If you are designing high speed high frequency board, or RF design like antenna, the impedance will a important issue the design. For the antenna design, we have a few wireless boards including RF24L01, CC1101, etc all have the on board PCB antenna. Also in the circuit design, trace width, trace isolation will also influence the impedance, for example the trace connects to the SIM908 or SIM900 RF pins, the impedance should be around 50ohm for both GSM and GPS.When the impedance does not match the requests, the final board will not work well.
Here is very useful impedance caculator, when you have your boards input, the tool will automatically calculate the trace impedance. After tested, it works pretty well. With our PCB service, the general value should be as below in millimeter:
- Trace (W – width): Depends on your design;
- Trace(W1- gross width): Add extra 0.005 normally on Trace(W)
- Thickness (Copper Thickness): normally 0.035mm
- Separation: The ground plane isolation with trace, it can be 6 mil (0.15mm) or 8 mil (0.2mm) normally.
- Dielectric: Use 4.5 mm default, I am not sure what it is.
And in the picture, you can see there is a few drawings describe about how the trace works.
For general 50ohm impedance, some common settings could like below: