PCB design at 10Ghz - Page 1
Forum section: electronics / projects

author: rfengg
author_rank: Regular Contributor
timestamp: November 08, 2021, 04:25:34 pm
content: Hi All,Taking my first stab at a PCB design at 10Ghz......my previous experience has been limited to 500Mhz.Need all your advice on the following as I have no access to any EM simulation software.a) As the board involves transmission lines between 50ohm paths, is the stripline, microstrip or the GCPW the best option on a RO-4350 substrate?b) Vias are inevitable on the RF tracks that I need to layout, which present a discontinuity of a new nH and this would kill the return loss at 10Ghz......is it better to use coaxial structures to form the vias rather than just a via alone from one layer to the other?c) Specifically to the GCPWG, where are the best places to place the vias on the ground tracks adjacent to the RF lines from the top ground layer to the next ground layer? Is it better to have the vias closest to the RF tracks or spaced some distance apart from the RF lines? Any rules of thumb in order to prevent higher modes due to the vias?

author: erikka
author_rank: Regular Contributor
timestamp: November 08, 2021, 06:30:07 pm
content: Ansys has a free version of their EM simulation software.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 08, 2021, 08:23:05 pm
content: Quote from: rfengg on November 08, 2021, 04:25:34 pmThat mostly depends on your stackup and your connectors. Some edge mount SMA connectors have giant center pins and widely spaced ground blades which are appropriate for a microstrip on a 62 mil substrate. Others have a tiny center pin and closely spaced ground blades that work well for GCPW. GCWP is the easiest to match to small surface mount components such as attenuators, small amplifiers, mixers, and AC coupling capacitors. Microstrip is the lowest loss and works with bigger components and if you have appropriately sized components will usually get better return loss. Stripline has the lowest crosstalk and dispersion and the uniform dielectric is important for structures built out of PCB traces such as couplers. Stripline has higher loss and requires vias to transition from launch and components.Both stripline and GCPW need stitching vias to connect upper and lower ground plane to prevent launching waveguide modes.QuoteThat is really in the realm where you need simulation to optimize. It's hard to make general statements about what works best without knowing the particular kind of signal transition you are making and what PCB structure you use.QuoteIf you are just talking about ground-to-ground stitching vias between two ground layers their placement is not critical. They are supposed to carry negligible current. They should be far enough from the signal trace to not materially effect the impedance and close enough spaced to avoid resonances. I've seen lambda/10 and lambda/20 guidelines. I've never simulated this, I have just done a simple calculation and never been able to measure a problem. Via design is much more important when you are making a transition between two transmission line structures. That is where simulation is key.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 08, 2021, 09:33:21 pm
content: Quote from: rfengg on November 08, 2021, 04:25:34 pmI’m curious, any special problems you found at 500MHz?