Appendix C: Transmission Line Primer; Background; Physical Transmission Line Types - Nvidia Jetson TX2 Manual

18.0 APPENDIX C: TRANSMISSION LINE PRIMER

18.1 Background

NVIDIA maintains strict guidelines for high-frequency PCB transmission lines to ensure optimal signal integrity for data
transmission. This section provides a brief primer into basic board-level transmission line theory.
Characteristics
The most important PCB transmission line characteristics are listed in the follow ing bullets:
▪
Trace w idth/height, PCB height and dielectric constant, and layer stack-up affect the characteristic trace
impedance of a transmission line.
L
Z
˜ =
0
C
▪
Signal rise time is proportional to the transmission line impedance and load capacitance.
Ris eTime =
˜
▪
Real transmission lines (Figure 50) have non-zero resistances that lead to attenuation and distortion, creating
signal integrity issues.
Figure 50. Typical Transmission Line Circuit
Source
Transmission Line
Z
S
Z
0
Transmission lines are used to "transmit" the source signal to the load or destination w ith as little signal degradation or reflection
as possible. For this reason it is important to design the high-speed signal transmission line to fall w ithin characteristic guidelines
based on the signal speed and type.

18.2 Physical Transmission Line Types

The tw o primary transmission line types often used for module board designs are:
▪
Microstrip transmission line (Figure 51)
▪
Stripline transmission line (Figure 52)
The follow ing sections describe each type of transmission.
Microstrip Transmission Line
Figure 51. Microstrip Transmission Line
W
Z
Dielectric 0
H T
▪
Z : Impedance
0
▪
W: Trace w idth (inches)
▪
T: Trace thickness (inches)
▪
Er: Dielectric constant of substrate
▪
H: Distance betw een signal and reference plane
Stripline Transm ission Line
JETSON TX2/TX2i OEM PRODUCT | DESIGN GUIDE | 20180618
1/2
Z * R
0 Term
* C
Load
Z + R
0 Term
Load
Z
L
87 5.98H
ln
=
Er + 1.414 0.8W + T
NVIDIA Jetson TX2/TX2i OEM Product Design Guide
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