Editing Image response (section)

{{Redirect|IMRR|the railroad|Illinois and Midland Railroad}}
[[File:Superheterodyne image problem.svg|thumb|upright=1.7|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 ''<span style="color:green;">(green)</span>'' from the antenna at the image frequency <math>f_\text{IMAGE}</math> is also heterodyned to the IF frequency <math>f_\text{IF}</math> along with the desired radio signal S1  ''<span style="color:blue;">(blue)</span>'' at <math>f_\text{RF}</math>, so they both pass through the IF filter ''<span style="color:red;">(red)</span>''.  Thus S2 interferes with S1.  ]]

'''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 receiver|superheterodyne]] principle. <ref> ''C-W and A-M Radio Transmitters and Receivers'', United States Department of the Army, 1952  page 229 </ref>

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 [[frequency|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 MHz, there are two incoming signal frequencies that can generate a 10 MHz IF frequency.  A signal broadcast at 100 MHz (the wanted signal), and mixed with the 110 MHz LO will create the sum frequency of 210 MHz (ignored by the receiver), and the difference frequency at the desired 10 MHz.  However, a signal broadcast at 120 MHz (the unwanted signal), and mixed with the 110 MHz LO will create a sum frequency of 230 MHz (ignored by the receiver), and the difference frequency also at 10 MHz.  The signal at 120 MHz is called the ''image'' of the wanted signal at 100 MHz. The ability of the receiver to reject this image gives the image rejection ratio (IMRR) of the system.