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2.2 The true sense of Wavelength 31 2.2 The true sense of Wavelength When the frequency of excitation of our circuits is relatively low, the effects of the connecting wires are limited to minor losses and can often be ignored. When working at higher frequencies however, the effect of such "wires" can no longer be ignored. In fact they are so significant that we do not use wires at Radio Frequency, but appropriately designed interconnecting structures called "transmission lines". Now where's the boundary between low and high frequency and when do we need to use transmission lines in place of wires? Well, this comes down to two factors: the maximum excitation frequency of our circuit and its dimensions. Let us consider a 100 km power line which carries a 50Hz signal, as shown in Figure 2.2-1. Figure 2.2-1 A 100 km power line Assuming that the signal will travel down the line at the speed of light 10 , it will get from the transmitting to the receiving end in a time equal to Now let us look at how quickly our signal changes. Well if the frequency is 50Hz then its period is equal to This means that every 20 ms our signal 11 starts from zero, goes up to a positive peak, then a negative peak and after 20 ms goes back to zero again and the same pattern repeats. Now in the that it has taken the signal to travel to the end of the power line, the voltage at the generator end has not changed by much. In fact, if we turn on our signal generator at time , by time the signal reaches the end of the line at , the signal generator has only gone through 6% of the signal period 12 . 10 The speed of light in vacuum is approximately equal to ⁄ 11 For simplicity assume a sinusoidal signal with an initial phase of zero degrees, ( ) 12 This figure is calculated as R L V S Power Line 100 km Conquer Radio Frequency 31 www.cadence.com/go/awr

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