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Microstrip versus coplanar waveguide

maticee , 07-05-2023, 02:16 PM
I am currently working on a project that uses a 2.4 / 5 GHz WiFi/BT transceiver that I am trying to route at a target impedance of 50 ohms on the top layer. This trace is routed to a U.FL connector.
This is being routed on a 12 layer board with material that has a DK / Er of 3.17.

See the following dimensions for the RF trace:
Trace width = 4.5 mil
Trace clearance to adjacent GND pour on same layer = 10 mil
Conductor height (L1 to L2) = 2.5 mil

I routed this originally as a microstrip design but have since come along many resources talking about CPWG and how it affects impedance. Saturn PCB gives me a microstrip impedance of 50 ohms and a CPWG impedance of 63 ohms. Which would should I design for? I have flexibility on trace thickness and spacing . . .

Additionally, at what point does coplanar waveguide become microstrip? Theoretically, CPWG with a very large gap would result in a microstrip trace but the tools do not seem to generate that outcome!​
robertferanec , 07-12-2023, 02:04 AM
I am curious to see answer on this question too
qdrives , 07-12-2023, 02:18 PM
I do not know if PCB toolkit is the best for CPWG.
If I enter for microstrip: 0.25mm trace 0.15mm above plane Er=4.6 18um+18um I get ~50 ohm
CPWG set gap to 10mm ~61ohm and 100mm ~63ohm.

I expect at those distances, the coplanar effect to be negligible. This is also the answer to your second question (when does it become a microstrip).
If it does have an effect, then every trace on that layer would have a impact on the impedance.
Just gone through a couple of other web calculators -- all the same results.
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