30 The PCB Design Magazine • October 2014
• The faster edge rate, for the same frequen-
cy and same length trace, creates ringing in the
un-terminated transmission line. This also has a
direct impact on radiated emissions.
• Transmission line effects become an im-
portant design consideration when the trace
length approaches 1/6 of the wave-length of the
signal being transported.
• Impedance is the key factor that controls
the stability of a design—it is the core issue of
the signal integrity methodology.
• Any mismatch in impedance along the
transmission path, will result in a reduction in
quality of the signal and possibly radiation of
noise.
• Series terminations are generally required
at fast edge rates to limit ringing.
• All drivers, whose trace length (in inches)
is equal to or greater than the rise time (in ns),
must have provision for termination.
• Differential pair return current actually
flows in the reference plane below each trace
not in the opposite signal.
PCBDESIGN
References
1. Barry Olney's Beyond Design Columns:
Practical Signal Integrity, Pre-Layout Simula-
tion, Intro to Board-Level Simulation and the
PCB Design Process, and Impedance Matching:
Terminations.
2. Howard Johnson: High-speed Signal Prop-
agation.
3. Henry Ott: Electromagnetic Compatibil-
ity Engineering.
The ICD Stackup and PDN Planner is distrib-
uted globally Altium.
Barry Olney is managing direc-
tor of In-Circuit Design Pty Ltd
(ICD), australia. This PCB design
service bureau specializes in
board-level simulation, and has
developed the ICD Stackup Plan-
ner and ICD PDn Planner software. To read
past columns, or to contact Olney, click here.
SIGNAL INTEGRITY, PART 1 OF 3 continues
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layers, though he is seeing a
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