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Image frequency
From Wikipedia, the free encyclopedia
Image frequency: In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces.
In a heterodyne receiver system, the mixer accepts two inputs, the signal to be detected and the local oscillator, at frequencies fs and fo, respectively. The frequency of the mixer's output is 
The output is then passed through a filter, which inherently has a bandwidth (or uncertainty if you will) denoted as Δ. Therefore, given an oscillator's frequency fo and the filter's bandwidth Δ, the receiver can detect signals of frequencies 
. Typically, one wants only one band, and the other unwanted band is called the image. The term image arises from the mirror-like symmetry of the detectable signal frequencies about the beating-oscillator frequency.