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Design >> RF Design >> image rejection
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Message started by aaron_do on Feb 24^th, 2009, 11:50pm

Title: image rejection
Post by aaron_do on Feb 24^th, 2009, 11:50pm

Hi all,

just curious, should we define image rejection ratio only when both the desired signal and the image signal are present? The reason i'm asking is as follows:

Assume your IF section consists of a complex filter and a limiting amplifier. The filter has a gain of 0 dB at the desired frequency and -30 dB at the image frequency. If the limiting amplifier gain is 50 dB, then it can easily saturate the image signal unless desired signal is present as well. BTW, if both are present, will the image signal still be -30 dB relative to the desired signal?

thanks,
Aaron

EDIT:
I thought about it a bit longer and I guess i have to re-phrase my question. If you have a filter followed by a limiting amplifier, then the overall gain will flatten out (won't follow the shape of the filter) somewhat depending on the strength of the out-of-band signals. How does this affect the output with and without an input signal?