From: Jim Thompson
Subject: Re: When does a filter become an oscillator?
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Date: Wed, 16 Oct 2002 21:42:42 GMT
NNTP-Posting-Date: Wed, 16 Oct 2002 17:42:42 EDT
Organization: Cox Communications
On Wed, 16 Oct 2002 20:56:13 GMT,
"John B" ,
In Newsgroup: sci.electronics.design,
Entitled: "When does a filter become an oscillator?",
Wrote the following:
|I have breadboarded a Sallen-Key filter, with center frequency 6.4 kHz and Q
|or about 5. I didn't expect it to oscillate, but it is, right at the center
|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?
Sallen-Key filters are notorious for their component sensitivities,
particularly at higher Q's.
Post your drawing on alt.binaries.schematics.electronic and we'll take
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
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