# Definition of Selectivity - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

morangie18
Post subject: Definition of selectivity Posted: Wed Jun 06, 2007 10:34 am

Lieutenant

Joined: Thu Oct 12, 2006 8:56 pm
Posts: 2
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?

Thanks.

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IR
Post subject: Posted: Wed Jun 06, 2007 3:29 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello morangie18,

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 that.

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.

Posted  11/12/2012

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