Coplanar Line with No Ground Plane | PCB Routing

In an earlier video (link below), Tech Consultant Zach Peterson explored PCB Routing on a 2-layer board. And during this examination, he laid out the need to use Coplanar Lines when routing USB on a 2-layer board. In this video, he dives further into Coplanar Lines and explores impedance, crosstalk, and more.
0:00 Intro
0:43 Revisiting the Coplanar Arrangement
5:48 What about Crosstalk?
7:11 Exploring Impedance
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[@dmitry.shpakov]

Wow! Good topic covered. Thank you Zach!

[@maks886]

Hi, could you show how to do PCB panelization in Altium for popular cheap PCB manufacturers (jlc, pcbway etc.)?

[@Zachariah-Peterson]

Oh that sounds like fun, sure I'll get to work on it.

[@user-ww2lc1yo9c]

why exactly is it that when we use a differential signal, we can have such a small width in the tracks?

[@Zachariah-Peterson]

The distance from the signal trace to the conductor carrying its return signal will determine the capacitance of an individual trace. In the case of a differential pair, the two conductors reference each other and the nearby plane, so both one of the traces will have its odd-mode impedance determined by the other trace and the nearby plane. Because each trace can reference the other trace, the impedance can be set by the other line even if there is no plane. Because presence of each trace adds some capacitance to the other trace, that would lower the trace's impedance below its target characteristic impedance (normally 50 Ohms). The result is that you have to make each trace a little thinner to have the odd-mode impedance be equal to your target impedance for an individual trace (again, normally 50 Ohms).

[@user-ww2lc1yo9c]

what is odd mode impedance?

[@Zachariah-Peterson]

The odd mode impedance is the actual impedance of an individual trace in a differential pair. The odd mode impedance is slightly lower than the characteristic impedance of the trace. These two values are sometimes said to be the same, but in fact they are not the same. The two traces in a differential pair have some mutual capacitance between them, and it is that mutual capacitance that causes the actual impedance of an individual trace to be slightly lower than its characteristic impedance. This means that, if you increase the spacing between the traces in a differential pair, the odd-mode impedance will eventually become equal to the characteristic impedance.

[@fredo514]

What’s the minimum width the coplanar ground planes need to be to still be effective? Would via fencing help in allowing narrower width of the coplanar planes? Thanks!

[@Zachariah-Peterson]

This is a really good question and I actually filmed a video yesterday that covers some of the aspects of grounded coplanar waveguides. I think you have to define what "effective" means. In the context of a 2-layer board, we're using the coplanar arrangement to try and set the required trace width to a smaller value, that way we can route longer traces and not have to worry about impedance mismatching. In general, placing the copper closer to the trace will give stronger capacitive coupling, so that will reduce the impedance for a narrower trace. So for example, if you start with a 12 mil wide trace on a 2 layer board, it will have high impedance, but that impedance will gradually reduce as you bring the pour closer to the trace. Eventually you can only get so close because you run up against fabrication limits and eventually you hit your impedance target for a given width, so you can't get any closer. As you bring the ground regions closer to the trace you also decrease the inductance with respect to the nearby pour, so you're getting more protection against crosstalk. In a multilayer board where the ground plane may be very close to the trace (like maybe a 3 mil dielectric?), now you'll probably be coupling more strongly to the ground plane than the nearby ground pour, so that will be more effective at suppressing radiation and crosstalk.

[@fredo514]

@@Zachariah-Peterson Thank you for taking the time to answer! I’m sorry I may have formulated my question wrong. I was trying to ask about the minimum width of the side ground traces. Do you have recommendations on that?

[@Zachariah-Peterson]

@@fredo514 I see, the side ground regions work best in terms of shielding effectiveness when placed as large copper pour regions. The coplanar waveguide impedance equation derived by Wadell assumes that the width of the ground region is at least 5x the distance between the edges of the ground regions. You can find this model on page 73 of Wadell's textbook (Transmission Line Design Handbook).

[@Zachariah-Peterson]

@@fredo514 For via fencing you can really place it anywhere, but where you place it will affect the bandwidth and shielding effectiveness of the structure. I'm about to upload a new blog to the Altium site that answers this as I was asked a very similar question during my AltiumLive Q&A session. You'll be able to see it in about an hour.

[@CatcatcatElectronics]

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