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The performance of a filter is normally specified in terms of a Rejection Ratio.
This indicates the filter's relative gain (‘loss’) at the center of the passband compared to just outside.
It is conventional to specify this in either of two ways:
the Adjacent Channel Rejection Ratio and the Alternate Channel Rejection Ratio.
The adjacent channels are those immediately on either side of the wanted
Selectivity is usually measured as a ratio in decibels (dBs),
comparing the signal strength received against that of a similar signal on another frequency.
If the signal is at the adjacent channel of the selected signal,
this measurement is also known as adjacent-channel rejection ratio (ACRR).
Adjacent Channel Power Ratio (ACPR):
The adjacent channel power ratio is computed as the ratio of the power in
the adjacent channel to the power in the base channel:
ACPR = Pavg,adj/Pavg,ch
where Pavg,adj and Pavg,ch are the average channel power in the adjacent
and base channels, respectively.