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CPWG transmission line - difference between TI reference and TXline calculation!

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shadewind:
I'm using the TI CC2500 in one of my designs and I'm quite the beginner on RF stuff. I'm sticking to the reference design as much as possible but one thing has me a bit confused.

This is a picture of the area on the reference design where the signal from the balun goes through the impedance matching network and on to the 50 ? transmission line to the antenna SMA connector:


I've figured out that this type of transmission line is a Co-planar Wave Guide with Ground. As can be seen in the picture, the spacing between the ground fill and the line itself seems to be intentionally larger than for the rest of the board. I measured it to 15 mil. The line itself is 55 mil wide and the board has a thickness of 62 mil. Copper thickness is standard 35 ┬Ám.

This is the same part on my board. I'm using a different conntector, an end launch RP-SMA which takes up less board space and fits with antennas which are more commonly available:


Now the funny thing is, checked the dimensions of the 50 ? line using the popular TXLine line calculator:


That's 55 ?! In my own design, I never saw the 15 mil spacing and used my default 10 mil spacing instead which resulted in the correct 50 ?:


So, should I follow the reference design or my calculations? Am I missing something?

Mechatrommer:

--- Quote from: shadewind on September 11, 2011, 04:27:20 pm ---So, should I follow the reference design or my calculations? Am I missing something?
--- End quote ---
its been 3 days while i was away... i think you are missing expert(s).

shadewind:
I got my answer from TI themselves!

http://e2e.ti.com/support/low_power_rf/f/155/p/133646/479912.aspx

Time:
Interesting.  Do they account for the solder mask effects everywhere on the board where there are t-lines or just in that one place?

shadewind:
I believe they simply measure the impedance and tweak it until it's right.

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