AWR eBooks

CHAPTER 2 - Conveying Power at Radio Frequency 56 Figure 2.5-24 Charge profile in the voltage wavefront region Let us zoom in to the spot on the top wire just underneath the voltage wavefront (Figure 2.5-24). To the left of the wavefront, the charge carriers appear pressed close together. To the right, they remain at a normal spacing. This subtle difference in carrier density on either side of the wavefront creates a small, local electric field pointing to the right. This field does not extend very far, it exists only underneath the rising edge of the travelling wavefront. The rightward-pointing field underneath the rising edge acts only on the charge carriers within its domain, accelerating them forward. On the bottom wire, the charge carriers are shoved in the opposite direction, peeling them away from the uniformly-distributed mass of particles to the right, accelerating them toward the battery. This is shown in Figure 2.5-25. Figure 2.5-25 Charge profile on the bottom wire corresponding to top wire region shown in Figure 2.5-24 Figure 2.5-26 Voltage wavefront reaches the end of the line When the charges reach the end of the line, they "hit" the end of the wire (Figure 2.5-26)! These charges carry with them a certain degree of momentum but there is nowhere left to go and hence they will have to stop, but they cannot stop instantaneously. There is no way to notify the source that a dead-end has been encountered. The source will therefore continue to pour charges into the line, piling them up at the end, for some time (Figure 2.5-27). + _ _ + _ + Vs Vs COMPRESSED RELAXED AT REST Electric Field + _ _ + _ + Vs Vs COMPRESSED RELAXED AT REST Electric Field AT REST + _ Vs Conquer Radio Frequency 56 www.cadence.com/go/awr

• Cover