The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
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 00:07:18 -0500
NNTP-Posting-Date: Thu, 05 Dec 2002 00:07:17 EST
Organization: Bell Sympatico
Gary Richardson wrote:
> "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
> > resistor.
> The max drive level of the C-2 type xtal is 1uW. Will an R2 of 10-12K
> satisfy that requirement?
Yes, this is bad advise. It probably will destroy the crystal.
Trying to get a 45 or 60 degree phase shift at the input to the pi network is a
myth. A simple SPICE analysis shows there is little or no phase shift across the
series resistor. The pi network is at resonance and is almost purely resistive.
Even when a phase shift is introduced at the output of the inverter, the crystal
maintains a 180 degree phase shift across the pi network. See
The Johnson noise of the series resistor is attenuated by the voltage divider
formed by the series resistance and the input impedance of the pi network. It is
small compared to the phase noise contributed by the inverter and the crystal
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup