Current flows in the traces (it is the movement of electrons). Energy
flows in the magnetic field around the traces.
The signal flows in one trace and the return flows on the other trace.
As long as the signals are "equal and opposite", no other return path is
necessary. If they are not equal and opposite (which happens when there
is a signal offset caused either by the components or by unequal trace
lengths (or unequal path lengths around a corner), then there is a
common mode (even mode) signal that is generated that MUST FLOW
SOMEWHERE. That is why a plane is (strongly recommended.) Thus, routing
on a layer without an underlying plane can be done, and field solvers
used to calculate impedance (and reflections can be controlled) as
described in my previous post. But the lack of a plane MAY cause a
problem related to common mode signals caused by signal offsets, or by
sensitivity to radiated EMI sources. Those are the tradeoffs.
Doug Brooks
(We are still running the promotional dpecial of providing a free pdf
copy of our latest book with the purchase of UltraCADs PCB trace calculator}
Kevin G. Rhoads wrote:
On 4/8/2017 2:15 AM, Binayak Shrestha wrote:
what is the actual return conductor for your signal Your signal is anelectromagnetic wave flowing in-between the signal conductor and its return
_________________
Duh. Pardon my French, but DUH. Strongly seconded.
As EEs are taught to think of voltages as signals, they are also not
taught with the same degree and amount of training that there are NO
voltages without CURRENTS.
Where is the return current going? What path(s) does it take? After
you consider the voltages (or voltage waves or electric fields) you also
need to consider the currents (and current waves and B-fields).
The current GOING and the current RETURNING. And whatever they will
interact with via capacitive and inductive coupling.
If your training includes electical field theory, great! Use it. Not to
try to solve things, but to visualize what this circuit is actually
going to be and do.
If your training does not include electrical field theory, MIT's OCW has
made a textbook available for non-commercial use:
http://www.scribd.com/doc/84765268/Emf-Theory-d-1
http://www.scribd.com/doc/84765360/Emf-Theory-d-2
http://www.scribd.com/doc/84765475/Emf-Theory-d-3
Mark Zahn's book is a good intro to electrical field theory. That was
my area of specialization, and I am still learning -- which is why I
lurk here on the SI list. SI has much to teach me, and even retired I
am still learning.
HTH
Kevin
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