SMA connector footprint design project - Page 1
Forum section: electronics / rf-microwave

author: null-a
author_rank: Newbie
timestamp: October 08, 2023, 11:21:33 am
content: I've designed a PCB footprint for an SMA connector as part of trying to teach myself something about RF electronics. I'm sharing some info about this here in case it's of interest. I'd also very much appreciate feedback on this from the experienced folk around here. Thanks!My goals were:Have the footprint accommodate the cheap SMA connectors commonly found on ebay etc.Design for JLCPCB's default 4 layer stack upTransition the footprint into a microstrip lineAim to get acceptable performance up to 8 GHzMy approach was to model the connector (simplified) & transition in openEMS (https://www.openems.de/), noodle with that to get s11/s21 looking as good as possible, transfer the design to KiCAD, order a board, and then measure it on my LiteVNA (https://www.zeenko.tech/litevna). Images corresponding to each of these steps are attached.A few notes on the footprint itself:I've removed the ground beneath the signal pin pad. I tuned the width of this cut-out using the simulation.There's a linear taper between the microstrip trace and signal pin pad. The length of this was tuned using the simulation. The ground cut-out also tapers.There's extra copper at the pcb edge end of the signal pin pad, because I discovered it improved the response at higher frequencies in the simulation. I guess what's happening is that this introduces extra capacitance, which compensates for (/resonates with) the inductance that arises as a result of the ground plane not extending all the way to the board edge.The end result is an s11 < 20 db up to 8 GHz (well, kinda...), for a board with two connectors, which seems usable for my needs.

author: simplelogin.96huy@simplel
author_rank: Newbie
timestamp: December 23, 2023, 01:46:34 am
content: Very nice result, looks promising on 8GHz!! Could you please share the kicad pcb project? Thanks.

author: null-a
author_rank: Newbie
timestamp: December 23, 2023, 02:40:37 pm
content: QuoteSure -- see attached. Note that this is a kicad v5 project. (I've never managed to get more recent versions running well on my machine.)I'd be curious to hear how things work out if you use this, and please do let me know if you find ways to improve it!

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 23, 2023, 11:51:15 pm
content: Pretty neat using open source tools!Do you have measurement results yet to compare?

author: null-a
author_rank: Newbie
timestamp: December 24, 2023, 08:34:58 am
content: QuoteThanks!QuoteThe s21 / s11 plot I sharedisa measurement of the physical board, not the output of the simulation. Maybe this wasn't clear and caused confusion?

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 24, 2023, 06:13:42 pm
content: Quote from: null-a on December 24, 2023, 08:34:58 amOh, I see, that's pretty good.I'm also curious how well your sim matches the measurement results.

author: null-a
author_rank: Newbie
timestamp: December 24, 2023, 09:58:52 pm
content: QuoteIt's a good question, but sadly I don't have a good answer.Part of the problem is that the I'm only simulating one (simplified) connector (in one direction), yet I'm measuring the combination of two connectors, a length of transmission line, and even an SMA-SMA adaptor. I did make a rough attempt at allowing for this (as best as I could, given I'm a novice at this!), and my sense was that the simulation was way too optimistic -- maybe as much at 8 dB better (on s11) at 8 GHz say. So, in general, I suspect the correspondence is poor.On the up-side, I'm confident the simulation did at least actually help, since this transition measures much better than what I was doing before.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: December 25, 2023, 12:08:23 am
content: I did mostly the same, not with simulation but with a TDR. The first try was mystandard SMA decal found in my Altium lib, probably for the normal 1.6 mm FR-4with thick center conductors on the sma. JL on sci.elecronics.design (usenet)wrote he measured IIRC 80-90 Ohms for these SMAs in free air.ed. not for the barrel but for the part that will end up on the board.Not much reserve for the local capacitance of the board.I did a 4 layer board at JLCPCB with 4 tiny LMX2594 synthesizers and had freespace for test structures. I tried 2 of the fat SMAs, interconnected with a 12 milline. 11.5 mil on top and GND on the inner layers are 50 Ohm according to theJLCPCB impedance calculator. And yes, the calculator is exact.On the TDR it showed that the 12 mil line was not too bad, but the SMAs hadlocally much too much capacitance (notch in the impedance/reflected voltageline). Even the amount of tin played a role.In V2 of the board, i made the center pad of the SMA much smaller and didcut outs on the inner layers. The notches were gone, very slightly inductive now,sorry, I cannot find the screen shot. Consider it slightly overcompensated, butMUCH better.But the SMA is now a Rosenberger and not cheap Chinese. The inner conductoris MUCH thinner. If you find a cheap SMA launcher with a thin inner conductor,I'm all ears.I did some fancy corners on the 12 mil line to see if I get punished, but no, incomparison to the SMAs, that's harmless. Only the first picture is with the fat SMA.The picture SMA_cutout.png is taken from the V2 synthesizer, not from the test structure.Apply the lessons learned immediately :-)On that exerpt of the amplifier output: (SMA_cutout.png)Red is top, green is bottom, blue is inner layer 1, layer 2 is invisible and unimportant.I don't think that the taper will play much of a positive role, it will in contrary increasethe capacitive load.regards, Gerhard

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 25, 2023, 03:20:20 am
content: Quote from: null-a on December 24, 2023, 09:58:52 pmYou can overcome that by useing a linear simulator (e.g. Qucs / QucsStudio) and cascading your connector model with a transmission line.This is a very good result at -20dB, but the point is to ensure your prediction ability based on simulation for the next design.Anyway, good job, again.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: December 25, 2023, 03:25:40 am
content: Quote from: Gerhard_dk4xp on December 25, 2023, 12:08:23 amThat 50G TDR module tho.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: December 25, 2023, 06:28:10 am
content: The TDR is 18 GHz only. PicoSecondPulseLabs had a proposal to usethat 50G plugin with one of their step generators as a TDR.When TEK bought PSPL, the app note was gone in record time.Usually, I avoid the 50G plug in for fear of killing it. The centerconductor is isolated by air, only held in position by a mica disk.The sampler is also quite delicate.

author: null-a
author_rank: Newbie
timestamp: December 25, 2023, 08:18:29 am
content: QuoteGreat. The "rough attempt at allowing for this" I alluded to was me doing something similar with scikit-rf. I'm aware of qucs but haven't tried it out yet, but this looks useful. Thanks.QuoteThank you.

author: eb4fbz
author_rank: Regular Contributor
timestamp: December 25, 2023, 09:32:42 pm
content: Quote from: Gerhard_dk4xp on December 25, 2023, 06:28:10 aman-05c-10ps-tdr.pdf(489.11 kB - downloaded 134 times.)

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: December 26, 2023, 12:08:17 am
content: The old retired heroes of PSPL and TEK seem to want to prove that the valueis in the brains and not in the sold company.<https://kh6htv.com/pspl-app-notes/>What a load of knowledge!<http://qucsstudio.de/de/start/> should be available in English, too.I get German language, probably by autodetect.The studio version is by the original author, still supported.There is a tutorial by Gunthard Kraus<http://www.gunthard-kraus.de/qucsstudio/index_qucsstudio.html>Gunthard went silent key a month ago, so grab it while you can.He will be missed.Gerhard, DK4XP