RF connector dual option - please critique design, selective population of board - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: August 25, 2014, 01:25:55 pm
content: Hi guys,For a product I need to be able to assemble two versions of a board - one with an SMA connector, the other one with an MMCX connector (which over the past few days I have come to despise on a personal level). The target frequency is around 868 MHz.Attached is my proposed implementation of this - the trace is as close to 50 Ohms as I can get on an FR4 according to the calculations. The feature I'm worried about is the selection of which connector to use - namely the JP2; C24; C26 and C25 parts. C24 will not be assembled unless required for some reason, it is just a placeholder as perhttp://www.artbrno.cz/download/telit/Telit_xE70_868_RF_Module_User_Guide_r0.pdfpage 35.When the SMA connector (J9) is used JP2 will be populated with a 0R resistor to the left, C25 will not be populated, C26 will be populated with a 0R resistor and J10 will not be populated.When the MMCX connector (J10) is used JP2 will be populated with a 0R resistor to the right, C26 will not be populated, C25 will be populated with a 0R resistor and J9 will not be populated.My question is: Will the proposed structure in some way impede good function of the RF pathway? If so, how can I minimize this or at least guess the extent of the problem?The scale of the attachment is one grid unit (white dots) = 25 mil.Thank you,David

author: daqq
author_rank: Super Contributor
timestamp: August 25, 2014, 01:29:42 pm
content: Note: The dark purple-ish circles are truhole drills.

author: KJDS
author_rank: Super Contributor
timestamp: August 25, 2014, 01:51:42 pm
content: I think you've made it more complex that it needs to be.No need to short the unused connectors to ground. I certainly would do away with C24 and the long trace that goes to it, it will have an effect on the matching.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2014, 05:43:18 pm
content: With a target frequency of only 868 MHz, none of your issues are critical. Just using surface mount parts and microstrip construction should provide enough design margin.

author: yramgu
author_rank: Contributor
timestamp: August 25, 2014, 08:23:11 pm
content: I agree with KJDS that there is absolutely no need to "ground" the unused connector. Although I strongly advise to keep the footprints Cxx : they form a Pi-shaped circuit with JP2 (C25-JP2-C24 for J9 and C26-JP2-C24 for J10), ALWAYS useful if you ever realise you actually need matching/filtering of spurious, etc.. I would also realign them.

author: ConKbot
author_rank: Super Contributor
timestamp: August 26, 2014, 05:10:47 am
content: Make the mmcx (or sma) sit straddle the transmission line, with a 0R right after it. When you want the mmcx, don't populate the sma and the 0R. When you want the sma, don't populate the mmcx. The through hole making a stub is short enough to get away with.

author: mark03
author_rank: Frequent Contributor
timestamp: August 26, 2014, 03:20:04 pm
content: Out of curiosity, what don't you like about MMCX? I was considering a pair of MMCX for mating two boards together, but I haven't held one in my hand yet.

author: KJDS
author_rank: Super Contributor
timestamp: August 26, 2014, 03:57:43 pm
content: Quote from: mark03 on August 26, 2014, 03:20:04 pmThey're entirely surface mount, so if a butterfly flaps its wings in a nearby room as you're mating or demating a cable into one, then you rip the pads off the board.

author: ConKbot
author_rank: Super Contributor
timestamp: August 26, 2014, 04:02:09 pm
content: Quote from: KJDS on August 26, 2014, 03:57:43 pmIf this is for messing around on your own, not production, then just use 5 minute epoxy to hold it down, the glob of 5 minute on the board has less effect on the trace than the trace being ripped offThough if youre doing board to board mounting, if you can, I'd use a through hole MMCX instead of a SMT. They should be plenty robust then, compared to SMT.

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2014, 05:23:16 pm
content: Quote from: KJDS on August 26, 2014, 03:57:43 pmAt least they are not like MCX connectors where misalignment bends and breaks off one of the outer contacts on the plug.I really like SMB connectors for test equipment as an alternative to SMA.

author: daqq
author_rank: Super Contributor
timestamp: August 26, 2014, 05:30:18 pm
content: Thanks guys for the suggestions!As to my opinion on them: I don't like the whole snap in concept in general, their non SMD versions are quite expensive compared to SMA and similar, I'm afraid of using SMD as you might have a vigorous user who'll use more strength then needed and rip off the connector with the connector. All in all, if it was not required I'd just slap a nice comfortable SMA and be done with it without the extra fuss.