Electronic – why does high speed current follows the path of least inductance? and how are the shape of lines are different at low and high speed


Does anybody know why the lines of electromagnetic field at high speed are bent and exactly following the shape of line, while in the low speed device the electromagnetic linesenter image description here are in the usual propagation shape.

Best Answer

This answer is a simplification, but is the best description I have seen.

First of all, "return current follows the path of least resistance" is actually a bit misleading.

The current returns by any and all means that it can. The path of "least resistance", or more accurately "path of least impedance", simply carries more current than any other path. However that may not be the majority of the current. That is, the path of least resistance may carry 25% of the current but all the other paths may carry 75% in total.

This is actually true regardless of the signal frequency.

However, each wire can be represented by the image below. Each point along the wire has a capacitance to the ground plane (Or to ground or whatever else is in the vicinity.)

enter image description here

As you know, with a DC signal those capacitors have no effect and can be ignored.

However, lets imagine a rising edge on a signal running down this line. As it passes each capacitor the voltage on the bottom end of the capacitor briefly jumps up to some fraction of the signal voltage.

Electrons are immediately drawn into the capacitor from the plane to charge the capacitor. When the signal reverses, the opposite is true.

These capacitances cause signal loss of course, but also has the effect of creating a potential well, or gulley in the plane which effectively becomes the new path of least impedance.