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From: "Jeff Verive"
References: <email@example.com> <_MgV9.firstname.lastname@example.org>
Subject: Re: Help analysing a CMOS ring oscillator
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 17 Jan 2003 13:04:45 -0600
NNTP-Posting-Date: Fri, 17 Jan 2003 13:04:46 CST
"Kevin Aylward" wrote in message
> Jeff Verive wrote:
> >> The basic problem isn't. Its actually quite complicated. It took me
> >> several pages of calculations to come up with a correct formula for a
> >> simple 3 stage feedback ring where there was only a millor capacitor
> >> and a gm included in the model. I'll see if I have time to post the
> >> details on my site.
> > Stop trying to model this as a linear AC amplifier. In a CMOS ring
> > oscillator of several stages, each stage spends very little of its
> > time in the active region.
> False. I'll send you a working SuperSpice schematic of a 2.5Ghz vco if
> you would like. The waveforms are resonable sinewaves.
I'll bet his CMOS ring oscillator is not running over 100 MHz. Mostly
> >Instead, each stage should be modeled as
> > two independent switches driving a small (albeit non-linear)
> > capacitance.
> Nope. This shows you have little knowlede of 3 ring oscilators. Indeed,
> you usually have to take its output to another gain stage to get a full
> rail swing.
Obviously, you've never built one of these. As relaxation oscillators, they
spend a lot of time at the rails. Very little time as linear amps. He
stated nothing of feedback to buy the extra gain, which is usually not
necessary in a 3-ring relaxation oscillator.
> I agree that the best thing to do is to model this in
> > SPICE and be done with it, but the OP didn't seem interested in doing
> > so. Nonetheless, analyzing this as a succession of linear AC
> > amplifiers is missing the point.
> Not at all. That's how these oscillators work. They don't work as large
> signal switches.
Sorry, but you've missed the boat. Stick to GHz RF is that's where your
"experience" is, and let the knowledgeable engineers help others in the
> Kevin Aylward
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.
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