The Bridge - Issue 2, 2022 - 18

Feature
Return Path Discontinuities and Common Return Path Issues
C. Considerations for Differential Signaling
Differential signaling utilizes at least three conductors
to transmit signals: two signal conductors and at least
one reference conductor. One might assume that
the reference plays no part in signal propagation, but
discontinuities can still affect the signal.
With differential signaling, the voltage and current
on one conductor of the pair are ideally equal and
opposite polarity of that of the other conductor.
Many layout features can unbalance the signal
in a differential pair. This unbalancing is more
appropriately called mode conversion. The differential
mode is converted to the common mode, which is
subject to the return path discontinuities in the same
way as a single-ended signal. EMC issues, such as
unintended electromagnetic radiation, may result. A
difference in the length of the differential pair, also
known as skew, is one layout issue that causes mode
conversion. As Section III-B explains, return path
discontinuities may also cause mode conversion.
Differential via transitions are subject to mode
conversion from asymmetric return currents, as well
[5]. Differential signals are less susceptible to return
path issues, but they are not entirely immune.
Fig. 4. Stripline cross-section with dominant signal and return current
paths with a critical return path discontinuity
Fig. 5. The current path across two PCBs with a missing return path
D. System-Level Return Path Considerations
While this article focuses primarily on return path
issues at the PCB level, the same concepts apply at
the system-level. The scale and complexity increase,
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but the same strategies may be employed. Currents
return to their sources; follow the current paths.
System-level has a different meaning depending on
the device under test. The system-level pertains to
buses routed between chips on the motherboard
and adapter cards or routed on cables internally
or to other frames for computers and servers.
Maintaining reference continuity for the return
path across these different types of connections is
critical. Miscommunications between designers can
lead to missing connections like the missing return
connection at the connector shown in Fig. 5. Because
the channel comprises multiple PCBs, the search
space is larger and fraught with chances to cast
blame and hurt feelings.
Historically, airplanes and automobiles have even
used the chassis as a return path. Since these
frames tend to be prominent conductors, they make
sufficient returns for low frequencies. However, the
paths may include large loops that invite coupling
interference even at low frequencies. These large
loops are extreme cases of what is presented later
in Section III-A. Higher frequency signaling should
maintain a separate return path for sensitive signals.
As more vehicles shift to composite materials, the
conduction paths used in the past are missing.
Designers must include return paths in the cable
harnesses and make sure that the returns isolate
sensitive signal paths from one another. One
temptation is to include a single return wire in a cable
bundle and declare it good. The harness will likely
have the same issues as a ribbon cable with a single
return. A frame return path has more in common
with a PCB reference plane than with a return wire.
Low frequency current can spread out more on a
frame. Beware! This spreading and exposure invite
interference from external signals. Higher frequency
currents should still have their own dedicated
return wires.
III. EXAMPLE RETURN PATH ISSUES
This entire article could easily focus on a single
geometry. However, this article aims not to promote
the best solution to a particular geometry, but rather
The Bridge - Issue 2, 2022

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