SparkFun Forums

[lain]

Hi, I'm working on a small board design using the GR-10 GPS receiver and the GPS chipscale antenna. In the datasheet for the antenna it mentions a 50ohm microstrip, I'm only vaguely familiar with microstrip traces - I understand the idea is to have a controlled-impedance trace, and I used a free online calculator to get the proper trace width, but I have some questions.
First, does the trace length matter? In the datasheet they give a specific length for the trace and I'm wondering if I need to match that, or if it can be any length?
Also, am I correct in assuming that microstrip should connect to the ANT pin on the GR-10, and if so, is it reasonable to assume that if I need to make a 90degree turn in the trace, I should do two 45-degree turns instead, to reduce impedance changes?
Lastly, should I have a stop layer over the microstrip so there is no solder mask over it, or is soldermask okay?

Thanks very much!

[leon_heller]

Microstrip is a track over a groundplane, separated by the PCB laminate.

With standard FR4, a track 0.1" wide over a groundplane will have a characteristic impedance of about 50 ohms. The trace length won't affect the impedance, but it will affect the losses. If the length is under 1/10 the wavelength of the signal, the impedance doesn't really matter. It's best to mitre the tracks.

Theoretically, solder mask over it will have an affect, but it will be minimal.

Leon

[wiml]

The length of the trace might matter if it's part of a tuned structure (like a series matching section), but if that's the case, I'd expect the reference design you're working from to say so. My guess is they just don't want the trace to be so long that it attenuates the signal, picks up or radiates noise, or things like that.

[ohararp]

Keep the trace as short as possibly and keep the trace away from any other digital lines.

Below google yielded a quick calculator. I think a 17 mil trace is about 50 ohm when using FR4 boards and 1 oz. copper.
http://circuitcalculator.com/wordpress/ ... alculator/

Can I ask which pcb design software you are using?

[leon_heller]

That calculator isn't suitable for microstrip design, it just gives the track resistance.

Leon

[ohararp]

Touche...based off this calculator http://divsys.com/Tools/CICalculator/Microstrip.asp a .128" trace will work for an impedance of 50 ohms and FR4 standards.

[Caffeine]

I've always used traces of 111 mils for my boards, with good results. This was based off information in some linx technologies datasheets (as I was using linx parts)

"For standard 0.062in thick FR-4 board material, the trace width would be 111 mils"

They have a quite usable calculator too.

http://www.linxtechnologies.com/Software/microclc.zip