Copyright: 1996 - 2024
BSEE - KB3UON
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Definition of selectivity - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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Forums activity dropped off precipitously. If the folks at
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but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
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Post subject: Definition of selectivity Posted: Wed Jun
06, 2007 10:34 am
Joined: Thu Oct 12, 2006
Hi everyone, just wondering if anyone knows
if there's an industry standard definition for filter selectivty?
For instance if I wanted to specify a filter with a certain
selectivty what other paramters would need to be known in order to provide
a complete definition of the filters response?
I imagine it to
be something like the passband bandwidth divided by the frequency range
from the band edge to the desire stopband rejection. So to understand
the full response I'd also have to spec the actual desired stopband
rejection and the loss reference I'm using for the passband edge (eg.
-3dB, -1dB, etc...)...make any sense? Is this the right definition?
Post subject: Posted: Wed
Jun 06, 2007 3:29 pm
Joined: Mon Jun 27,
2005 2:02 pm
Filter's selectivity defines how much the filter will reject
unwanted frequencies. When you design a filter you should know the desired
attenuation of the unwanted frequencies. For example: -40dBc@100MHz
from the cutoff frequency. Here at an offset of 100MHz from the cutoff
frequency, the filter'S rejection would be 40dB lower than the insertion
loss of the filter's passband.
It is possible to synthesize
a filter by using a simulation tool and define a list of attentuations
at given offsets. The SW will calculate the filter's order based on
This property will have of course a direct impact on the
filter's order. As steeper is the desired rejection as higher is the