Image frequency

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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 $f s$ and $f o$ , respectively. The frequency of the mixer's output is $f_{if} = f_{s} \pm f_{o}$ 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 $f o$ and the filter's bandwidth $Δ$ , the receiver can detect signals of frequencies $f_{s} = f_{o} \pm \Delta$ . 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.