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can you kindly explain to me this part of this text book kinda narrow it down.

can you kindly explain to me this part of this text book kinda narrow it down.

Introductory Circuit Analysis (13th Edition)
Introductory Circuit Analysis (13th Edition)
13th Edition
ISBN:9780133923605
Author:Robert L. Boylestad
Publisher:Robert L. Boylestad
Chapter1: Introduction
Section: Chapter Questions
Problem 1P: Visit your local library (at school or home) and describe the extent to which it provides literature...
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can you kindly explain to me this part of this text book kinda narrow it down.

4.2.1 Transmission Lines In order to understand the concept of transmission lines, consider a long straight wire or trace with its return wire or trace nearby. The wire has some inductance along its length. There is also some capacitive coupling between the wire and its return. Figure 4.10 shows what is commonly called a lumped model of the wire pair, because the capacitors and inductors are shown as lumped components. In reality, the inductance and capacitance are spread continuously along the wires. In concept, the wire is infinitely long and the figure only shows an infinitely small part of the total length. L/2 roo roor m oooo 8000 C weee I weee I weee Ive Ive I elle меее Fig. 4.10 Transmission line: made up of an infinitely long network of capacitors and inductors If these wires are infinitely long, it is obvious that there will be no reflection at all from its far end. Also, if the wires are absolutely uniform, even then there will be no reflections. Therefore, one way to avoid reflections is to use an infinitely long, absolutely uniform wire or trace pair. Such a wire or trace pair has been given the special name Transmission Line (Brooks, 2002). If we look into the front of this wire pair, there is input impedance which can be mathematically calculated. It is represented by a symbol Zo, and is called the intrinsic or characteristic impedance of the line. By calculating the lumped values of inductance (L) and capacitance (C), the impedance would be then calculated as: Z= √LIC Referring to Figure 4.11, let us take the infinitely long transmission line and break it into two parts. If we look at the second part, it also looks like an infinitely long transmission line with an impedance of Z. It thus turns out that a transmission line of finite length, terminated in its characteristic impedance Zo looks like an infinitely long transmission line. Therefore, even though it has a finite length, it will still have no reflections as all the energy traveling down the line is exactly absorbed or dissipated in the termination and there is no energy left to reflect back and no reflection to worry about.
Transcribed Image Text:4.2.1 Transmission Lines In order to understand the concept of transmission lines, consider a long straight wire or trace with its return wire or trace nearby. The wire has some inductance along its length. There is also some capacitive coupling between the wire and its return. Figure 4.10 shows what is commonly called a lumped model of the wire pair, because the capacitors and inductors are shown as lumped components. In reality, the inductance and capacitance are spread continuously along the wires. In concept, the wire is infinitely long and the figure only shows an infinitely small part of the total length. L/2 roo roor m oooo 8000 C weee I weee I weee Ive Ive I elle меее Fig. 4.10 Transmission line: made up of an infinitely long network of capacitors and inductors If these wires are infinitely long, it is obvious that there will be no reflection at all from its far end. Also, if the wires are absolutely uniform, even then there will be no reflections. Therefore, one way to avoid reflections is to use an infinitely long, absolutely uniform wire or trace pair. Such a wire or trace pair has been given the special name Transmission Line (Brooks, 2002). If we look into the front of this wire pair, there is input impedance which can be mathematically calculated. It is represented by a symbol Zo, and is called the intrinsic or characteristic impedance of the line. By calculating the lumped values of inductance (L) and capacitance (C), the impedance would be then calculated as: Z= √LIC Referring to Figure 4.11, let us take the infinitely long transmission line and break it into two parts. If we look at the second part, it also looks like an infinitely long transmission line with an impedance of Z. It thus turns out that a transmission line of finite length, terminated in its characteristic impedance Zo looks like an infinitely long transmission line. Therefore, even though it has a finite length, it will still have no reflections as all the energy traveling down the line is exactly absorbed or dissipated in the termination and there is no energy left to reflect back and no reflection to worry about.
m m weee I weee I weee I weee I wee мее Fig. 4.11 Termination of an infinitely long transmission line to its characteristic impedance, oz Zo It is exactly this technique which is used to control reflections on PCB. It is required to make the traces look like transmission lines and to terminate them in their characteristic impedance Zo. Certain types of transmission lines are commonly encountered. The coaxial cable leading to the cable TV is a 70 ohm transmission line. The 10 base 2 coax cable for networking is a 50 ohm transmission line. From the above, it is clear that PCB traces can take on the characteristic of transmission lines. The point at which this happens (the critical length) is usually defined as when "the two-way delay of the line is more than the rise time of the pulse". For example, the critical length is approximately 3" for a signal with a 1ns rise time in FR4. The characteristic impedance of a transmission line is a function of its geometry. In PCB applications, it is a function of several variables, two of which are the width of the line and the height of the line above the plane. If the signal trace length is greater than the critical length, and if there is no claim for the trace to reference to, it is likely that there could be no control over Zo, no way to terminate the trace, and therefore, no way to control reflections. The reflections which are likely to occur in such cases can cause noise voltages and false signals that will cause the circuit to fail.
Transcribed Image Text:m m weee I weee I weee I weee I wee мее Fig. 4.11 Termination of an infinitely long transmission line to its characteristic impedance, oz Zo It is exactly this technique which is used to control reflections on PCB. It is required to make the traces look like transmission lines and to terminate them in their characteristic impedance Zo. Certain types of transmission lines are commonly encountered. The coaxial cable leading to the cable TV is a 70 ohm transmission line. The 10 base 2 coax cable for networking is a 50 ohm transmission line. From the above, it is clear that PCB traces can take on the characteristic of transmission lines. The point at which this happens (the critical length) is usually defined as when "the two-way delay of the line is more than the rise time of the pulse". For example, the critical length is approximately 3" for a signal with a 1ns rise time in FR4. The characteristic impedance of a transmission line is a function of its geometry. In PCB applications, it is a function of several variables, two of which are the width of the line and the height of the line above the plane. If the signal trace length is greater than the critical length, and if there is no claim for the trace to reference to, it is likely that there could be no control over Zo, no way to terminate the trace, and therefore, no way to control reflections. The reflections which are likely to occur in such cases can cause noise voltages and false signals that will cause the circuit to fail.
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