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Tips and questions relating to the GPS modules from SFE
By jconradt
#33774
Hi GPS guys,

sorry, this might be a completely stupid question, but I am not a HF person at all. So my apologies ahead :-)

I am thinking of reducing weight to the smallest possible, thus using the tiny GPS chip scale antenna. The data sheet talks about a "50 Ohm micro strip" input... What is this? A PCB trace that has 50 Ohm resistance? At what frequency? How do I design that? Is there a rule-of-thumb?

Also, people have recommended using an LNA (e.g. RF Micro Devices RF2373) to amplify the antenna-signal. Does this go *before* or *after* the 50-Ohm strip? In the data sheet (http://tinyurl.com/2ahxeb) on page 4-5 they show an example circuit, but it's not clear to me how this interacts with the 50Ohm trace... Is such a "complex" circuit required at all? Can I 'just' use the amplifier?

Ideally, I'd just have a tiny GPS modul (about 3grams) and LNA+passive antenna <0.5grams)... HELP ... please ;-)

Or does anyone have a very simple design example? Doesn't have to be perfect.

Thanks a lot!

Jorg
User avatar
By leon_heller
#33782
A 50 ohm impedance microstrip line on FR4 is approximately 0.1" wide over a ground plane on the other side. You really need the PCB characteristics to calculate it properly, but 0.1" should work OK. It's what I'm using with a Telit GE863-GPS.

if you don't know anything about microwave design, I wouldn't bother trying to design an LNA, it just won't work.

Leon
By jconradt
#33870
Hi Leon,

thanks for your kind reply. I get the idea of a 50 ohm trace. Unfortunately, however, I am not using FR4. I am using 0.3mm PCB material in order to save weight. I assume all the other parameter (copper thickness, material) are identical to FR4. Would you know a recommended wire-width for 0.3mm material as well? Or a tool to calculate?

Thanks a lot! Jorg
User avatar
By leon_heller
#33874
You need to know the permittivity of the board material. Given that and the thickness, you can calculate the microstrip width. You can find the formula with Google, or use an on-line calculator.

Leon
By velocet
#34123
You can download AppCad for windows and in the passive component tab you can enter your board details to calculate the strip dimensions.

http://www.hp.woodshot.com/appcad/appcad.htm

Hope this helps.
By Krogoth
#34136
The U-Blox website has masses of data on all things to do with GPS and high frequency RF design - you have to register to access the PDFs, but they're invaluable when it comes to designing a GPS board and all the information is presented in a practical, reader-friendly way.
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