Post subject: Definition of selectivity Posted: Wed
Jun 06, 2007 10:34 am
Joined: Thu Oct
12, 2006 8:56 pm
Hi everyone, just wondering if
anyone knows if there's an industry standard definition for filter
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
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
This property will have of course a direct impact
on the filter's order. As steeper is the desired rejection as higher
is the filter's order.