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From: Winfield Hill
Subject: Re: circuit wanted-SQUARE PULSE GENERATOR
Date: 26 Nov 2002 10:56:26 -0800
Organization: Rowland Institute
References: <1038144646.265208@athprx02> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
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Jim Thompson wrote...
> I've been using the 1X/2X approach for at least 35 years in on-chip
> VCM's (voltage-contolled multivibrators).
> Here's a question for all you lurkers:
> Delay in the comparators (in a 555 or otherwise) affect the linearity
> of the control curve, particularly at high frequencies (I've built
> these at frequencies as high as 50MHz).
> Can anyone here mathematically describe the effect as a function of
> delay and operating frequency? And then offer a solution? I can.
Well let's see, there's a delay in the integrator dV/dt output
(if one is implemented as an opamp integrator) and there's a
delay in the comparator and current-switching circuitry, let's
call all these delays td. If I is the charge/discharge current
and V is the low-frequency charging-voltage excursion, then we
get f = I / 2C V (1 + 2td f), which equivalent to the comparator
threshold voltage growing by the fraction 2td/T where T = 1/f is
the output period. So it would seem a correction that slightly
reduces the threshold voltage as a function of I would do the job.
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