The Bridge - Issue 2, 2022 - 17

Return Path Discontinuities and Common Return Path Issues
Feature
where
is a step function. The corresponding
current solution is
This solution is valid until
when the
wave is incident at the end of the first line.
The boundary condition at
is
is evaluated to
determine the reflected wave on line 1 and the
forward wave on line 2. The condition on the current
at
due to the continuity requirement. The boundary
condition on the voltage can be determined by a KVL
expression and is
This boundary condition is the key concept in this
section. Given the form of (4), there is no way to
tell whether the voltage drop associated with the
inductive discontinuity is in the signal path or the
return path.
Time domain reflectometry (TDR) is one of the best
measurement techniques to diagnose discontinuities
in signal paths. However, the incident wave used by
TDR is simply a less ideal version of the transmission
line solution in (1). The TDR measurement will see
the discontinuity in a circuit like Fig. 2, but it will
not indicate that the discontinuity is in the return
path. Such will be the case with most return path
discontinuities. The TDR response will still show the
delay to the location of the discontinuity. Then, the
search for the cause of the disturbance can start,
investigating the signal path and any possible
return paths.
B. Considerations for Stripline Signaling
Microstrip and stripline structures are the most
common signal configurations in PCB designs.
Microstrip utilizes a single conductor for the signal
and a reference plane for the return path. All return
currents utilize this single reference plane and
any break in the path will lead to a discontinuity
as discussed in Section II-A. Stripline uses a single
signal conductor sandwiched between two reference
planes. As a result, return currents may use either
reference, but the references may not be used
equally, depending on the overall signal path.
Figs. 3 and 4 illustrate two different stripline crosssections
with return path discontinuities. The
dominant return current paths are drawn for both
cases. The first in Fig. 3 is positioned in the lower
reference. There is a clear path for the return currents
to mirror the signal currents without encountering
the discontinuity in the lower reference. While there
may still be a disruption in the stripline propagation
mode due to the break in the lower reference, the
discontinuity may be tolerable.
Fig. 4 includes a break in the upper reference. This
break disrupts the lowest impedance return path for
the currents. The current will find a path back to the
source although possibly through a high impedance
path. The question marks in Fig. 4 are there because
the return path is unclear from what has been drawn.
Placing return vias near the two signal vias shown in
the cross-section may be sufficient to mitigate this
return path discontinuity for lower speed signals.
Without additional return vias, the return currents
will find the nearest existing return vias and pass
by displacement current paths. Neither will be as
low impedance as a solid reference path or wellplaced
return vias.
Since return paths can be difficult to predict for
complex designs with many PCB layers and signals,
design rules commonly require reference vias within
a certain radius of any signal via transition. Even
without a break in either reference plane, return vias
are beneficial. The return vias ensure that return
currents have a better path to either reference, which
is expected in the stripline propagation mode.
Fig. 3. Stripline cross-section with dominant signal and return current
paths with a noncritical return path discontinuity
HKN.ORG
17
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The Bridge - Issue 2, 2022

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The Bridge - Issue 2, 2022 - Cover1
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