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CHAPTER 2 - Conveying Power at Radio Frequency 38 Even if we assume that we are using wires with zero resistance, there would still exists some capacitance along the cable due to the fact that any pair of conductors separated by an insulating medium creates capacitance between those conductors (Figure 2.3-3). Figure 2.3-3 Equivalent circuit showing stray capacitance between conductors. When en electric filed is applied to our "pair of wires", the capacitance which exists between them means that a current proportional to the rate of change of voltage over time will be drawn. This is described by the equation (1.4-1)(b), ⁄ . This capacitance stores the energy provided by the electric field created by the voltage source. According to the equation, an instant rise in applied voltage (produced by a perfect switch closure) gives rise to an infinite charging current. However, the current drawn by a pair of parallel wires will not be infinite, because there exists series impedance along the wires due to inductance (Figure 2.3-4). Figure 2.3-4 Equivalent circuit showing stray capacitance and inductance Such an inductance comes from the fact that current through any conductor develops a magnetic field of proportional magnitude and energy is stored in this magnetic field (Figure 2.3-5). This stored energy comes from the electric field created by the source and this transformation of energy from electric into magnetic manifests itself as a voltage drop governed by the inductance equation (1.4-1)(c), ⁄ . This voltage drop limits the rate of change of voltage across the distributed capacitance, preventing the current from ever reaching an infinite magnitude. Infinite Length Switch DC Source Conquer Radio Frequency 38 www.cadence.com/go/awr

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