Why return current is directly under the signal trace
Q
Hi,
The return current for the high speed part of the signal will take the path of least inductance. At high speeds, return current stays tightly bunched under signal trace. Why is this directly beside the signal? Please explain.

Best Regards,
Balamanikandan.K
A
Less loop area, hence least impedance if the return current flows directly under the trace.
A
The dreaded electron hose explanation is that the signal current induces the return current. The closer the metal the stronger the induction. The E/M wave explanation is that the metal of the signal trace and the
return path form reflection boundaries.
A
Hi Bala,
This is a resultant of skin depth phenomenon. Lower the frequency, higher will be the penetration capability of charges in a conductor and higher the frequency lower will be the penetration area. For microstrip TL case, the current distribution will be spread to a larger area beneath the trace when compared with that of higher frequencies. This link below may give you better insight.
http://www.rfcafe.com/references/electrical/skindepth.htm
Warm regards,
Nithin
A
The return current typically wants to travel under, not next to, the signal trace in order to minimize the mutual inductance between the two paths. This assumes a signal return plane under the signal trace. Dr. Clayton Paul has the best explanation for this in his textbook, Introduction to Electromagnetic Compatibility (2006).
A
Kenneth Wyatt
Wyatt Technical Services LLC
Woodland Park, CO
You are correct that the current follows the path of least inductance. If you have a trace directly over a (related, continuous) reference plane, the path of least inductance is the path with the lowest loop area. This is also the path with the lowest mutual inductance (hence lowest overall impedance). The proof involves a lot of math, but the qualitative explanation is that easy.
One way to think about it is this: The signal couples to the return path and boosts the return. The return couples to the primary signal path and boosts that signal. The closer are the paths, the greater the "boost," therefore the lower the energy that is required to move the signal, therefore the lower the impedance. This is the SAME effect that leads to lower differential impedance if differential traces are moved closer together, increasing the coupling between
them and lowering the differential impedance.
Lower impedance (and therefore lower inductance, in this case) also leads to lower EMI. So there are lots of reasons to route traces close to (continuous, related) underlying planes. In fact, all this leads to the first fundamental rule for good signal integrity control: Route every (critical) trace as close as practical over a continuous, related, underlying plane.
All this is covered in my now book "PCB Currents: How They Flow, How They React" (Prentice Hall) to be published 5/17 13. I will have a "blurb" up about it on the UltraCAD web pages by the end of this week.
Doug
A
With all due respect, this is not (at least directly) related to skin effect at all.But within the topic of skin effect, there are effects called the "Proximity Effect": and the "Ground Effect." THESE effects are caused by exactly the same phenomenon that causes the return path to be directly on the plane directly underneath the trace: that is the point of lowest mutual inductance.
The skin effect IS caused by inductance, but is not related to the return current path,
I have a chapter on the skin effect in my forthcoming book "PCB Currents: How They Flow, How They React" (Prentice Hall) due out on 5/17/13. I will have a blurb on it posted on the UltraCAD web pages by the end of the week.
Doug Brooks
A
To be even more correct we should use the term "path of least impedance"instead of "path of least inductance" to quantify the signal/return path. The path of least impedance is the path of lowest energy, which is the one that all systems strive to achieve. This accounts for current spreading all the way down to DC, and resonant phenomena.
A
Interesting question !
Well you know at the frequencies of interest the current has to close in a loop upon itself because the divergence of the current density can be approximated to zero at the rates we are talking about. That infers that for every current
there has to be an image/return somewhere, whether that is called induction or conduction or displacement or combination. In the case of an active trace and a ground plane, the discontinuity of tangential magnetic field through the ground plane will induce a current that is consistently in the opposite direction of the active trace current; that is how waveguides in general propagate a signal, a discontinued magnetic field at the copper interface will generate a looping current on the waveguide walls. For a higher frequency the fields will be more localized and hence the return will be closer to the trace, a concept consistent with the path of least inductance.
A
Why the return current density is more localized at higher frequency: since more charges are now on the surface of the trace they induce more negative charges on the ground below. These charges are consistent with the presence of
a normal Electric field on the ground surface which in turn is consistent with a tangential Hfield and an opposite return current density)
A
Hi Antonis,
Interesting topic!
Is there a link you can recommend with illustrations in order to better understand by the visualization of this description?
_____________________________________________
The simplest case I know that exhibits a concentration of returning signalcurrent near a signal trace happens in a ribbon cable.
Imagine a 50conductor ribbon cable connecting point A to point B. From point A, drive signal line #25 (positioned near the middle of the cable),using all the remaining ribboncable conductors as ground returns (all conductors 124 and 2650 tied together to digital ground). After taking into account the series resistances and mutual inductance of all the wires, you will find that above some critical frequency the returning signal current associated with signal wire #25 flows most strongly on wires #24 and #26, and the intensity of that current falls off quadratically with distance (pretty much) as you move to other return conductors remote from the central
position. I like this form of the problem because it is discrete.
If you simplify further, using just three wires (one signal and two parallel returns), above a critical freuqency the bulk of the returning signal current flows in the wire closest to the signal wire.
The critical frequency is that point above which the inductance of thecircuit exerts more influence on the flow of current than the resistance.
In a solid plane, you may divide the plane layer into a number of conductive strips with uniform current density within each strip and the mathematics works out nearly the same. The general name for this effect is "Proximity
Effect". These general discussions may help:
Proximity Effect www.sigcon.com/Pubs/news/4_1.htm
Proximity Effect II www.sigcon.com/Pubs/news/4_3.htm
Proximity Effect III www.sigcon.com/Pubs/news/4_8.htm
In 2005 I found a way to directly observe, visually, the path of returning signal current. I would like to express my profound gratitude to Mr. Robert Trautman of Owego, NY for putting together the technical apparatus necessary
for making this experiment and taking these extraordinary photographs.
http://www.sigcon.com/Pubs/news/8_08.htm
Other discussions of returning signal current are here:
http://www.sigcon.com/Pubs/pubsKeyword.htm (see keyword: "Return Current")
Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 5099970750, howie03@xxxxxxxxxx
www.sigcon.com  HighSpeed Digital Design seminars, publications and films