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This chapter is from the book

7.2 The Signal

As we will see, when a signal moves down a transmission line, it simultaneously uses the signal path and the return path. Both conductors are equally important in determining how the signal interacts with the interconnect.

When both lines look the same, as in a twisted pair, it is inconsequential which we call the signal path and which we call the return path. When one is different from the other, such as in a microstrip, we usually refer to the narrow conductor as the signal path and the plane as the return path.

When a signal is launched into a transmission line, it propagates down the line at the speed of light in the material. After the signal is launched in the transmission line, we can freeze time a moment later, move along the line, and measure the signal. The signal is always the voltage difference between two adjacent points on the signal and return paths. This is illustrated in Figure 7-3. Figure 7-3 Map of the signal, frozen in time on a transmission line. The signal is the voltage between two adjacent points on the signal and the return paths.

It is important to distinguish, and pay attention to, the voltage on the signal line, which is what would be measured by a scope probe, and the propagating signal. The propagating signal is the voltage pattern that is moving down the transmission line and is dynamic.

When the signal passes by a point on the transmission line, the voltage a scope probe would measure is the same magnitude as the signal. However, if there are multiple signals on the transmission line propagating in opposite directions, the scope probe cannot separate them. The voltage that would be measured is not the same as the propagating signal.

These general principles apply to all transmission lines—single ended and, as we will see, differential transmission lines.