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

["JaMi Smith" <jamismith@xxxxxxxxxxxxx>]

   . . . but then for the exception to the rule, there is the
'non-inductively wound' 'wire wound resistor' (say that five times fast),
which is intentionally wound in a manner that cancels any magnetic field (or
most of it) that would allow it to act as an inductor, as can a standard
'wire wound resistor' can, and usually does.

For those who have never seen such behavior, compare a RWR81wire wound power
resistor, to a RWR81N non-inductively wound wire wound power resistor.

In the hands of the right person, anything can become a tool, and in the
right circumstances, even a 'guard trace' can be made to work as a 'guard
trace' . . .

But then again, in the wrong hands . . .

You know what they say about a mind being a terrible thing to waste . . .

And regarding the transformer - motor analogy:

Take two 1000 foot spools of wire (standard Belden metal spool preferred).
If you can, find the inner end of the wire on one spool, and then along with
the outer end, hook it up across a low voltage adjustable DC power supply
(still on the spool), and measure the resultant magnetic field. Now take the
other spool of wire, and un-roll all of the wire into a massive pile of wire
on the floor in the middle of the room, and carefully bunch it all together
like a giant 'brillo-pad', and then carefully place the empty spool centered
on top of the 'brillo-pad', and now once again find the two ends, and hook
it up across a low voltage adjustable DC power supply, and once again
measure the resultant magnetic field . . .

 . . . duh !

JaMi


----- Original Message ----- 
From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
To: "Doug Smith" <doug@xxxxxxxxxx>; <a.ingraham@xxxxxxxx>
Cc: "si-list" <si-list@xxxxxxxxxxxxx>
Sent: Monday, January 08, 2007 10:19 AM
Subject: [SI-LIST] Re: Shielding clock traces on PCB's


> 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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