I am currently working on a design in which one of my ICs specifies the use of a 50 ohm trace. The answer to this question, Characteristic impedance of a trace, shows that a 120 mil trace is required to get this impedance.
The IC only has room for 18.8 mil traces, and that is assuming no space between traces. So, how can I actually design with that trace impedance kept in mind? Obviously I can decrease the board thickness or increase the copper height, but only to some extent and I would like this to be fabricated for somewhat cheap. How is this usually dealt with?
The IC that I am using is the MAX9382 which can operate up to 450 MHz, I will probably be using it around 400-450 MHz. The data that is being used is initially analog, but has to be hard limited to become digital in order to be used with that IC.
Best Answer
Use a 4 layer stackup.
Calculating the trace width needed is pointless unless there is a solid ground plane under it, with a 2 layer design you may need to route traces on the other side which then pretty much ruins your impedance if they come anywhere close to your trace.
At 450Mhz you really should have solid, continuous, properly decoupled power and ground planes. This will improve noise performance, EMI issues, give you better impedance control, etc. Fabbing a 4 layer board isn't that much more to expensive than a 2 layer.
Use a 4 layer like:
Spacing could change a little based on your copper thickness choice.
That will give you something like 10-20mil for your 50ohm trace on Signal 1/2 depending on final dielectric and copper thickness on the Signal layers.