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Wi-Fi & BT Perhipheral Board Progress

A project log for Journey SBC

My educational journey into the development of a single board computer.

Kevin NeubauerKevin Neubauer 11/14/2018 at 05:252 Comments

Why not start with the second hardest concept? I've never done RF design before, so this took quite a bit of reading. I read (mostly read, some skimmed) several white papers on RF PCB design before tackling this board. "Antenna Design and RF Layout Guidelines" by Cypress Semiconductor was approachable and well written. 

Four layer board stack-up:

Components:

Via Stitching/Fencing:

Other Notes:

[Edit: The below calculation was wrong! I should have trusted my intuition and done more digging. :-) User @amorim317 helpfully pointed out that the below calculation does not factor in the substrate and its dielectric permittivity. Click here for new numbers.]

It seemed very difficult to find clear guidance on via fencing & stitching for RF layout. Most Internet sites refer to the frequency wavelength/20 guidance for determining spacing. However, they don't specify units and don't give examples! Frustrating to say the least, but it turns out it doesn't matter. It literally is as simple as finding your wavelength and dividing by 20... However, at the time I didn't think anything in RF design could be that simple, so I traced back to the source of this guidance. It originated from an IEEE Press white paper published in 1999 that was titled "EMC and the Printed Circuit Board. Design, Theory, and Layout Made Simple" by Mark Montrose. Section 2.9 of this paper gives the equation used along with a reference table of miscellaneous frequencies. Using this information, I was able to calculate the via stitching needed for 2.4 GHz. I used the middle of the band @ 2.45 GHz. The wavelength for 2.45 GHz is 122.45 mm. Take that divided by 20 and you get 6.1225 mm. My fence around my RF trace needed to have vias spaced 6 mm apart. After looking at 6 mm on my board and seeing how far apart it was compared with commercially produced 2.4 GHz PCBs, I decided to follow existing examples as closely as I could and use 0.75 mm stitching for both my RF fence and around the board edges and 2.5 mm stitching to join the ground pours with the ground plane. Extra vias on a small run board equals cheap insurance...

Discussions

Kevin Neubauer wrote 11/15/2018 at 04:45 point

Thanks for the info! I'll check it out tomorrow.

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Jarrett wrote 11/14/2018 at 07:51 point

If you haven't already (and if it's not too late), check out Michael Ossmann's 5 simple rules for RF design:

https://www.youtube.com/watch?v=TnRn3Kn_aXg

His approach is pretty good, because he's basically a lay-person that developed a few rules to allow him and others to blindly follow to reliably get RF designs working.

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