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From: Winfield Hill
Subject: Re: circuit wanted-SQUARE PULSE GENERATOR
Date: 26 Nov 2002 13:48:05 -0800
Organization: Rowland Institute
References: <1038144646.265208@athprx02> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
X-Newsreader: Direct Read News 4.11
>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.
Ah, a resistor in series with the capacitor! Does the same job as
lowering the threshold voltage as a function of current, nice!!!
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