Joined Nov 17, 2003
What is the selectivity of a filter?
The selectivity of a filter describes the filters ability to pass frequencies of a specified range and reject all other frequencies that may/may be adjacent in the frequency spectrum. It is essentially a performance measurement for a particular filtered channel.

What is the mathematical expression for selectivity?
It is expressed as the ratio of the signal strength of the frequencies passed against the signal strength of a similar frequency on an adjacent frequency which is filtered. It is typically written as decibels (dB).



Joined Jan 23, 2006
LC circuits are often used as filters; the L/C ratio determines their selectivity. For a series resonant circuit, the higher the inductance and the lower the capacitance, the narrower the filter bandwidth. For a parallel resonant circuit the opposite applies.
[reference: wikipedia]

Thread Starter


Joined May 29, 2007
Thanks for ur answers!!!

I read somewhere( a paper downloaded from net) that in Lowpass filter prototype it is the ratio of stopband to passband frequencies, but I am not very sure whether it is correct or not?

Do anyone of you have any info for this??