Electronic – How to design a PCB to connect the GSM modem with 50Ohm track impedance

pcb-design

In the system integration manual of the Leon G100 GSM modem (http://www.u-blox.com/images/downloads/Product_Docs/LEON-G1-G2_SystemIntegrationManual_(UBX-13002023).pdf) it’s written to carefully design the 50 Ohm micro strip or stripline on the pcb (“The transmission line up to antenna connector or pad may be a micro strip or a stripline. In any case must be designed to achieve 50 Ω characteristic impedance”).
I design a 2-layer pcb and before I got to chapter 2.2.1.1 of the mentioned document I carelessly attached the ANT with the signal pin of the SMA.

I cannot use the stripline as it’s a 2-layer PCB, instead I used a coplanar waveguide with ground. Using a KiCad Pcb Calculator I ended up with the following conclusions: to have 50 Ohm at FR4 PCB of 1.5mm thickness and freq of 900MHz it has to be a 1.25mm wide path separated from the gnd paths by 0.25mm. The Electrical length 22 degrees.

And here I have some questions:
– for which frequency, 900 or 1800MHz shall I calculate the parameters?
– what is the 22 degrees electrical length about?. This value increases proportionally with frequency and at 1800MHz I get 44 degrees
– how shall I design the vias for the gnd and sma signal pin? As far as I know the SMA connector is “through pcb” not SMD.

Best Answer

The electrical length is the phase shift that the design frequency experiences when travelling down the trace. In this case, it is irrelevant. For some designs, these delays must be known adjusted to specific values. Quarter wave transmission lines of specific impedances can be used for impedance matching and in the construction of various RF components. For this, you need an electrical length of 90 degrees. In your case, the length is irrelevant. For the ground vias - just position them in some what that they don't interfere with the impedance controlled trace. The signal via probably won't be very close, but at 1800 MHz, it's too small relative to the wavelength to be much of a problem. I wouldn't worry about the connector too much either for the same reason. At 2 GHz, the wavelength is about 15 cm. If any 'discontinuities' are less than 1 cm (1/16 wavelength), you should be fine.