22 DESIGN007 MAGAZINE I MARCH 2022
rent, using good stackup design practices, and
by lowing the AC impedance of the PDN by
using distributed embedded capacitance.
Key Points:
• e first rule of stackup design is that all
signal layers should be adjacent to, and
closely coupled to, an uninterrupted
reference plane, which creates a clear
return path.
• Return path discontinuities (RPDs) can
create large loop areas that increase series
inductance, degrade signal integrity, and
increase crosstalk and electromagnetic
radiation.
• Common mode radiation is the result
of parasitics in the circuit that emanate
from the unwanted voltage drops in the
conductors.
• If the return path of a common mode
current is far from the signal path, then
the common mode current will radiate.
• ECM technology provides an effective
approach for decoupling high-perfor-
mance ICs whilst also reducing elec-
tromagnetic interference. Coupling the
planes very close together creates high
capacitance.
• ECM provide distributive decoupling
capacitance that take the place of
conventional discrete decoupling
capacitors over 1 GHz.
• Differential mode propagation can be
converted to common mode by parasitic
capacitance or any imbalance caused by
signal skew, rise/fall time mismatch, or
asymmetry in the channel.
• In the case of differential pairs, the
transformation from differential mode to
common mode typically takes place on
bends and non-symmetrical routing, near
via and pin obstructions, but can also be
caused by small changes in impedance
due to RPDs.
• Differential signals that are closely coupled
will operate mainly in the differential
mode with some common-mode radiation
from imbalances in the signals.
• If the two traces of a differential pair are
separated enough, to prevent coupling,
then both act as single-ended signals.
• As signal traces come into proximity of
an aggressor signal, part of that signal is
unintentionally electromagnetically
coupled into the victim trace as noise,
due to the overlapping of EM fields.
• e easiest way to reduce crosstalk from
a nearby aggressor signal is, of course, to
increase the spacing between the signals
in question.
• By varying the trace height, one can also
control the coupling—hence crosstalk.
If real estate is limited, then this may be
a better solution.
• Crosstalk is typically picked up on long
parallel trace segments. ese can be on
the same layer but may also be broadside-
coupled from the adjacent layer. DESIGN007
Columns by Barry Olney
• "Stackup Configurations to Mitigate Crosstalk"
• "The Dark Side—Return of the Signal"
• "Stackup Planning, Part 1"
• "The Curse of the Golden Board"
• "Material Selection for SERDES Design"
• "Common Symptoms of Common Mode
Radiation"
Barry Olney is managing
director of In-Circuit Design
Pty Ltd (iCD), Australia, a PCB
design service bureau that
specializes in board-level sim-
ulation. The company devel-
oped the iCD Design Integrity
software incorporating the
iCD Stackup, PDN, and CPW Planner. The software
can be downloaded at www.icd.com.au. To read
past columns or contact Olney, click here.
