SparkFun Forums

[TheRidDler]

Hi! I dont know if this is the right place to post this, but im really stumped when it came to this. I am making my own pcb for the first time (my first project as well!) and i have to connect a passive antenna to a gps module using a 50ohm microstrip. I have no idea how to create a track so that it is exactly 50 ohms! anyone have any advice to give me on this plz reply )) Is there a nice little application out there somewhere that can calculate the dimesions of the strip from other data? im really stuck! Thnx alot!

[bigglez]

Hi! I dont know if this is the right place to post this, but im really stumped when it came to this. I am making my own pcb for the first time (my first project as well!)

Greetings TheRidDler,

Sure! I might have choosen this forum instead:

i have to connect a passive antenna to a gps module using a 50ohm microstrip. I have no idea how to create a track so that it is exactly 50 ohms! :oops: anyone have any advice to give me on this plz reply :))) Is there a nice little application out there somewhere that can calculate the dimesions of the strip from other data? im really stuck!

The characteristic impedance of the physical trace is given by:

where weff is the effective width, which is the actual width of the strip, plus a correction to account for the non-zero thickness of the metallization. The effective width is given by:



with:

Z0 = impedance of free space,
dielectric constant of substrate,
w = width of strip,
h = thickness ('height') of substrate and
t = thickness of strip metallization.

It is claimed that for most other cases, the error in impedance is less than 1%, and is always less than 2%.

I've found that 10% is a practical tolerance for many PCB fab houses. A work around is to make a test coupon on the PCB, measure it with a TDR or network analyzer, and either accept or reject the PCB lot. This is way beyond BatchPCB's mission, so I'd suggest taking your best guess and living with the results. IMHO, for minor trace lengths I wouldn't worry about controlled impedance too much.

There's an on-line microstrip design tool here.

Comments Welcome!

[leon_heller]

A track 0.1" wide with a ground plane on the other side is about 50 ohms, with standard FR4.

Leon