The Bridge - Issue 2, 2022 - 16

Feature
Return Path Discontinuities and Common Return Path Issues
like two points with an open field between them. The
logical direction is to travel in a straight line from start
to finish on the other side. However, if the straightline
path is blocked (impeded), a longer route must
suffice. A longer dry route would still be quicker and
less messy than a shorter route through a mire, for
example. Of course, there is not just one possible
alternative route but numerous possible routes.
Another complication is that the return path does
not always mean the reference plane. A voltage
supply can be a return path, as well [2]. A push-pull
driver topology is an example of a structure that
utilizes both voltage supply and the reference. One
way or another, the current will return to its source.
The loop may not be obvious, but the current will
find a way back to its origin. How much current
flows is dependent on the impedance of the entire
path. Pick the path, or the current will, whether by
conduction or displacement. [3] employs a detailed
plot of the return currents to analyze the return path
discontinuity in the transition from PCB to an edgemount
coaxial connector. Such current paths are
key to understanding any return path discontinuity.
The remainder of this article will discuss return path
discontinuities and the issues they present through
basic theory and several modeling examples.
II. RETURN PATH DISCONTINUITIES
Return path discontinuities differ from signal
discontinuities only in that they occur in the return
part of the signal path. Return paths tend to have
significantly different geometries than signal paths.
The current is less constrained than on a signal trace,
as is indicated by Fig. 1. The spread of the current
is highly dependent on frequency. High frequency
currents will ideally mirror their signal counterpart,
which minimizes the inductance in the current loop.
Low frequency currents spread out across reference
conductors to reduce resistive losses, because
inductance is less dominant. As a result, return path
discontinuities can be subtle, like a trace near the
edge of a reference plane, or they may be significant
and acute like a missing return via a connector.
Return path discontinuities may be misidentified as
THE BRIDGE
Fig. 1. A microstrip geometry with approximate current profiles overlaid
issues in the signal path. They also affect different
signaling structures in different ways.
A. Misidentification
Misidentifying return path discontinuities as
discontinuities in the signal path is a nontrivial
problem. A transmission line model implies
transverse electromagnetic (TEM) propagation
(or quasi-TEM). One of the conditions for TEM
propagation is cross-sectional invariance. The crosssection
of the conductors cannot change along the
length of the model. Deviation from the cross-section
is, by definition, a discontinuity. Another property of
the transmission line formulation is that a bundle
of N conductors can support N-1 TEM modes [4].
The other way of viewing this condition is that any
transmission line structure must have at least two
conductors to support a TEM mode. Assume one of
these conductors is a signal, and the other is a return.
A change to the shape of either conductor introduces
a discontinuity. The TEM mode requires both
conductors to be cross-sectionally invariant to avoid a
disturbance in propagation; however, changes to the
signal path are less likely to be overlooked than those
in the return path.
Fig. 2. A transmission line circuit with a parasitic inductance
in the return path
Consider the transmission line circuit in Fig. 2. This
circuit has an inductance in the return path between
the two transmission line elements. The procedure
for solving this circuit begins at finding the solution for
the voltage on the first line for
which results in
The Bridge - Issue 2, 2022

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