Free Books

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[htbp] \includegraphics{eps/sineamfd}

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[htbp] \includegraphics{eps/sineamgfd}

Next Section:
Bessel Functions