From: Jim Thompson
Subject: Re: Help analysing a CMOS ring oscillator
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Date: Tue, 21 Jan 2003 19:43:06 GMT
NNTP-Posting-Date: Tue, 21 Jan 2003 14:43:06 EST
Organization: Cox Communications
On Tue, 21 Jan 2003 13:19:07 -0600,
"Jeff Verive" ,
In Newsgroup: sci.electronics.design,
Entitled: "Re: Help analysing a CMOS ring oscillator",
Wrote the following:
|"Jim Thompson" wrote in message
|> I kept waiting for someone to jump in here and point out that the
|> oscillation frequency of CMOS ring oscillators is only dependent on
|> the device gm/C ratio and not the size of the devices.
|Of course, which is to say that gm*Xc is the voltage gain for each stage for
|low loading and low source (generator) impedance. The value of C was what
|the OP was trying to get a reasonable guess for, and since it appeared he
|was doing an IC design analysis, the physical size of the devices has an
|impact on the capacitances (gate and substrate).
What I should have elaborated on is that the gm/C term is a constant
for a given process; which is why the oscillation frequency is
constant independent of device sizing.... increasing gm causes a
proportionate increase in C.
I regularly use ring oscillator configurations to qualify processes
for maximum attainable speed... I'm fond of lots of margin so I like
operation at no higher than 1/4 of the ring oscillator frequency,
simulated with the SLOW/HOT process corner ;-)
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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