38 The PCB Design Magazine • February 2014
minators close to the source to dampen the re-
flections.
Points to Remember
• A series terminator is best for high-speed
transmission lines.
• Bifurcated transmission lines—traces that
are split into two or more T-sections—are some-
times used to distribute signals to multiple
loads.
• At the branch, the two 50 ohm traces in
parallel equate to a 25 ohm equivalent trace—
and a mismatch in impedance.
• Trace width is not directly proportional to
impedance.
• Bifurcated transmission lines with matched
impedances are a better choice.
• The end terminators ensure that the trans-
mitted pulse progresses once down the line and
then stops, preventing reflection.
• Matching branched lines are not often
used due to the difficulty of fabricating traces
of widely varying impedance on the same sub-
strate.
• If one of the bifurcated traces is located on
a different layer to the other, the return current
will be referenced to different planes potentially
creating a large loop area and EMI.
• Fly-by topology employs a series termina-
tor close to the driver and a parallel 100 ohm
VTT pull-up at the end of the loads.
• Star routing is ideal for distributing clocks
to multiple loads and is also used for power dis-
tribution on signal layers.
• Reflections occur if the trace lengths of star
routing are unmatched.
• The choice is this: Match the lengths or
use individual series terminators, close to the
source, to dampen the reflections of a star route.
PCBDESIGN
References
1. Barry Olney—Advanced Design for SMT,
Beyond Design: Impedance Matching: Termina-
tions,
Beyond Design: Differential Pair Routing,
PCB Design Techniques for DDR, DDR2 &
DDR3, Part 1,
PCB Design Techniques for DDR, DDR2 &
DDR3, Part 2
2. Howard Johnson—High-Speed Digital
Design
3. Mark Montrose—EMC and the Printed
Circuit Board
4. The ICD Stackup Planner and PDN Plan-
ner—www.icd.com.au
Barry Olney is managing di-
rector of in-Circuit Design Pty
ltd (iCD), Australia. This PCB
design service bureau special-
izes in board-level simulation,
and has developed the iCD
Stackup Planner and iCD PDn
Planner software. To read past columns, or to
contact Olney, click here.
beyond design
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