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The Physical Basis of Transmission Lines

Prolific author and instructor Eric Bogatin goes into detail of what a real transmission line is and its properties.

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We hear the term transmission line all the time, and we probably use it every day, yet what really is a transmission line? A coax cable is a transmission line. A PCB trace and the adjacent plane beneath it in a multilayer board is a transmission line.

As we will see, a transmission line is used to transport a signal from one point to another. Figure 7-1 illustrates the general features of all transmission lines. To distinguish the two conductors, we refer to one as the signal path and the other as the return path.

Figure 7-1

Figure 7-1 A transmission line is any two conductors with length. We label one of the conductors the signal path and the other the return path.

A transmission line is also a new ideal circuit element with very different properties than the three previously introduced ideal circuit elements: resistors, capacitors, and inductors. It has two very important parameters: a characteristic impedance and a time delay. The way a signal interacts with an ideal transmission line is radically different from the way it interacts with the other three ideal elements.

Though in some cases we can approximate the electrical properties of an ideal transmission line with combinations of Ls and Cs, the behavior of an ideal transmission line matches the actual, measured behavior of real interconnects more accurately and to much higher bandwidth than does an LC approximation. Adding an ideal transmission line circuit element to our toolbox will dramatically increase our ability to describe the interactions of signals and interconnects.

7.1 Forget the Word Ground

Generally, the term ground is reserved for the conductor with the lowest voltage in the circuit compared to any other node in the circuit. Only a voltage difference is ever measured. When the ground node is selected as the reference point, all other circuit nodes are at a higher voltage. There is nothing special about this conductor labeled as ground other than that it is the lowest voltage in the circuit, and all other nodes are at a higher voltage. This type of ground is referred to as the circuit ground.

This is distinct from the chassis ground and from earth ground. Chassis ground is a special conductor. It refers to the connection to the product’s metal enclosure. This conductor is unique.

Earth ground is also a special connection. Ultimately, any conductor connected to earth ground can trace its path to a copper rod driven at least 4 feet into the ground. Many building codes specify the details of how the rod is placed to be considered an earth ground.

The round socket of three-prong AC power plugs is connected to earth ground. Usually, as a safety precaution, chassis ground is connected to earth ground. This is required by Underwriters Laboratory (UL) specifications, for example.

In bipolar circuits, the ground node is often the node that has a voltage halfway between +Vcc and −Vcc.

Too often, the other line in a transmission line is referred to as the ground line. Using the term ground to refer to the return path is a very bad habit and should be avoided.

One of the most common ways of getting into trouble with signal-integrity design is overusing the term ground. It is much healthier to get into the habit of calling the other conductor the return path.

Many of the problems related to signal integrity are due to poorly designed return paths. If we are always consciously aware that the other path plays the important role of being the return path for the signal current, we will take as much care in designing its geometry as we take for the signal path.

When we label the other path as ground, we typically think it is a universal sink for current. Return current goes into this connection and comes out wherever there is another ground connection. This is totally wrong. The return current will closely follow the signal current. As we saw in Chapter 6, “The Physical Basis of Inductance,” at higher frequencies, the loop inductance of the signal and return paths will be minimized, which means the return path will distribute as close to the signal path as the conductors will allow.

Further, the return current has no idea what the absolute voltage level is of the return conductor. The actual return conductor may in fact be a voltage plane such as the Vcc or Vdd plane. Other times, it might be a low-voltage plane. That it is labeled as a ground connection in the schematic is totally irrelevant to the signal, which is propagating on the transmission line. Start calling the return path the return path, and problems will be reduced in the future. This guideline is illustrated in Figure 7-2.

Figure 7-2

Figure 7-2 Forget the word ground, and more problems will be avoided than created.

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