The Bridge - Issue 2, 2022 - 19

Return Path Discontinuities and Common Return Path Issues
Feature
to survey several potential trouble solutions.
The following sections investigate geometries
where the return structure has a significant effect
on signal integrity, EMC, or both.
Fig. 6. Parallel wire (ribbon cable) configurations with references (red) and signals (blue)
A. Ribbon Cables
Ribbon cables were once a common structure to
connect storage media to motherboards. Some
ribbon cables are designed with many signal
conductors and a single return conductor. All return
currents flow on one single return wire. As a result,
all signals are inductively coupled. (The signals are
also capacitively coupled, but the focus will be on the
inductive aspect here.)
Early revisions of the parallel ATA bus ran at relatively
low baud rates with low slew rates, leaving most of
the energy at lower frequencies. Since the crosstalk
from mutual inductance is more pronounced at
higher frequencies, one reference could be shared
by several signals. As baud rates increased in later
revisions of the ATA standard, this crosstalk became
an issue. The solution to the inductive coupling
problem was to introduce one return wire for every
signal wire. Fig. 6 shows three configurations: one
with one return for each signal (1:1 ratio) and two
others with one return for three signals (3:1 ratio).
Fig. 7 shows the far-end crosstalk (FEXT) for the
three configurations from Fig. 6. The FEXT of the
configuration with multiple references is one order of
magnitude lower than the other two configurations
through most of the modeled bandwidth. Even so,
individual return conductors are not always necessary.
Next, the FEXT was evaluated with a transient model
for another perspective. The FEXT was measured
on the middle signal conductor and plotted in Fig.
8. The left-most conductor of three channels was
excited with a 1-volt, 4 Mbps signal and a 40 Mbps
signal. The shape and amplitude of the crosstalk
are affected by the source and load impedances at
either end of the conductors (Fig. 8 includes 20 Ω
at the source and 1 kΩ at the load). The crosstalk
for any of the configurations is low enough to be
ignored at 4 Mbps. Even at 40 Mbps, the crosstalk is
likely tolerable, but the FEXT of the configuration with
multiple returns is negligible,
whereas that of the other two
is on the order of
40 mV.
Coupling of the same type as in these ribbon cable
geometries can occur in PCB structures. Replace the
ribbon cable geometry with an array of vias. When
return currents for multiple signals are forced to share
return vias, there will be coupling. The dominant
form of this coupling is inductive; the current paths
have a common segment and will have magnetic
flux coupling as a result. Connector footprints are
another common geometry where signals may share
return paths. In another example, the area within the
footprint of large ASICs can become so perforated
with antipads that narrow return path channels may
be shared by some signals unfortunate enough to be
buried in the interior of the pin array. These examples
represent some but not all possible shared return
path structures.
B. Splits in Reference Planes
A classic example of a return path discontinuity is
crossing a split in a reference plane. The issue is the
same whether the geometry is called a split, a slit, or
a void. The current return path has an obstruction to
conduction that must be circumvented or bypassed.
Circumvention extends the current path away from
the signal current path, which can be represented by
a parasitic inductance, as in Fig. 2. If the capacitive
coupling from one edge of the split to the other
is strong enough or the signal frequency is high
enough, currents may pass by displacement.
Fig. 9 shows the layout of the geometry considered
in this section. The two microstrip traces shown are
routed 40 mil above a reference plane with a 500
µm slit. The primary trace is centered vertically on the
PCB. A secondary trace, used as a victim for crosstalk
experiments, is below the primary with a dotted
outline. The ideal return path is directly beneath the
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The Bridge - Issue 2, 2022

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