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7.3 Uniform Transmission Lines

We classify transmission lines by their geometry. The two general features of geometry that strongly determine the electrical properties of a transmission line are the uniformity of the cross section down its length and how identical are each of its two conductors.

When the cross section is the same down the length, as in a coax cable, the transmission line is called uniform. Examples of various uniform transmission lines are illustrated in Figure 7-4.

Figure 7-4

Figure 7-4 Examples of cross sections of uniform transmission lines commonly used as interconnects.

As we will see, uniform transmission lines are also called controlled impedance lines. There are a great variety of uniform transmission lines, such as twin leads, microstrips, striplines, and coplanar lines.

Nonuniform transmission lines exist when some geometry or material property changes as we move down the length of the line. For example, if the spacing between two wires is not controlled but varies, this is a nonuniform line. A pair of leads in a dual in-line package (DIP) or quad flat pack (QFP) are nonuniform lines.

Adjacent traces in a connector are often nonuniform transmission lines. Traces on a PCB that do not have a return-path plane are often nonuniform lines. Nonuniform transmission lines will lead to signal-integrity problems and should be avoided unless they are kept short.

Another quality of the geometry affecting transmission lines is how similar the two conductors are. When each conductor is the same shape and size as the other one, there is symmetry, and we call this line a balanced line. A pair of twisted wires is symmetric since each conductor looks identical. A coplanar line has two narrow strips side by side on the same layer and is balanced.

A coax cable is unbalanced since the center conductor is much smaller than the outer conductor. A microstrip is unbalanced, since one wire is a narrow trace, while the second is a wide trace. A stripline is unbalanced for the same reason.

Whether the transmission line is uniform or nonuniform, balanced or unbalanced, it has just one role to play: to transmit a signal from one end to the other with an acceptable level of distortion.

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