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

[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 µm.

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]

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

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.

[Bambur]

I got my answer from TI themselves!

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

Thanks, shadewind, for posting the link! Please post more in the future. I'm interested in the microwave design very much.

I've heard that the TXLine doesn't give very accurate results sometimes. There is a book which has a lot of information on the transmission line design, it might be worth checking out. It's not difficult to find the book.

Transmission Line Design Handbook, Brian C. Wadell
http://www.amazon.com/Transmission-Handbook-Antennas-Propagation-Microwave/dp/0890064369

Besides, you should specify the substrate material and loss tangent correctly, since those will define the models used to calculate the transmission line parameters. I suppose at 2.4 GHz it matters very much.

[shadewind]

Thanks Bambur, I will try to post more when I've got the board assembled and got it up and running if you want. I'm a complete beginner when it comes to everything RF but I'm learning things all the time.

According to the guys at TI, the substrate material does not affect the impedance of the transmission line as much as you'd think, at least in the range of values possible for "regular" FR4. This is also evident when you play with the dielectric constant value in TXline.

[vk6zgo]

The difference between 50 & 55 Ohms is not going to have any practical effect.
Even 75 Ohms probably wouldn't make that much difference!
It is highly unlikely that the impedance matching network presents an exact 50 Ohm output impedance,anyway.

VK6ZGO

[Bambur]

vk6zgo, you're absolutely right! It won't do any difference in this case.