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From: firstname.lastname@example.org (carltons)
Subject: Re: When does a filter become an oscillator?
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Thu, 17 Oct 2002 08:53:31 GMT
Date: Thu, 17 Oct 2002 08:53:32 GMT
In article ,
email@example.com (Bob Wilson) wrote:
> In article ,
> firstname.lastname@example.org says...
> >I have breadboarded a Sallen-Key filter, with center frequency 6.4 kHz and
> >or about 5. I didn't expect it to oscillate, but it is, right at the
> >I have derived the transfer function for the filter. Let's say I examine
> >its mathematical behavior using Mathcad.
> >Are not the general criterea for oscillation:
> >1) a gain greater than 1; and
> >2) the absolute value of the phase shift greater than 90 degrees and less
> >than 270 degrees?
> The rule of thumb is this:
> Build a filter (any type will do). Then walk back and forth loudly
> proclaiming that this is a FILTER. Then test it. The damn thing will sure as
> shooting oscillate.
If your dealing with op amps at a gain of 1 driving R's and C's,
oscillation is very possible depending on the op amp. You are considering
theoretical gain based on closed loop gain, your op amp still has a gain
in the 1000's at your Fo. The opamp also has an output resistance and
this may be the problem depending on its loading. You can plug and chug
different opamps or you can simulate to find a solution. Plug and chug is
probably faster and most opamps have the same pin outs. Look for the
opamps with guaranteed stability for a Av of 1 and driving a high C load.
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