Image response
Encyclopedia
Image response is a measure of performance of a radio receiver, particularly one that operates on the super-heterodyne
principle.
In such a radio receiver, a local oscillator
is used to heterodyne
or "beat" against the incoming signal frequency
, generating sum and difference frequencies. One of these will be at the intermediate frequency
(IF), and will be selected and amplified. The radio system is responsive to any signal at the IF frequency, including unwanted signals. For example, if the wanted signal is 100.0 MHz, and the IF is 10.7 MHz, the local oscillator may be tuned to 110.7 MHz, creating the sum (210.7 MHz) and difference (10.7 MHz). However a signal at the input at a frequency of 121.4 MHz will create sum (232.1 MHz) and difference (10.7 MHz) frequencies, of which the latter signal will be selected and amplified by the IF stages of the radio receiver. 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 IMRR of the system.
IMRR is measured in dB
, giving the ratio of the wanted to the unwanted signal to yield the same output from the receiver. In a good design, ratios of >60 dB are achieveable. 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.
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
principle.
In such a radio receiver, a local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
is used to heterodyne
Heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
or "beat" against the incoming signal frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, generating sum and difference frequencies. One of these will be at the intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF), and will be selected and amplified. The radio system is responsive to any signal at the IF frequency, including unwanted signals. For example, if the wanted signal is 100.0 MHz, and the IF is 10.7 MHz, the local oscillator may be tuned to 110.7 MHz, creating the sum (210.7 MHz) and difference (10.7 MHz). However a signal at the input at a frequency of 121.4 MHz will create sum (232.1 MHz) and difference (10.7 MHz) frequencies, of which the latter signal will be selected and amplified by the IF stages of the radio receiver. 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 IMRR of the system.
IMRR is measured in dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
, giving the ratio of the wanted to the unwanted signal to yield the same output from the receiver. In a good design, ratios of >60 dB are achieveable. Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity
Selectivity
Selectivity may refer to:* Selectivity , in radio transmission* Binding selectivity, in pharmacology* Functional selectivity, in pharmacology* Socioemotional selectivity theory, in social psychology...
); 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.