Image response

Image response (or more correctly, image response rejection ratio, or IMRR) is a measure of performance of a radio receiver that operates on the superheterodyne principle.

[1] In such a radio receiver, a local oscillator (LO) is used to heterodyne or "beat" against the incoming radio frequency (RF), generating sum and difference frequencies.

The image rejection ratio, or image frequency rejection ratio, is the ratio of the intermediate-frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency.

The image rejection ratio is usually expressed in dB.

When the image rejection ratio is measured, the input signal levels of the desired and image frequencies must be equal for the measurement to be meaningful.

IMRR is measured in dB, giving the ratio of the wanted to the unwanted signal to yield the same output from the receiver.

Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity); the signal yields a perfectly valid IF frequency.

Rather, it is the measure of the bandpass characteristics of the stages preceding the IF amplifier, which will consist of RF bandpass filters and usually an RF amplifier stage or two.

The Image Frequency Rejection Ratio (IRR) is characterized by its RF filter which can be determined on the basis of its relative response of a parallel tuned circuit.

The Image Rejection Ratio for a given value of gain imbalance

This article incorporates public domain material from Federal Standard 1037C.

Graphs illustrating the problem of image response in a superheterodyne. The horizontal axes are frequency and the vertical axes are voltage. Without an adequate RF filter, any radio signal S2 (green) from the antenna at the image frequency is also heterodyned to the IF frequency along with the desired radio signal S1 (blue) at , so they both pass through the IF filter (red) . Thus S2 interferes with S1.