Mixed Signal PCB layout - RF Cafe Forums
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Post subject: Mixed Signal PCB layout
Unread postPosted: Thu Jun 02, 2005 8:27 am
We make a pCB layout of a mixed signal board (High-speed digital and RF up to 400MHz). The board has 14 layers. The board is divided into 2 sections by a slot across the board. At he RF side we use only in 6 layers for the signals and grounds. My question is can we use the other layers as GND planes and stitch them with vias to the already used GND layers ? Is it recommended?
Unread postPosted: Fri Jun 03, 2005 7:08 am
It is recommended. At 400MHz there is not alot of concern about stray RF, but the additional ground palnes will serve as a heatsink for the rest of the board. It is there, so you should use it.
Post subject: ground planes
Unread postPosted: Wed Jun 08, 2005 11:23 am
Do remember that the vias which do the stitching together of the ground planes need to be spaced adequately close: well less than 1/10 of a wavelength at c in the medium (and that's not c in free space, but c/epsilon for the board material you're using. So if you're using a board material with a dielectric constant epsilon of 4, at 400 MHz one wavelength of a buried trace is 0.18 meters, so stitching vias should be absolutely no farther than 18 mm apart. I'd actually recommend much closer - say 2 mm - since you may have harmonics, or need a low-impedance connection.)
Unread postPosted: Wed Jun 08, 2005 1:50 pm
Many thanks for your help!
I will do as you advised.
Thanks again!!! :-D
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