FreeLists / si-list / [SI-LIST] Re: Shielding clock traces on PCB's

["Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>]

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 <doug@xxxxxxxxxx>
> To: <a.ingraham@xxxxxxxx>
> Cc: si-list <si-list@xxxxxxxxxxxxx>
> 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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