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Z

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1.41

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1.9 2 H T

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Z

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s

0.96

H

diff

0

Z

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1 0.48

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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

13

DSLVDS1002

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SNLS619 – JULY 2018

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Copyright © 2018, Texas Instruments Incorporated

When two signal lines are placed close by, they form a pair of coupled transmission lines. Figure 6 shows

examples of edge-coupled microstrip lines, 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, S is less than 2W, 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 6. Controlled-Impedance Transmission Lines