[SI-LIST] Re: Shielding clock traces on PCB's
- From: Kim Helliwell <kimgh@xxxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx, Doug Smith <doug@xxxxxxxxxx>, a.ingraham@xxxxxxxx
- Date: Mon, 8 Jan 2007 10:58:19 -0800 (PST)
I suppose the notion is to enclose the guarded traces in Faraday cage. However,
a guard trace is not a Faraday cage, because it's 2D, while the fields are 3D.
It's one of those tempting ideas that people try because it looks like it
should work, but both experiment and maturer reflection prove that it doesn't.
Lee Ritchey <leeritchey@xxxxxxxxxxxxx> wrote: The underlying assumption with
guard traces is that they somehow stop
electromagnetic fields from propagating to neighbors. That suggests that
wires can block electromagnetic fields. Last time I looked, this was not
so. If it were, how would transformers and motors work?
Lee
> [Original Message]
> From: Doug Smith
> To:
> Cc: si-list
> Date: 1/7/2007 5:34:14 PM
> Subject: [SI-LIST] Re: Shielding clock traces on PCB's
>
> Hi Andrew,
>
> In general I agree but there are assumptions made there that are not
> explicit and if these don't hold the conclusion does not either. I
> think the assumptions are:
>
> - similar signals between the traces (such as logic signals)
> - possibly multi-layer boards combined with the above
> - frequencies involved such that the guard traces are a significant
> portion of a wavelength
>
> I have seen cases where a guard trace would have been one solution of
> several possible.
>
> The world is full of signals that are not small ones. For instance,
> a board I have seen (years ago) had a small switching supply well into
> the board from the edge connector. The supply was powered from -48
> Volts. The -48 Volt trace to the supply ran parallel to a +5 Volt lead
> for a few inches from the same edge connector. This was a two layer
> board. Many modern products use them still such as DVD players - lots
> of fast logic, two layer board, class B).
>
> The inrush current into the switching supply and its input caps
> magnetically induced a 7 Volt spike into the 5 Volt trace in a phase
> as to add to 12 Volts being sent to some devices on the 5 Volt rail.
> About every tenth board insertion, one or another 5 Volt device would
> blow. A ground trace connected on both ends between the -48 and +5
> leads would have prevented this (not a unique solution).
>
> If the length of the guard trace (between its ground connections) is
> somewhat less than 1/4 wavelength at the highest frequency in the
> nearby traces, filtering should not be a problem.
>
> Just an example I thought might be interesting.
>
> Doug
>
> Andrew Ingraham wrote:
> >>Guard traces ar great ways to make accidental band pass filters. Why
> >>would anyone advise using them to control cross talk?
> >
> >
> > Because the word hasn't gotten around to most electrical engineers that
it
> > doesn't work like they think it should. So it continues to be done, and
> > many engineers wouldn't think there is anything wrong with it.
> >
> > On first glance it seems like a reasonable thing to do, akin to putting
a
> > shield around a wire, turning it into coax. Reality is another matter.
> >
> > Andy
> >
> >
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