From: "Jeff Verive"
References: <3E225B37.A41@sneakemail.com> <3E230680.firstname.lastname@example.org> <3E23128D.email@example.com>
Subject: Re: SMD Cap Test Fixture
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Date: Mon, 13 Jan 2003 15:06:40 -0600
NNTP-Posting-Date: Mon, 13 Jan 2003 15:06:40 CST
"Mike Monett" wrote in message
> Mike Monett wrote:
> > firstname.lastname@example.org wrote:
> > [...]
> > > I've got the same circuit somewhere, though I used a 4011, and two
> > > feedback resistors (switchable) in the megaohm and kiloohm range.
> > > Worked well down to pf.
> > > And of course if you've got a frequency meter which can measure
> > > periods, it can be set to give a direct readout.
> > > http://inquisitor.i.am/ | mailto:email@example.com | Ian Stirling.
> Ian, just out of curiosity, I looked up the CD4011 - it is a dual input
> How did you get it to oscillate? With negative feedback, I would have
> assumed the circuit would converge to a stable operating point. With
> positive feedback, it would simply latch up.
> I'm mystified!
The oscillations can come from floating inputs (a CMOS no-no), or from the
phase lag that the negative feedback resistor adds when taken with the input
capacitance. The 4011 has three buffer stages, each with its own
propagation delay. If the RC phase delay mentioned above plus the total
propagation delay adds up to 360 degrees of feedback (it will at several
frequencies) *and* if the circuit has at least unity gain left in it at one
of these frequencies - Voila! - you have an oscillator. Now go in a dark
closet and curse!