Coplanar waveguide on a 4-layer PCB

Question

I have a 4-layer PCB where

• L1 = Sig/Pwr
• L2 = GND
• L3 = GND
• L4 = Sig/Pwr.

I need to route a 50 Ω coplanar waveguide for an RF 4G-LTE antenna on L1. I know how to make the calculations for the impedance and all, but does it matter if I use L2 or L3 as GND plane for the waveguide?

If I use L2 the dielectric thickness would be 8 mils and I could design the waveguide with 12 mils trace thickness and 4 mils gap.

However, if I choose to use L3 as GND plane the distance to it becomes 32.8 mils and the RF trace will need to be 25 mils with 4 mils gap.

I would prefer to use L2 as GND plane because of the slimmer traces, but only if it doesn't degrade performance too much.

Answer 1

The devil is in the detail.

• what size components do you need to connect to your CPW?
• is your stackup a central L2/L3 core with two foil outer layers (sounds like it, given your dimensions) or two L1/L2 and L3/L4 cores stuck together with a single prepreg?
• larger structures have less sensitivity to etching tolerances, and will typically be lower loss.

Look at all the components you'll be connecting to the ends, and even points in the middle, of the line. Avoiding changes of width to make a connection would be a good plan.

Generally, core dielectric will give you better loss and better tolerance on thickness and composition, so repeatability. If you have a central core, run L1 to L3, so that the bulk of the dielectric is core.

You can get cores that process like FR4, but have a much better loss and frequency range, like Rogers 4350. It's what all my mixed technology boards were made with when I was working.

However an antenna is a fairly poorly controlled structure, and line deficiencies are unlikely to be the biggest issue you have.