Image response is a measure in decibels of how components in radio electronics function. It provides a ratio of the signals that are intended to pass through the radio receiver to unwanted signals. A signal’s image is generated as a result and needs to be rejected to minimize interference. Also called the Image Response Rejection Ratio (IMRR), image response measures the characteristics of the signal before the intermediate frequency (IF) is amplified. The phenomenon is more often applied to super-heterodyne radios, which use an oscillator to create two frequencies out of an incoming signal; a sum and a difference frequency.
An unwanted signal can be removed by including the appropriate circuitry into a radio receiver. The circuit removes the signals at a particular frequency. Other channels of the radio signal are left alone. To be effective, the nature of the image response is that the separated signals have to be equal to the intermediate frequency or be double the rate. Otherwise the interference and changing pitches associated with receiver tuning will reduce the quality of radio communications.
The ratio of image response defines the level of IMRR that a radio features. To get a measurement of it, the wanted signals and their associated image needs to be defined at the particular frequency on which the radio operates. Image response typically varies according to the frequency, and the difference between the signals as a percentage is less as the frequency gets higher.
Managing the image frequency can become complicated when antenna signals interfere with intermediate frequencies. This interference is known as IF breakthrough. To combat this issue, frequencies that don't have these tendencies are usually chosen.
A radio with an image response ratio insufficient for a radio communications application will have too much interference. It will seem to have more signals than it does, which are components of the non-rejected signals. The interfering signals are not actual frequencies the radio is tuned to, but act to reduce the quality of radio transmissions and the sounds heard by radio operators and listeners.
Image response is a characteristic measured on both radio transmitters and receivers that operate on the super-heterodyne principle. In some cases, multiple intermediate frequencies are used to compensate for IMRR concerns in the system. The IMRR also measures how the radio oscillator is stabilized, even within 21st century frequency synthesizer technology and software-defined radios in computer applications.
By Andrew Kirmayer