[SI-LIST] plane cutouts under ac-coupling cap pads

  • From: Matthew Severini <MSeverini@xxxxxxxxxxxx>
  • To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 10 Jul 2013 23:10:38 +0000

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Hello experts,

I have a problem that has me totally stumped, so I thought you may have som=
e ideas. We have some signal lanes with dc-blocking caps, that run at about=
 10ghz. I noticed that the planes had not been cutout underneath the pads o=
f these caps, which has obvious issues. I simulated the cap structure in HF=
SS to find an optimum cutout size, and rolled it into the next spin of the =
board. All of my simulations claim that the new structure should have a hug=
e positive influence on the si of these lanes. Unfortunately, when we measu=
re the eye, it seems to be about the same, or ever so slightly worse! I did=
 a tdr on the cards with and without the cutout, and the cutouts definitely=
 seem to flatten the impedance curve along the trace. I haven't been able t=
o do a vna measurement yet because I cannot access one side of the trace. I=
 have ordered boards with only those caps on them so that I can make this m=
easurement, but all the initial data seems to say that the transmission mea=
surements will also be better with the cutouts. I was wondering if anyone h=
ad some possible explanations as to why the eye would be a little worse, in=
 almost every measurement (according to sig test), when the impedance is fl=
atter.

Thanks for your help,

Matt Severini

Signal Integrity Engineer
mseverini@xxxxxxxxxxxx<mailto:mseverini@xxxxxxxxxxxx>
(801) 987-7127


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