October 2017 • The PCB Design Magazine 25
• The characteristic impedance (Zo), is de-
fined as the instantaneous impedance of
a lossless transmission line.
• The most fundamental signal integrity
analysis involves defining the board
stackup, which is essential to generating
accurate results.
• Dielectrics, specifically designed to be
used for high-speed design, have a very
low dielectric constant.
• Closed-form equation based impedance
calculators are extremely limited in
accuracy and the use of a precision field
solver is mandatory.
• A series resistor can be used to match the
driver to the impedance of the transmis-
sion line.
• Termination is required if the trace length
exceeds one sixth of the electrical length
of the rising edge rate.
• High-speed digital design is all about
controlling impedance.
PCBDESIGN
References
1. Barry Olney's Beyond Design columns:
Controlled Impedance Design, Impedance Match-
ing: Terminations, Signal Integrity Parts 1, 2 & 3
2. What's the Difference Between Signal In-
tegrity and Power Integrity?
by Patrick Carrier
3. The Critical Length of a Transmission Line,
by Eric Bogatin
4.
Bandwidth Basics, Wavelength Electronics
5. IPC-2251–Design Guide for the Packaging
of High-Speed Electronic Circuit
6. High-Speed Digital Design, by Howard
Johnson
Barry Olney is managing director
of In-Circuit Design Pty Ltd (iCD),
Australia, a PCB design service
bureau 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.
High-speed digital design is all about con-
trolling impedance. The impedance of the
transmission lines needs to be matched and
maintained at a constant value along the en-
tire length of the interconnect. Also, the power
distribution network needs to provide a low
impedance path, through the planes, across the
entire frequency bandwidth of the signal. These
seemingly unrelated disciplines control the sta-
bility and reliability of the product. Get it right
and your high-speed design is off to a great
start; get it wrong and you are in for a (real)
world of pain.
Points to Remember
• At high frequencies the PCB trace now
behaves as a distributed system with para-
sitic inductance and capacitance charac-
terized by delay and scattered reflections.
• The transmission line effects create under
and over shoot resulting in ringing in the
signal.
•
The Fourier series expansion of a square
wave is made up of a sum of odd harmonics.
• A trace becomes a transmission line sim-
ply when the length is comparable to the
signal rise time.
• All drivers whose trace length (in inches)
is equal to or greater than the rise time
(in nanoseconds) should be considered
critical and treated as high-speed
transmission lines.
• Impedance is the key factor that controls
the stability of a design; it is the core
issue of both the signal and power
integrity methodology.
• For perfect transfer of energy, the
impedance at the source must equal the
impedance at the load.
• Terminations are generally required, at
fast edge rates, to match the impedance
and limit ringing.
• As core voltages drop, rise times become
faster and frequency increases, and a
lower impedance is more desirable.
• The measured input impedance of a
transmission line looks remarkably
similar to the AC impedance of a plane
cavity's resonance which also has no
termination.
WHEN DO TRACES BECOME TRANSMISSION LINES?
