Post subject: Mixed Signal PCB layout
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
Post subject: ground
Unread postPosted: Wed Jun 08, 2005 11:23 am
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.)
postPosted: Wed Jun 08, 2005 1:50 pm
Many thanks for your help!
I will do as you advised.
Thanks again!!! :-D