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 super-heterodyne principle.

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. One of these will be at the intermediate frequency (IF), and will be selected and amplified. The radio receiver is responsive to any signal at its designed IF frequency, including unwanted signals. For example, with a LO tuned to 110.7 MHz, there are two incoming signal frequencies that can generate a 10.7 MHz IF frequency. A signal broadcast at 100 MHz (the wanted signal), and mixed with the 110.7 MHz LO will create the sum frequency of 210.7 MHz (ignored by the receiver), and the difference frequency at the desired 10.7 MHz. However, a signal broadcast at 121.4 MHz (the unwanted signal), and mixed with the 110.7 MHz LO will create a sum frequency of 232.1 MHz (ignored by the receiver), and the difference frequency also at 10.7 MHz. The signal at 121.4 MHz is called the image of the wanted signal at 100.0 MHz. The ability of the receiver to reject this image gives the image rejection ratio (IMRR) of the system.