[SI-LIST] Re: Routing Signals Between PWB Layers - Part 2
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: GStokes@xxxxxxxxx
- Date: Thu, 04 May 2006 09:10:00 -0400
Geoff, Doug,
This example nicely shows the importance of proper power distribution
design.
The emission peaks correspond to the modal resonances of the PCB planes.
As Doug points out in the article, moving the planes closer would reduce the
emission about proportionally. However, if one wants to stay with a
four-layer
construction, reducing the plane separation is not a viable solution: it
would
either result in an unmanageably thin board, or if we add the tickhness
back to
the top/bottom dielectrics, in a dramatically reduced routing density.
Nearby grounding vias would help, but in practical cases the two planes
are at
different DC potentials, so it must be a bypass capacitor rather than a
stitching
via. A single bypass capacitor will do little in the hundreds of MHz
frequency range,
even if it serves only one signal. Also, if we have wide busses instead
of a single
trace, trying to provide a return path locally to all of the layer
transitions becomes
much harder to implement.
A more realistic and straightforward solution is to design the
power-distribution
network in such a way that it does not exhibit large resonances. In
multi-layer
boards, where we may also have ground-ground planes creating such
cavities, the
solution is stitching vias, but not necessarily near the layer
transitions. We would
just need to make sure that the large resonating cavity is broken up to
sufficiently
small blocks such that the lowest resonance frequency is safely above
the highest
frequency of excitation.
Regards,
Istvan Novak
SUN Microsystems
Geoff Stokes wrote:
>Hi Doug
>
>In this experiment, you appear to have no plane vias adjacent to the
>signal vias. What would happen if a few plane vias were added around
>the transitions and fairly close by?
>
>Geoff Stokes
>Systems Engineer
>Zetex Semiconductors plc
>Zetex Technology Park
>Chadderton
>Oldham
>OL9 9LL
>UK
>=20
>+44-161-622-4857
>www.zetex.com
>www.zetex.cn
>=20
>
>=20
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Doug Smith
>Sent: 03 May 2006 21:41
>To: SI-List
>Subject: [SI-LIST] Routing Signals Between PWB Layers - Part 2
>
>Hi All,
>
>I have been writing and recording again and have posted part 2 of my=20
>article on signals that change layers in a PWB.
>
>Abstract: Printed wiring board, PWB, signal paths must often change=20
>layers in a board stackup. Under some conditions this can cause=20
>problems. An emissions example is used to illustrate the conditions=20
>where changing layers can cause problems.
>
>The link to the article is the picture of the emissions plot at the=20
>bottom of the home page at http://emcesd.com .
>
>There is also an audio discussion of this article on my podcast site:=20
>http://emcesd-podcast.com where the direct link to the article is:
>
>http://emcesd-podcast.com/2006/may/2006-0503.mp3
>
>Can't download mp3 files? Download the following instead:
>
>http://emcesd-podcast.com/2006/may/2006-0503.dcs
>
>After download, change the extension from .dcs to .mp3 and the file=20
>will then be able to play on most computers. Since the last HFNews=20
>there have been two new podcasts posted.
>
>Doug
>--=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
>-------------------------------------------------------
>
>
>
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