[sdiy] Steep bandpass filters / SVF leakage?

Neil Johnson neil.johnson97 at ntlworld.com
Wed Aug 22 11:13:15 CEST 2012


Karl Ekdahl wrote:
> Hi list, so i'm in need of a steep bandpass filter with high q capability. I was toying around with a regular SVF (despite it being only 2-pole) and i'm not sure if i did something incorrect in my circuit but the bandpass output seems to leak in a lot of frequencies that it shouldn't - is this normal? (i seem to recall this not being that unusual). That's a no-go for me if it's expected.

A 2-pole SVF bandpass will have -6dB/oct slopes on either side.  Can't
say much more since you haven't defined what you mean by leakage.  How
much is your "lot"...?

> Anyway what i'd prefer is a 4-pole, or even 8-pole filter that does *not* have to be CV-controlled - any common topologies around? My google skills are failing.. mostly i can only find SVFs..

An SVF bandpass is really an inverse-notch.  A proper bandpass will be
specified by two frequencies, the passband attenuation, the stopband
attenuation, and how much ripple in both.

If you don't need CV control then Sallen-Key is the usual candidate,
although at 8 poles you need to be careful will component tolerances.

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