[SI-LIST] Re: Shielding clock traces on PCB's
- From: "Christopher McGrath" <christopher.mcgrath@xxxxxxxxxxx>
- To: <william.coombs@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 2 Jan 2007 13:15:38 -0800
What was the distance to the guard trace vs. the distance to the nearest
reference plane?
I have never found guard traces to be very useful when dealing with
dialectrics in the 4-5 mil range since you'll get even better noise
immunity due to the plane and, with thin dialectrics, you'll be closer
to the plane than the guard trace. Rather than use the guard trace,
just put in an extra keepout area that takes up the same amount of
space. For instance, if you have a 5 mil space to the guard trace, a 5
mil guard/ground trace, and a 5 mil space to the next trace on that
layer, just get rid of the guard trace and put a 15 mil keepaway. With
a 5 mil dialectric, that's a 3H spacing. You get the same effect but
save a couple of drills. (Simple is better, right?)
On critical clock traces, I usually shoot for 4H or 5H, but that isn't
achievable in many cases, so 3H is a common minimum that I use and I
don't bother with the guard trace. However, I do make sure that I have
a nice ground via stitching the ground planes together as close as
possible to the high-speed signal or clock via(s).
-Chris
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of Coombs, William B. (US SSA)
>Sent: Tuesday, January 02, 2007 12:03 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Shielding clock traces on PCB's
>
>=3D20
>Hello all,
>
>In regards to noise reduction, how effective do guard traces
>seem to be?
>
>Iv'e experimented with Hyperlynx by simulating my clock lines=3D20
>on the spectrum analyzer. These clock lines were simulated with=3D20
>and without grounded guard traces wrapped around the clock traces.=3D20
>The results showed little to no noise reduction with the addition
>of the guard traces.
>
>The clock lines & guard traces were routed on an internal layer in=3D20
>a dual-stripline Stackup.
>
>Thanks.
>
>William B. Coombs
>BAE Systems
>(973)636-7463
>Email:William.Coombs@xxxxxxxxxxxxxx
>
>
>
>
>
>
>
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