60 The PCB Design Magazine • November 2017
Figure 3 illustrates the transition from a mi-
crostrip transmission line to a SIW. The propa-
gating electromagnetic wave, which is guided by
the microstrip trace, travels through the dielec-
tric, solder mask and air. However, as the wave
enters the SIW, it begins to tunnel between the
ground planes and as such, the dispersion losses
are solely based on the losses of the substrate
material. A homogeneous, ultra-low loss dielec-
tric provides the best frequency response.
Elaborating on how the requirements, of the
transition, are calculated is beyond the scope of
this column however, the associated equations
are provided in a paper by Kumar et al
[3]
. Al-
though, I can confirm that the impedance of
the microstrip trace is 50.45Ω
(simulated by the
iCD Stackup Planner), one would expect the
impedance to remain constant at ~50Ω through
the SIW to perfectly transfer the energy. The
simulation, of the electric field, shows how the
losses reduce as the electromagnetic wave en
-
ters the SIW. Here the field become more in-
tense, and less distributed, providing clarity of
signal and thus higher bandwidth. Obviously,
another similar transition back to microstrip,
at the other end to receive the signal, is also
required.
Substrate integrated waveguides are low loss
structures that provide high bandwidth and
eliminate the need for both differential serial
(SERDES channels) and space consuming paral-
lel busses. They exhibit similar performance to
traditional waveguides but, can be built as pla-
nar PCB structures. This greatly reduces the cost
and tremendously improves the performance,
of data transfer, compared to the traditional
PCB interconnect to 100GHz and beyond.
Points to remember:
• Conductor size, dielectric loss, copper
roughness, and data transfer capacity im-
pact on the performance of copper inter-
connects at high frequencies.
• Recently substrate integrated waveguides
(SIW) structures have emerged as a viable
alternative.
• SIW are planar structures fabricated using
two periodic rows of PTH vias or slots con-
necting top and bottom copper ground
planes of a dielectric substrate.
• SIW retain the low loss property of con-
ventional metallic waveguides.
• PCB interconnects have limited current
carrying capacity, high dielectric loss,
rough copper surfaces and restricted signal
data transfer capacity.
• SIW propagating modes are very close to,
but not identical to, those of the rectangu-
lar waveguides.
• The most distinguishing characteristic of
the SIW is the current distribution of the
vias, which is limited to the vertical direc-
tion only.
• Microstrip to SIW transition is undoubt-
edly the simplest to implement.
• Substrate integrated waveguides are low
loss structures that provide high band-
width and eliminate the need for both
differential serial (SERDES channels) and
parallel busses.
PCBDESIGN
References
1. Barry Olney's Beyond Design columns:
Microstrip Coplanar Waveguides, Effects of Sur-
face Roughness on High-Speed PCBs, Transmis-
sion Line Losses.
2. SI List forum: Scott McMorrow, Yuriy
Shlepnev.
3. A Review on Substrate Integrated Wave-
guide and its Microstrip Interconnect, by Ku-
mar, Jadhav, Ranade.
4. Substrate-Integrated Waveguide Transi-
tions to Planar Transmission-Line Technologies,
by Taringou, Dousset, Bornemann, Wu.
5. Design for Tapered Transitions from Mi-
crostrip Lines to Substrate Integrated Waveguide
at Ka Band, by Mehdi, Keltouma, Mohammed.
Barry Olney is managing director
of In-Circuit Design Pty Ltd (iCD),
Australia, a PCB design service bu-
reau that specializes in board-level
simulation. The company developed
the iCD Design Integrity software
incorporating the iCD Stackup, PDN and CPW
Planner. The software can be downloaded from
www.icd.com.au.To contact Olney, or read past
columns, click here.
NEXT-GEN PCBS—SUBSTRATE INTEGRATED WAVEGUIDES
