svg This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).


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