[SI-LIST] Re: Resonance due to stitching via ground stubs
- From: "Antonis Orphanou" <orphanou@xxxxxxxxxxxx>
- To: "Jason.Ellison@xxxxxxx" <Jason.Ellison@xxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 5 Apr 2013 19:23:34 +0000
The ground is also carrying current and along with the signal structure they
form an impedance controlled waveguide. A resonance does not distinguish which
structure the current is on whether that is a signal or a ground...the ground
structure is not a whole hard reference zero potential for your pcb or trace
waveguide and I think this is where your question comes from....
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jason.Ellison@xxxxxxx
Sent: Friday, April 05, 2013 6:28 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Resonance due to stitching via ground stubs
Hello,
We recently found a somewhat interesting outcome of using stitching vias
without having a ground plane on the top and bottom of the PCB . In our
case, we voided ground in a footprint for impedance reasons and did not
put ground on the top layer.
We backdrilled the signal vias, but left all the grounds alone. An
unexpected resonance in IL that appeared to be from a stub was observed.
We found the quarter wave length related to the resonance frequency in
our board material. We found that the distance was 4 mils from the
stitching via stub length. We modeled this geometry in a 3D full wave
solver and sure enough we recreated the problem.
The question I have for the group is, why does this happen with a ground
stub? We all know that quarter wave stubs on signals lines cause this
kind of response, but it is not intuitive that a ground would do the
same thing.
Thanks!
Jason Ellison
FCI - Signal Integrity Engineer
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