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Superheterodyne Spurious Responses

July 12th Fact-of-the-Day

The intermediate-frequency filter selectivity of a superheterodyne receiver is not a complete indication of its actual selectivity under all conditions, because a superheterodyne receiver is susceptible to the reception of interfering signals outside its normal reception bandwidth. That susceptibility results from frequency conversion. Frequency conversion is accomplished by mixing a signal from a local oscillator in the receiver with incoming signals. Unfortunately, signals on two different incoming frequencies are converted to the same intermediate frequency by that process and the undesired signal or the two must be filtered. The undesired signal is never filtered to zero amplitude in practical receivers, so all practical superheterodyne receivers receive at least two frequencies simultaneously. However, a well-designed receiver is much more sensitive to signals on one frequency than the other. The unwanted frequency is called the 'image frequency.' Multi-conversion superheterodyne receivers are susceptible to interference on multiple image frequencies. Those undesired sensitivities are known as spurious responses. ©2005 Martek International All rights reserved.


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