Example AM Spectra

Equation (4.4) can be used to write down the spectral representation of $x_m(t)$ by inspection, as shown in Fig.4.12. In the example of Fig.4.12, we have $f_c=100$ Hz and $f_m=20$ Hz, where, as always, $\omega=2\pi f$ . For comparison, the spectral magnitude of an unmodulated $100$ Hz sinusoid is shown in Fig.4.6. Note in Fig.4.12 how each of the two sinusoidal components at $\pm100$ Hz have been ``split'' into two ``side bands'', one $20$ Hz higher and the other $20$ Hz lower, that is, $\pm100\pm20=\{-120,-80,80,120\}$ . Note also how the amplitude of the split component is divided equally among its two side bands.

Figure 4.12: Spectral magnitude representation of the sinusoidally modulated sinusoid $\sin (40\pi t)\sin (200\pi t)$ defined in Eq.(4.3). Phase is not shown.

Recall that $x_m(t)$ was defined as the second term of Eq.(4.1). The first term is simply the original unmodulated signal. Therefore, we have effectively been considering AM with a ``very large'' modulation index. In the more general case of Eq.(4.1) with $a_m(t)$ given by Eq.(4.2), the magnitude of the spectral representation appears as shown in Fig.4.13.

Figure 4.13: Spectral representation of the sinusoidally modulated sinusoid $[1+ \sin (40\pi t)]\sin (200\pi t)$ from Eq.(4.1), with $\alpha =1$ , and $a_m(t)$ given by Eq.(4.2).