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Definition of selectivity - RF Cafe Forums

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 you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the 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

Site Admin


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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