Importance of return current

[arduino_guy]

Hi,
So far I have been thinking most of the electronics in terms of voltage. But now when I tired to learn a few things about industrial pcb design, it seems that this so called return current is so important and everyone is suggesting that it is best to have the return current coupled with the original signal current. This seems to be due to a wide variety of reasons. So please do chip in your thoughts on why it is best to have a return current coupled to the original signal and other mysteries surrounding return current.

[dmills]

Current flows in loops (always), and the amount of magnetic energy stored in the resulting field is proportional to the loop area, so keeping the enclosed area of the loops small minimizes radiation, susceptibility and signal integrity headaches (Three good reasons to pay attention to this stuff).

Fortunately at any frequency above `almost DC` the presence of a reference plane on an inner layer will automatically do this as the path of least impedance will be the one that minimizes loop area for any given loop, there is a reason you only see 'star earths' used on near DC boards, everyone else uses ground planes.

73 Dan.

[arduino_guy]

Hi dmills,

Thanks for the reply. It makes sense that a large loop area will increase radiation emissions. But how does that affect signal integrity?

[basinstreetdesign]

A large loop area constitutes a larger inductance around that path since the larger area causes larger magnetic fields to be created.  These larger magnetic fields impede the changes in current around that same loop.  This is seen by any signal that is trying to traverse that path as an AC impedance that rises with frequency.  All fundamental stuff, right?

So, say you have a fast signal that is to go from A to B along a pcb track.  The signal travels as a charge wavefront along that track.  But it also has to find a a return path to the source as all signals must.  This is true be it a fast digital pulse or an RF signal or audio or whatever.  If the least-impedance path is on a ground plane layer right under the track in question, the loop is small (squished flat), has low inductance and the impedance of the entire loop is relatively low.  All signals can travel that path with little loss or distortion as long as the terminating impedance is matched to the transmission line.  If that path has a slot in the ground plane partway along or the track take a detour away from an easy return path then the return current must find a path around that slot, extending its length and enlarging the round-trip loop.  Bigger inductance and losses result.  Also all signals will reflect from the change in impedance that occurs at every change in path.  This causes reflections, more radiation, etc, etc.  For fast digital stuff that spells possible data loss, for high-quality high-bandwidth (video) analog it means distortion and even for high-end audio it can mean a not-so-great frequency response.

If that wasn't bad enough then there is another effect that shows up as a dual of the radiation situation.  Any element that can radiate energy can intercept it as well.  So any bad pcb layout that allows signals to radiate can be a victim of EMI susceptibility of the same kinds of signals.  A wire that transmits at X MHz  will also receive the same.  Usually this isn't so bad since the signal level that will be captured on such a wire from an outside source is much lower than the signals it normally carries but it can happen anyway.

[Harsh Chandola]

Hi,
So far I have been thinking most of the electronics in terms of voltage. But now when I tired to learn a few things about industrial pcb design, it seems that this so called return current is so important and everyone is suggesting that it is best to have the return current coupled with the original signal current. This seems to be due to a wide variety of reasons. So please do chip in your thoughts on why it is best to have a return current coupled to the original signal and other mysteries surrounding return current.

One of the reasons are to cancel out the Electromagnetic radiation, so whatever radiation your signal path creates would be cancelled out by your return current path if they are close together.
if you have high frequency AC going through your signal path, the path'll also have constantly changing magnetic field around it, so when you couple your return path with the signal path, your return path will also have changing magnetic field around it but in opposite direction which would cancel out the Electromagnetic radiation from your tracks which otherwise could have interefered with other nearby devices or could have induced current in your own circuit.
Also try to have smaller loops of path on your PCB to avoid making it an antenna :p

Correct me if I'm wrong.

[arduino_guy]

Hi,

Thanks for your replies. Now things make more sense. Let's keep this open for people to chime in their ideas and what they know about this topic.