The Bridge - Issue 2, 2022 - 21

Return Path Discontinuities and Common Return Path Issues
Feature
the pitch of the connector, but the vias may also
require a significant antipad to minimize the excess
capacitance in the via transition. This issue is
common with differential connectors using compliant
pin technology. Often signal vias of a differential
pair share a larger common antipad. Regardless, if
these large antipads are combined with excessive
misregistration between layers in the PCB, signal
traces may inadvertently cross the antipad.
This asymmetry will unbalance a differential pair, and
a single-ended line will see an impedance change
within the via array.
C. Return Via Discontinuities
While splits in return planes are one of the more
apparent return path discontinuities, issues with
return vias can be more challenging to diagnose.
More complex does not mean subtle. The effect will
be pronounced if the return via is missing entirely,
including a sharp inductive discontinuity in a TDR
trace. However, the TDR trace may not clarify whether
the issue is the signal path or the return path. Fig. 5
showed a missing return via. Signals may also share
return vias as part of the return path, like the crosstalk
issues discussed in Section III-A.
Fig. 8. Far-end crosstalk with different signaling rates for
the different ribbon cable geometries
Fig. 17 shows a microstrip-to-microstrip transition
from the side and top. The signal path is clear.
Presumably, the return path is clear as well. There is
a return via placed at some distance from the signal
via. The geometry in Fig. 17 was modeled at multiple
frequencies and multiple via spacings. Current was
measured using an Ampere's law calculation around
the signal via and return via individually. At 10 kHz,
the ratio of the return via current to the signal via
current ranged from 0.55 at a 30 mm separation
to 0.75 at 2.5 mm separation. At 1 GHz, the ratio
of the return via current to the signal via current
ranged from 0.08 at a 30 mm separation to 0.37 at
2.5 mm separation. These estimates are subject to
meshing and modeling errors. The exact values are
less important than the trend they indicate. A closer
return via will pass a higher percentage of the return
currents than one farther away. When the return via is
too far away, higher frequency currents find another
route to complete the loop displacement current
between the planes. Placing one or more return vias
near the signal via will mitigate, if not eliminate, most
return path coupling and discontinuity effects.
Fig. 9. Top view of a microstrip line crossing a split in
the reference plane.
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21
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The Bridge - Issue 2, 2022

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