AWR eBooks

2.5 Reflection of DC voltage in Transmission lines 55 2.5.1.2 Physical Approach – Charge carriers and Electric Fields Let us now look at the phenomena that take place along an open-circuited line from a more fundamental and physical viewpoint. For simplicity, throughout this section we will stick with the convention of current being the motion of positive charges. We will also assume that thermal agitation is negligible. The green dots represent mobile charge carriers distributed evenly throughout an invisible metallic lattice (Figure 2.5-22). Figure 2.5-22 Open-circuited line with no stimulus, charge carriers evenly distributed throughout metallic lattice As the switch is closed, charge is injected from the generator into the line. This makes the charges bunch up and creates a sort of a wave of compression which travels, along the signal wire, from the source towards the end of the line (Figure 2.5-23). Figure 2.5-23 Voltage wave travels down the line Figure 2.5-23 shows what happens shortly after closing the switch. On the top wire, the charges on the left-hand side of the voltage wavefront are squeezed together, creating an excess of charge, whereas on the right-hand side of it the charges are undisturbed. In the lower left portion of Figure 2.5-23, the bottom wire (also called the return wire) exhibits a deficit of charge carriers. This happens because the battery, which stuffs positive carriers onto the top wire, creating an excess, must draw those charges from the bottom wire, creating a deficit. In the region to the left of the travelling wavefront, the excess of charges in the top wire, combined with the deficit of charges in the bottom wire, creates an electric field (E-field) that points straight down, from the top wire to the bottom one (Figure 2.5-23). + _ + _ _ + _ + _ + _ + E Field Vs Vs COMPRESSED RELAXED Conquer Radio Frequency 55 www.cadence.com/go/awr

• Cover