I attempted to design a 4-layer PCB with the first layer as signal, the second as ground, the third as ground, and the bottom as signal. Additionally, I incorporated an RF design using a CPW structure and FR4 material for the PCB, with a dielectric constant of 3.48. The thickness of the material, which is 0.5mm, was used as the distance between layers. I chose the second layer as the reference ground plane. However, during the Gerber review, it was noted that for a 1mm signal width and 0.2mm clearance, the second layer should not be used as the reference layer. For better signal integrity and return path, should the PCB stack-up be signal/ground/ground/signal? Should I remove the second layer as ground? Additionally, what is the solution to achieve 50 ohm impedance in this scenario?
I am assuming we are only dealing with the first and second layers here. From what you describe a quick analysis yields 46.99 ohms. By upping the gap from 8 mils to 12 mils you can get 49.7 ohms. Alternatively, if you decrease trace width from 39.4 mils to 35 mils you can hit 50.0 ohms. Your stackup/layer selection should be just fine as is.
Thanks for your response. I am a beginner in electronic circuit and PCB design. I would like to clarify if it is okay to use the first layer as the signal layer and the second layer as the ground reference when constructing a coplanar waveguide (CPW) in a 4-layer PCB. I am designing a passive antenna connection for an LTE module. To learn, I usually depend on blogs and YouTube videos. Any kind of help is appreciated
Well, let me say that’s the way I’d do it. I don’t actually know if there’s a better way, but then again there are only two or three ways you could morph CPW.
As with any transmission line structure, cleanliness (of layout) and uniformity (material and copper geometry) are the gospel. I think there are several advantages to good old microstrip; 1) Easiest to design (ok, I admit, I’m lazy) 2) easiest to tune, especially when prototyping or first article efforts (ditto), 3) Easier to fabricate - being on the outer layers allows both in-process monitoring and in-process tweaking of trace geometry to reach target impedance, 4) Easier to measure and couple in and out of, and 5) Can be further adjusted by small increments using soldermask or even conformal coating.
But, back to CPW. The key things to watch carefully the exact geometry of the traces. If you keep the thickness of the copper, the trace width and the spacing to the coplanar grounds closely in mind as you explore your design options then you get good results. I don’t mention the height above adjacent ground layer or anything about the dielectric, because other than initially choosing the laminate there will be little you can control or rely on. But if your work gets you to 90% of perfect, the PCB shop can bump you that last five percent to get an outstanding design and product.
If you email me directly I’ll gladly copy you on any technical articles I might have.
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