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Mixed Signal PCB layout - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


Guest
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?


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Guest
Post subject:
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.


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Another Guest
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.)

Good luck!


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Guest
Post subject:
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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