"Jared Cabot via groups.io" <jaredcabot@...>
writes:
Alrighty, now Christmas is done, it's almost time to order the final revision of the PCB's.
I have one last thing to figure out (If it even matters at 3GHz?)
To keep impedances as close to 50 ohms and VSWR low as possible etc etc etc, I have seen that for a through-hole RF connector into a coplanar waveguide, the ground plane should be pulled back around the connector's center pin.
Does anyone know how to calculate the required clearance? I'm using a standard 2 layer 1.6mm FR-4 PCB with 1oz copper.
Here's a picture of the board so far, you can see I pulled the ground plane back by an arbitrary amount to the left of the PCB for now, I just need to get that right and the PCB will be done ready for production.
The proper way to calculate this is with an electromagnetic field
solver. OpenEMS is free and open-source and more than suitable for this
task. Others exist too (e.g., meep), but OpenEMS is probably the best
first choice (easiest to learn).
Learning to perform EM simulations such as this is not a trivial task,
but is well worth the effort and frees you up so that you don't need to
rely on calculators for this sort of thing, which don't exist for many
geometries and configurations and suffer accuracy limitations outside
the conditions in which they were designed to provide a reasonable
approximation. And, once you learn the process, they're not all that
hard to use.
At 3 GHz, you might be able to get away with ignoring the issue or just
guessing. And, with FR-4, you face significant uncertainty in terms of
material permittivity, so a nominal EM simulation will only be so useful
(you'd really want to perform a tolerance analysis).
The added metal from the center pin and via adds capacitance to the
transmission line, lowering the characteristic impedance, hence the need
to pull back the ground plane nearby. You might be able to try to
calculate this effect for your particular connector and setup, but it
would be easier and more accurate to just fire up an EM simulator.
I appreciate that this probably isn't the easy answer you were looking
for, but it is the best answer. This is how this is done professionally.
Some possibly useful resources:
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Good luck,
Matt
