IEEE Circuits and Systems Magazine - Q3 2020 - 36

Debugging such problems can be active, when the
board is assembled and operational, or passively, with a
bare PCB. Active testing, can be done with a high-speed oscilloscope to detect, for example, glitches in a digital signal
due to coupling. Passive testing can be done with a TimeDomain Reflectometer (TDR) or a Network Analyzer (NA).
In both cases, a suspected PCB trace is excited, and the
response is monitored to identify potential issues.
Recently, professional PCB design tools became capable of performing advanced signal integrity simulations, which help in the prognosis of potential problems
and avoiding them.
III. On-Board Transmission Lines
With the increasing speed of on-board signals, it is essential to analyze signal propagation from a transmission
line perspective. As a result, this section summarizes
the properties of the most common planar transmission
lines, in addition to current return paths and matching. It
is important to mention here that many other structures
of planar transmission lines exist [14], [15]. The routing
concepts, however, are identical.
For the scope of this tutorial, a transmission line is a
two-conductor structure, with an unvarying cross section.
This will ensure that the electric and magnetic fields dis--
tribution is undisturbed during signal transmission. The
relationship between the electric and magnetic fields is determined by the physical dimensions of the transmission
line, and is quantified as Z0, the characteristic impedance
(typically a real value). For the best performance in any
Single-Ended
S

Differential
S+ S-

(a)
G

G (Optional)

S+

S-

G
(c)

Figure 2. Structures of common planar transmission lines in
SE and DIFF format (a) Microstrip, (b) Coplanar Waveguide,
and (c) Stripline. Many other structures exist [14], [15].

A. Transmission Line Types
1) Microstrip
Fig. 2(a) shows the structure of a microstrip for Single
Ended (SE) and Differential (DIFF) applications. Due to
the dielectric-air interface boundary conditions, a small
electric and magnetic fields exist longitudinally [17]. Since
there is no exact definition for the characteristic impedance (Z0) for such hybrid-mode of propagation, approximate equations are used. Typically, Z0 for a microstrip is
found using Finite Element Method (FEM) simulators. For
a given substrate and copper thicknesses, the impedance
can be controlled using the line width.
In addition to ohmic and dielectric losses, microstrip
lines lose energy through radiation [14]. If the radiation loss
is dominant, which can occur at high frequencies, it can be
reduced by using a high dielectric constant fr, and a thinner substrates. This ensures that the majority of the fields
are within the substrate, reducing radiated energy.
The structure of a microstrip allows for an easy series components. Shunt components, on the other hand,
require through hole vias, which can limit the performance of the line.
2) Coplanar Waveguide (CPW)
The CPW was first presented by C.P. Wen in [18]. The
structure shown in Fig. 2(b) for SE and DIFF CPW. Similar to
microstrip lines, CPWs do not support a pure TEM mode.
As a result, either approximate equations, or FEM simulations are used to find Z0. CPWs give a two-dimensional control over the impedance, for a given substrate and copper
thicknesses. The width of the line and the gap between the
line and the ground can both affect the impedance. As a
result, there is no unique design for a given impedance.
Adding series or shunt components to a CPW lines
does not require any vias, which can be advantageous
for certain circuits. Furthermore, CPWs provide a convenient structure for probing signals on board (using
GSG probes for example).

G (Optional)

(b)

G
Optional Vias

S+ S- G

circuit, Z0 and the load impedance should be equal. Further
reading material on transmission lines include [1], [16].

3) Stripline
Striplines, shown in Fig. 2(c), can support the TEM mode.
Also, since the fields are contained in the dielectric material, no dispersion occurs, in contrast to microstrips
and CPWs.
While there are exact equations for the impedance of
a strip line [19], they assume zero-thickness lines. To
account for copper thickness, approximate equations
are used. Consequently, designers still rely on FEM simulators to calculate Z0.

IEEE CIRCUITS AND SYSTEMS MAGAZINE

THIRD QUARTER 2020

IEEE Circuits and Systems Magazine - Q3 2020

Table of Contents for the Digital Edition of IEEE Circuits and Systems Magazine - Q3 2020