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From: "Gary Richardson"
Subject: Re: 100Hz from 25.6kHz crystal.
Date: Wed, 4 Dec 2002 18:37:55 -0800
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"Fred Bloggs" wrote in message
> Peter Nolan wrote:
> > Hello,
> > The circuit now works with the following values:
> > Rbias=4.7MOhms and R2=750kOhms.
> > C2=100pF and C3(the trimmer) around 30pF.
> > Beautiful.
> > 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
The max drive level of the C-2 type xtal is 1uW. Will an R2 of 10-12K
satisfy that requirement?
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