s own provides little information other than its presence or its absence. So we must find some Amplitude modulation (AM) is the method used in method of modulating our information on to the radio medium-wave and short-wave radio broadcasting. carrier wave. We can change: Figure 2.1-3 shows what happens when we apply am- its amplitude (Vc) according to our information - this plitude modulation to a sinusoidal carrier wave. is called amplitude modulation and will be described Figure 2.1-3(a) shows the modulating wave on its in Section 2.1.3.1; own. 6 Figure 2.1-3(b) shows the carrier wave on its own. 4 Fourier analysis will be explained fully in a later section. A sinusoidal wave is a generic name for a sine or cosine wave. In many cases, cosine waves are used because of ease in mathematical manipulation. I have used a cosine wave here because you will see later when we use Fourier analysis that waveforms, no matter how complicated, can be resolved into a series of d.c., sine and cosine terms and their harmonics. CHAPTER 2. Basic features of radio communication systems Figure 2.1-3(c) shows the resultant wave. The resultant wave shape is due to the fact that at times the modulating Carrier wave and the carrier wave are adding (in phase) and at Lower Upper other times, the two waves are opposing each other (out sideband sideband of phase). Amplitude modulation can also be easily analysed (fc-tm) (fc + fm) mathematically. Let the sinusoidal modulating wave be Frequency described as Bandwidth Vm = Vmcos(Wmt) (2.1.2) Fig. 2.1-4 Frequency spectrum of an AM wave. where Vm = instantaneous modulating amplitude (volts) resultant wave shape is due to the fact that the carrier Vm = modulating amplitude (peak volts) wave frequency increases when the modulating signal is Wm = angular frequency in radians and Wm = 2m positive and decreases when the modulating signal is negative. Note that in pure FM, the amplitude of the where carrier wave is not altered. fm = modulating frequency (hertz) The frequency deviation (Afc) of the car