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From: Fred Bloggs
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Subject: Re: 100Hz from 25.6kHz crystal.
References: <5Q1H9.32847$zX3.email@example.com> <3DEE2C3D.A5902908@rica.net>
Date: Wed, 04 Dec 2002 15:47:57 GMT
NNTP-Posting-Date: Wed, 04 Dec 2002 07:47:57 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Peter Nolan wrote:
> The circuit now works with the following values:
> Rbias=4.7MOhms and R2=750kOhms.
> C2=100pF and C3(the trimmer) around 30pF.
> Many thanks.
> Peter Nolan.
> To email please remove x from firstname.lastname@example.org
The 750K will result in quite a bit of added phase noise beyond a more
sensible value. The original configuration was not working because you
were exciting a parasitic mode or maybe power pin feedback. Go for 60
degree phase shift across R2-C2 at the operating frequency. You want to
use a temperature stable C2 so maybe 1000p in C0G is convenient. Then
you have TAN(60)=2 x pi x 25.6K x R2 x 1000p which solves for R2=10-12K
for an RMS Johnson reduction of nearly 40dB per root-Hz due to that
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