GPS and 50 Ohm passive antenna connetion?

GPS/GNSS Development Boards
1

Hi GPS guys,

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

I am thinking of reducing weight to the smallest possible, thus using the tiny GPS chip scale antenna. The data sheet talks about a ā€œ50 Ohm micro stripā€ inputā€¦ 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 itā€™s not clear to me how this interacts with the 50Ohm traceā€¦ Is such a ā€œcomplexā€ circuit required at all? Can I ā€˜justā€™ use the amplifier?

Ideally, Iā€™d just have a tiny GPS modul (about 3grams) and LNA+passive antenna <0.5grams)ā€¦ HELP ā€¦ please :wink:

Or does anyone have a very simple design example? Doesnā€™t have to be perfect.

Thanks a lot!

Jorg

2

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. Itā€™s what Iā€™m using with a Telit GE863-GPS.

if you donā€™t know anything about microwave design, I wouldnā€™t bother trying to design an LNA, it just wonā€™t work.

Leon

3

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

4

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

5

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.

6

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 theyā€™re invaluable when it comes to designing a GPS board and all the information is presented in a practical, reader-friendly way.