Doug - Nice rational analysis. Best Regards Charles Grasso Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Pager/Short Message: 3032042974@xxxxxxxxx Email: charles.grasso@xxxxxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Lee Ritchey Sent: Monday, January 08, 2007 11:19 AM To: Doug Smith; a.ingraham@xxxxxxxx Cc: si-list 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?=20 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=20 > explicit and if these don't hold the conclusion does not either. I=20 > 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=20 > portion of a wavelength > > I have seen cases where a guard trace would have been one solution of=20 > 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=20 > 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=20 > board. Many modern products use them still such as DVD players - lots=20 > of fast logic, two layer board, class B). > > The inrush current into the switching supply and its input caps=20 > magnetically induced a 7 Volt spike into the 5 Volt trace in a phase=20 > as to add to 12 Volts being sent to some devices on the 5 Volt rail.=20 > About every tenth board insertion, one or another 5 Volt device would=20 > blow. A ground trace connected on both ends between the -48 and +5=20 > leads would have prevented this (not a unique solution). > > If the length of the guard trace (between its ground connections) is=20 > somewhat less than 1/4 wavelength at the highest frequency in the=20 > 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? > >=20 > >=20 > > 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. > >=20 > > 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. > >=20 > > Andy > >=20 > >=20 > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >=20 > > or to administer your membership from a web page, go to: > > //www.freelists.org/webpage/si-list > >=20 > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >=20 > > List FAQ wiki page is located at: > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > >=20 > > List technical documents are available at: > > http://www.si-list.org > >=20 > > List archives are viewable at: =20 > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > =20 > >=20 > >=20 > > --=20 > ------------------------------------------------------- > ___ _ Doug Smith > \ / ) P.O. Box 1457 > =3D=3D=3D=3D=3D=3D=3D=3D=3D Los Gatos, CA 95031-1457 > _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 > / /\ \ ] / /\ \ Mobile: 408-858-4528 > | q-----( ) | o | Email: doug@xxxxxxxxxx > \ _ / ] \ _ / Website: http://www.dsmith.org > ------------------------------------------------------- > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: =20 > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu