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From: Fred Bloggs
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Subject: Re: 100Hz from 25.6kHz crystal.
References: <5Q1H9.32847$zX3.firstname.lastname@example.org> <3DEE2C3D.A5902908@rica.net> <3DEE23DC.email@example.com>
Date: Thu, 05 Dec 2002 16:59:13 GMT
NNTP-Posting-Date: Thu, 05 Dec 2002 08:59:13 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Gary Richardson wrote:
> "Fred Bloggs" wrote in message
>>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.
>>>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?
Ahhh- good point, Gary. This will depend upon his Vdd. I will run it
through the sim and see what the limits are....
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