Re: [SI-LIST] : Ferrites on power leads

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From: Ray Anderson (raymonda@ha1mpk-mail.eng.sun.com)
Date: Tue Aug 01 2000 - 12:10:10 PDT


Isn't the primary effect of noise ingress on the clock signal a bit
different when the path of the ingress is via a power pin as opposed
to via an output trace.

If the noise gets into an oscillator or synthesizer via the power
supply there is potential for the noise to modulate the circuitry
both in amplitude and frequency/phase with the result on the output signal
being some sort of degradation by some non-linear process.

In the case of clock signal contamination when the impinging noise is
picked up by a clock distribution transmission line, I'd be more inclined
to expect a linear addition of the noise to the clock signal. The reverse
isolation of the output driver serves to isolate the internal oscillator or
synthesizer circuitry somewhat from effects of noise on the output line.

Of course this is a lot of hand-waving theorizing because each case is
a bit different, but in general I'd expect noise arriving via the
power supply path to be more damaging as it's potential for inducing
jitter and timing problems would be greater than the noise added
more or less linearly by output trace contamination.

These comments are anecdotal based on many years of designing
communications oscillators and synthesizers for satcom applications,
but I think the same end effect follows. Comments ?? Anyone know of any
studies on the effects of clock degradation caused by different paths
of noise ingress ?

-Ray

>
> Ellis,
>
> Noise on the power pin of a device usually perturbs the output signal.
> Such a perturbation may be significant, especially if the output is a
> clock signal. In this context, if radiated noise impinging on traces is
> a concern, my point was that the clock quality is affected more by the
> noise picked up by the clock trace , than by the noise picked up by the
> power trace of the clock driver.
>
> Thanks,
> Vinu
>

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