I am designing a four-layer board on which I have to run two 50 Ω traces, one on the top and one on the bottom side of the board. I have decided to use a coplanar wave guide for both the traces. I am skeptical about calculating the impedance of the trace on the bottom side since the adjacent plane is a power plane. I have attached the stack up image of the board.

enter image description here

Calculated impedance using APPCAD:

50 ohm trace with reference to ground

50 ohm trace with reference to vcc

  • \$\begingroup\$ Why are you skeptical? \$\endgroup\$
    – Andy aka
    Nov 15, 2014 at 11:17
  • \$\begingroup\$ About the return path of the current of bottom trace near vcc plane \$\endgroup\$
    – karthik
    Nov 18, 2014 at 9:02
  • \$\begingroup\$ @karthik What´s the question? \$\endgroup\$ Nov 18, 2014 at 16:39
  • \$\begingroup\$ @JesúsCastañé I have added the calculated impedance of the coplanar line(according to my board stack up) along with my question ,check it out and give your input on this \$\endgroup\$
    – karthik
    Nov 22, 2014 at 8:19

1 Answer 1


First I would ask why a coplanar waveguide? Given that you have a four layer board I would think using micro strips would be easier / better.

Based on your Stackup you are really talking about a conductor backed coplanar waveguide. In that case return current will flow on both your side conductors and the "reference" plane underneath them. I say "reference" to emphasize that on the bottom return current will flow in your vcc plane and your side conductors. In a micro strip it would flow just on vcc. This is because this path is the path of lowest impedance for the current to follow.

Now current flows in a loop so when it gets back to the source it must find a way back to gnd to complete the circuit so it will find a way. In a well decoupled board that path will be the nearest decoupling cap(s).

A good source to learn more is the handbook of black magic by Dr Howard Johnson, his website, Eric Bogatin's books or website, and Brian Waddel's transmission line book.

  • \$\begingroup\$ I have added the snap shot of the calculated impedance of the coplanar line along with my question ,check it out and give your input on it.... \$\endgroup\$
    – karthik
    Nov 22, 2014 at 8:00
  • \$\begingroup\$ @karthik please update the impedance calculator link above \$\endgroup\$
    – gzix
    Nov 22, 2014 at 8:06

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