56 The PCB Design Magazine • April 2017
• Each signal layer should be adjacent to,
and closely coupled to, an uninterrupted refer-
ence plane, which creates a clear, uninterrupted
return path.
• Although power planes can be used as ref-
erence planes, ground is more effective as local
stitching vias can be used, for the return current
transitions, rather than stitching decoupling ca-
pacitors which add inductance.
• The return current of a high-speed, fast rise
time digital signal will always follow the path of
least inductance.
• RPDs tend to divert the return current in-
creasing the loop area, inductance and delay–
which is not desirable.
• A via that provides the connection between
signal traces, referenced to different planes, cre-
ates RPDs.
• The skin effect forces the return current to
the surface closest to the signal trace.
• It is important to have a clearly defined
return current path and to know exactly where
the return current will flow.
• RPDs can never be totally eliminated but
we can take steps to minimize them significantly.
• It is all about inductance! If the return
path loop area is increased, in any way by RPDs,
then the inductance will also increase.
• The use of a number of central GND planes,
with signals on either side, will mitigate the RPDs.
• By reducing the size of the cavity with a
thin, high dielectric constant (Dk) material,
ringing at low frequencies is reduced and the
cavity resonance moves in to the upper band.
PCBDESIGN
References
1. Barry Olney Beyond Design columns:
Stackup Planning, Part 3, The Dumping Ground.
2. Power Integrity Modeling and Design for
Semiconductors and Systems, by Madhavan
Swaminathan.
3. Qualifying the Impact of Non-Ideal Re-
turn Path, by Byers and Hall.
4. High-Speed Digital Design, by Howard
Johnson.
Barry Olney is managing director
of In-Circuit Design Pty Ltd (ICD),
Australia. This PCB design service
bureau specializes in board-level
simulation, and has developed the
ICD Stackup Planner and ICD PDN
Planner software. The software can be download-
ed from www.icd.com.au. To read past columns,
or to contact Olney, click here.
RETURN PATH DISCONTINUITIES
Harnessing wave energy by
localizing it and suppressing its
propagation through a medium
is a powerful technique. Now, Al-
agappin Gandhi and Png Ching Eng Jason from the
A*STAR Institute of High Performance
Computing
have calculated a design that localizes light in tiny
loops, within a two-dimensional structure created by
merging two lattices of slightly differing periodicities.
The new technique is not limited to light, and
may enable the design of systems that can precisely
control wave energy in any realm and at any scale.
The ability to create resonators in which light is
localized on the surface of a device also has applica-
tions in quantum computing components based on
light, such as defects in diamond.
Gandhi and Png designed the
structures by superimposing lattices
of small circular dielectric materials
with periods in a simple ratio R:R-1 — for example
one lattice is merged with another whose spacing is
4/3 as big, or 5/4, 6/5 etc.
"It creates a two-dimensional effect similar to
beats between two waves of very close frequency,"
Gandhi said. "Where there are antinodes the light is
localized in the form of a closed path."
Gandhi and Png ran numerical simulations of
the propagation of light in a range of wavelengths
slightly below that of the lattice spacing, and calcu
-
lated the energy band structure.
The Perfect Pattern to Trap Light
