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

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Tue Aug 01 2000 - 18:15:03 PDT


Any hard limiting amplifier (read logic device) will produce intermodulation of
two linear signals present at it's input. The oscillator is more susecptible
because power supply voltage variations can directly change the operating
frequency (FM).

---Bob Lewandowski

Vinu Arumugham wrote:

> Depends on the which circuit the power pin is supplying. If the power pin
> supplies the internal PLL of a clock device, I would expect the output to be
> affected through a non-linear process. If the power pin supplies the clock
> output (TTL/LVTTL type) buffer, I would expect it to be linear addition.
>
> Vinu
>
> Ray Anderson wrote:
>
> > 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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