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Ebin42 , 11-28-2024, 05:19 PM

Recently i watched a video about the return current path where @Robert Feranec mentioned the return current goes around the gap if there is a discontinuity in the plane. That got me thinking what if there is one more layer near to the reference plane without any gap? Does the return path take the next nearby layer or does it go around the gap?
What do you think?

QDrives , 11-28-2024, 07:02 PM

'Energy' cannot just magically swap layers when it encounters a discontinuity.
So if you have 2 reference planes, and one of them has a gap, then place vias there so the energy can swap layers.
Alternatively, do some local co-planar waveguides.

Ebin42 , 11-28-2024, 08:06 PM

But current always tends to take the less inductive path right. Especially when it comes to the high speed signals. Going around the gap is true for low speed signals. I was wondering if displacement current takes the next best layer to find the low impedance path. I would like to know if @Robert Feranec did any simulations or experiments with it. I remember he mentioned it.

QDrives , 11-28-2024, 08:16 PM

Current does not know what the least impedance path is. It takes the path of least impedance as it sees it at that instance.
So if you have 2 planes, and one has a gap later on, current will flow in both. When it reaches the gap in the one plane, that current needs to go somewhere. However, when the current started at the beginning of the signal, it did not yet know of the gap.

Ebin42 , 11-28-2024, 08:34 PM

Could you tell me what you meant by current doesn't know the least impedance path? Also do you mean current travels in both planes from the beginning to end of the transmission line?

QDrives , 11-28-2024, 09:18 PM

When the signal starts, it does not know that down the road (signal path) there is a discontinuity, so it travels in both planes from the beginning.
There is a video from Robert with Eric Bogatin. It may be this one: https://www.youtube.com/watch?v=icRzEZF3eZo
And then this one: https://www.youtube.com/watch?v=LdEHJXs5xpw

Ebin42 , 11-28-2024, 11:39 PM

My original question is related to the first video itself. The discussion with Eric Bogatin. Thanks for looping it.

QDrives , 11-29-2024, 02:55 PM

But is that video the one where Eric says something like: "the signal sees 50 ohm and continues to see 50 ohm until it hits the end"?
I have seen to many Eric Bogatin videos to keep them apart.