Author Topic: Microwave/RF through a via  (Read 2687 times)

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Offline itdontgoTopic starter

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Microwave/RF through a via
« on: August 29, 2012, 11:02:52 am »
Hi,

It would have been convenient on occasion to use a via to connect very sensitive RF parts of a circuit but I have always designed around it.  For example it would be beneficial in one project I worked on to have a surface mount GPS antenna feed in to a module which was mounted on the other side of the board but instead I designed everything on the one side.

Presumably a via would act like an inductor (and capacitor where the boards overlap) and you would therefore require different matching components (in this case) from the aerial to the module to compensate for the inductance.  Beyond that though I would expect the return loss to show a significant decrease but I cannot quantify it.  Would anyone like to hazard a guess in the GPS/GSM 1500MHz+ range?

I never designed a board to test the real world effect as I always assumed it would be a poor solution.  I also dont have easy access to Agilent ADS/Gensys or similar so have never 'wasted' any time on it.

Offline nctnico

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Re: Microwave/RF through a via
« Reply #1 on: August 29, 2012, 01:17:26 pm »
You could download a free limited functionality copy of Sonnet in order to simulate the effect. IMHO what you basically want is that the via has the same impedance as your trace to minimize the mismatch and reflections. I found the following article (use the 'pdf version' button to get the PDF version with pictures):
http://www.edn.com/design/components-and-packaging/4330788/Designing-controlled-impedance-vias
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline itdontgoTopic starter

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Re: Microwave/RF through a via
« Reply #2 on: August 30, 2012, 05:49:15 am »
Thanks for that very interesting.  I didn't think of doing that!


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