How ground planes work

I came across and this video which has some interesting information about the properties of ground planes, especially the routes taken by return currents. Although it is aimed at EMC problems in high frequency digital circuits,much of what it contains is relevant to audio.

https://www.youtube.com/watch?v=M-JYy-Y9fz0

Cheers

Ian

Everything I love about gdiy.  Thank you Ian!

I don't understand. Why is the inductance less under the send trace?

squarewave said:

I don't understand. Why is the inductance less under the send trace?

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I am not sure i do either but it seems to be the case. I must have to do with the current taking the path of least inductance but why that is directly below the trace I don't know.  What surprised me is the effect occurs at audio frequencies.

Cheers

Ian

squarewave said:

I don't understand. Why is the inductance less under the send trace?

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I haven't seen the video yet, but I guess it's because the flux are opposing, just like in a common-mode choke. The send current and the return current produce opposite flux, so the coupled inductance is less than the non-coupled.

abbey road d enfer said:

I haven't seen the video yet, but I guess it's because the flux are opposing, just like in a common-mode choke. The send current and the return current produce opposite flux, so the coupled inductance is less than the non-coupled.

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Yes.  In a sort of hand wavy explanation the magnetic fields cancel out for differential signals and that means the inductance is minimised. Think of it as being dependent on the 'loop area' so the closer the traces the smaller the loop area and the smaller the inductive impedance. For some numbrs to go with the theory I'd suggest a look at whatever the current (no pun intended edition is of 'EMC for Product Designers' (author Tim Williams)

Newmarket said:

Yes.  In a sort of hand wavy explanation the magnetic fields cancel out for differential signals and that means the inductance is minimised. Think of it as being dependent on the 'loop area' so the closer the traces the smaller the loop area and the smaller the inductive impedance. For some numbrs to go with the theory I'd suggest a look at whatever the current (no pun intended edition is of 'EMC for Product Designers' (author Tim Williams)

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This is correct, the return signal travels through the path of least impedance, at low frequencies resistance dominates, so signal travels through least resistance but, after, say 10kHz, the inductance of the trace dominates, and the signal travels through the path of least inductive reactance, this just happens to be right below the signal carrying trace which might not necessary be the closest path between source and reciever. May I also recommend Electromagnetic Compatibility Engineering by Henry Ott.

user 37518 said:

May I also recommend Electromagnetic Compatibility Engineering by Henry Ott.

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Yeah - the Henry Ott stuff is good too and he has some good info on the web IIRC.