can you kindly explain to me this part of this text book kinda narrow it down. 4.2.1 Transmission LinesIn order to understand the concept of transmission lines, consider a long straight wire or trace withits return wire or trace nearby. The wire has some inductance along its length. There is also somecapacitive coupling between the wire and its return. Figure 4.10 shows what is commonly called alumped 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, thewire is infinitely long and the figure only shows an infinitely small part of the total length.L/2roo roormoooo8000Cweee I weee I weee Ive Ive IelleмеееFig. 4.10 Transmission line: made up of an infinitely long network of capacitors and inductorsIf these wires are infinitely long, it is obvious that there will be no reflection at all from its farend. 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. Sucha 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 mathematicallycalculated. It is represented by a symbol Zo, and is called the intrinsic or characteristic impedanceof the line. By calculating the lumped values of inductance (L) and capacitance (C), the impedancewould be then calculated as:Z= √LICReferring to Figure 4.11, let us take the infinitely long transmission line and break it into twoparts. If we look at the second part, it also looks like an infinitely long transmission line with animpedance of Z. It thus turns out that a transmission line of finite length, terminated in itscharacteristic impedance Zo looks like an infinitely long transmission line. Therefore, even thoughit has a finite length, it will still have no reflections as all the energy traveling down the line isexactly absorbed or dissipated in the termination and there is no energy left to reflect back and noreflection to worry about. mmweee I weee I weee I weee I weeмееFig. 4.11 Termination of an infinitely long transmission line to its characteristic impedance, ozZoIt is exactly this technique which is used to control reflections on PCB. It is required to make thetraces look like transmission lines and to terminate them in their characteristic impedance Zo. Certaintypes of transmission lines are commonly encountered. The coaxial cable leading to the cable TV isa 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 delayof the line is more than the rise time of the pulse". For example, the critical length is approximately3" 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 lineabove the plane. If the signal trace length is greater than the critical length, and if there is no claim forthe 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 cancause noise voltages and false signals that will cause the circuit to fail.

Question

can you kindly explain to me this part of this text book kinda narrow it down. 4.2.1 Transmission LinesIn order to understand the concept of transmission lines, consider a long straight wire or trace withits return wire or trace nearby. The wire has some inductance along its length. There is also somecapacitive coupling between the wire and its return. Figure 4.10 shows what is commonly called alumped 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, thewire is infinitely long and the figure only shows an infinitely small part of the total length.L/2roo roormoooo8000Cweee I weee I weee Ive Ive IelleмеееFig. 4.10 Transmission line: made up of an infinitely long network of capacitors and inductorsIf these wires are infinitely long, it is obvious that there will be no reflection at all from its farend. 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. Sucha 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 mathematicallycalculated. It is represented by a symbol Zo, and is called the intrinsic or characteristic impedanceof the line. By calculating the lumped values of inductance (L) and capacitance (C), the impedancewould be then calculated as:Z= √LICReferring to Figure 4.11, let us take the infinitely long transmission line and break it into twoparts. If we look at the second part, it also looks like an infinitely long transmission line with animpedance of Z. It thus turns out that a transmission line of finite length, terminated in itscharacteristic impedance Zo looks like an infinitely long transmission line. Therefore, even thoughit has a finite length, it will still have no reflections as all the energy traveling down the line isexactly absorbed or dissipated in the termination and there is no energy left to reflect back and noreflection to worry about. mmweee I weee I weee I weee I weeмееFig. 4.11 Termination of an infinitely long transmission line to its characteristic impedance, ozZoIt is exactly this technique which is used to control reflections on PCB. It is required to make thetraces look like transmission lines and to terminate them in their characteristic impedance Zo. Certaintypes of transmission lines are commonly encountered. The coaxial cable leading to the cable TV isa 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 &#34;the two-way delayof the line is more than the rise time of the pulse&#34;. For example, the critical length is approximately3&#34; 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 lineabove the plane. If the signal trace length is greater than the critical length, and if there is no claim forthe 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 cancause noise voltages and false signals that will cause the circuit to fail.

Accepted Answer

Transmission lines.