Electronic – Characteristic impedance of a trace

I'm trying design a video transmitter around an 5.8GHz Airwave module. The documentation says the trace to the antenna must have a characteristic impedance of 50 ohms. How do I calculate the characteristic impedance of my trace?

Best Answer

As a quick rule of thumb for 50 ohms, you want the ratio of the trace width to the height above the ground plane (probably your board thickness) to be around 1.8 with 2-oz copper. With a standard 0.063" thick PCB, that means a trace width around 120 mil. (Fat!)

Formulas for calculating this can be found in various places. If you access to an engineering library, try chapter 4 of High Speed Digital Design by Johnson and Graham. (Equation 4.90 on p. 187 is the key one.)

Here's an online calculator: http://referencedesigner.com/tutorials/si/si_06.php

Related Solutions

Electronic – Impedance Matching and Large Trace Widths

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:

>----------------Signal 1
8.3 mil
>----------------Ground
39 mil
>----------------Power
8.3 mil
>----------------Signal 2

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.