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0.96

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Co-Planar Coupled Microstrips

Broad-Side Coupled Striplines

Edge-Coupled

Edge-Coupled

Single-Ended Microstrip

Single-Ended Stripline

W

H

T

W

T

H

H

S

H

Differential Microstrip

Differential Stripline

S

H

S

H

H

G

G

W

W

W

S

22

SN65MLVD206B

SLLSEX9A – DECEMBER 2016 – REVISED FEBRUARY 2020

www.ti.com

Product Folder Links: SN65MLVD206B

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Copyright © 2016–2020, Texas Instruments Incorporated

9.2.3.10 PCB Transmission Lines

As per SNLA187, Figure 19 depicts several transmission line structures commonly used in printed-circuit boards

(PCBs). Each structure consists of a signal line and a return path with uniform cross-section along its length. A

microstrip is a signal trace on the top (or bottom) layer, separated by a dielectric layer from its return path in a

ground or power plane. A stripline is a signal trace in the inner layer, with a dielectric layer in between a ground

plane above and below the signal trace. The dimensions of the structure along with the dielectric material

properties determine the characteristic impedance of the transmission line (also called controlled-impedance

transmission line).

When two signal lines are placed close by, they form a pair of coupled transmission lines. Figure 19 shows

examples of edge-coupled microstrips, and edge-coupled or broad-side-coupled striplines. When excited by

differential signals, the coupled transmission line is referred to as a differential pair. The characteristic impedance

of each line is called odd-mode impedance. The sum of the odd-mode impedances of each line is the differential

impedance of the differential pair. In addition to the trace dimensions and dielectric material properties, the

spacing between the two traces determines the mutual coupling and impacts the differential impedance. When

the two lines are immediately adjacent; for example, if S is less than 2 × W, the differential pair is called a tightly-

coupled differential pair. To maintain constant differential impedance along the length, it is important to keep the

trace width and spacing uniform along the length, as well as maintain good symmetry between the two lines.

Figure 19. Controlled-Impedance Transmission Lines