I'd like to match the impedance on one of my PCB traces.

For this I chose to use TNT Field Solver.

Suppose that the trace itself looks like this (trace width currently set to 0.8128mm = 812.8 micron):

enter image description here

If I use the TNT like this:

enter image description here

It will iterate the trace width to match 50 Ohms, but in this case trace width is recommended to be 3082 micron (>3 mm) which is really huge so I think this is not the right way to do this.

Now I can add a GND plane also to the top like this, but how can I handle the 'air' between the trace and the top GND plane?

I have 2 questions here:

1.) How can I insert 'air' between the trace and the GND plane? (obviously it is not air, but there is no metallic conductor there)

2.) How to include vias in this calculation?

enter image description here


I've included now the ground stiching and the top-layer ground plane, however ground plane fills between the stiching and the signal wire which is not correct. For ground planes it is not possible to set the width or position, so I would need to do something around the red circles here:

enter image description here


A rough rule of thumb is that a 50 Ohm microstrip will be close to the same width as its distance to the reference plane. So your simulation isn't that surprising.

You can reduce the microstrip width by choosing a thinner board, or using top copper fills as you show in your question. The copper needs to be close to the microstrip to have an effect. And it needs to be well stitched to ground. Google "coplanar waveguide".

A 4-layer board will give you a much closer reference plane :)

As far as TNT, try this: to your cross-section, add "New Rectangle Conductors" and name them "groundWires" (the capitalization is intentional, but don't use the quotes). Then, in the details, choose number=2 and adjust the spacing accordingly. The spacing won't be intuitive :)

  • \$\begingroup\$ Okay, thanks. I can do the stiching with the Rectangle Conductors, however I added also a ground plane to the top. But in this case the top ground plane fills between the stiching and the signal copper. That needs to be improved. Please see my update in the question for details. \$\endgroup\$
    – Daniel
    Jun 23 '19 at 15:27
  • \$\begingroup\$ @Daniel If you named the rectangle conductors "groundWires" the simulator assumes it is well stitched. That is all you need; you shouldn't add an additional ground plane on top. I think the simulator rule is to start a name with "gr" to assume an ideal (stitched) ground. \$\endgroup\$
    – bitsmack
    Jun 24 '19 at 3:49
  • \$\begingroup\$ @Daniel Oh, and you want the depth of the rectangle conductors to be the same (17.5) as the microstrip. It's all the same copper :) \$\endgroup\$
    – bitsmack
    Jun 24 '19 at 3:52
  • \$\begingroup\$ Thanks, will try that! I've also found a new tool named 'AppCAD'. It has a builtin tool for calculating cpwg: portal.u-blox.com/sfc/servlet.shepherd/version/… \$\endgroup\$
    – Daniel
    Jun 25 '19 at 9:30

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