Hello Everyone,
I had some questions regarding microstrip (or Coplanar waveguide CPW) design. Since it's my first project using a RF microstrip I was wondering if you could give me some advice on this subject.
My design incorporates a GSM modem. Earlier designs had the MC35i from siemens, this modem had an easy accessible RF connector where the antenna connector was placed on directly. Now, the design needs to be reduced in size and a smaller modem is to be used. My choice will be the Enfora Enabler IIIG [1] or the Telit GE864 [2]. Where the enfora has an RF connector, it's almost impossible to use this for a direct connection to the antenna connector. The Telit is a BGA module and has no RF connector at all.
The idea of my microstrip is to simply make a connection for the antenna on my PCB which can easily be reached. Basically this comes down to getting the RF from under the modem, to an 'open space' on the PCB. I think it will be no longer then 10mm. I will be working with a 4 layer board [3] and my prototype PCBs will come from Eurocitcircuits. I have used a piece of software called TXLine [4] to do some calculations for me on the CPW. I want to route the RF on the top layer (enclosed by GND Copper) and the layer(s) below will be ground as well. I am neglecting the effects of the substrate and copper below the 2nd layer. I chose the Gap between strip and copper to be 0.2mm and the program calculated for me, that in order to get a 50Ohm impedance, the width of the strip should be 0.66mm. A RF connector will be placed on the end of the strip. The frequencies on the strip will be GSM900 and GSM1800 (and perhaps later GSM850 and 1900).
How critical is making such a strip? Is my approach sufficient to stay out of trouble? Or am I overlooking things? I have never used RF on a board, and rather not blow up my modems or degrade the output power. Any advice is welcome.
Thanks in advance!
Jasper Keuning
[1]Cu (17u), Prepreg (100u), Prepreg (200u), Prepreg (200u),
Cu (17u), Core (550u) Cu(17u),
Prepreg (200u), Prepreg (200u), Prepreg (100u), Cu(17). (all Material is Fr-4)
[4]