From: Jim Thompson
Subject: Re: Help analysing a CMOS ring oscillator
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Date: Sat, 18 Jan 2003 01:04:17 GMT
NNTP-Posting-Date: Fri, 17 Jan 2003 20:04:17 EST
Organization: Cox Communications
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.
I thought this was correct, but had to wait to do a web update to my
PSpice which didn't like a particular parameterized library that would
help demonstrate the effect.
I just did a seven-inverter ring oscillator. I sized the individual
inverters for mid-supply thresholds, then added a parameter "K" to
scale the sizes up and down uniformly.
Varying the parameter "K" as 1, 2, 4, 8, the frequency remained at
1.325GHz within a fraction of a percent. (107ps delay per inverter.)
Keep in mind that this 7-stage ring oscillator has no output loading.
Adding a buffer will slow the oscillation slightly.
| 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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