Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
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page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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