•   Wikis

# Image frequency: Wikis

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# Encyclopedia

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

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