Two Modulating Oscillators

Just as the number of carriers can be increased, so can the number of modulating oscillators.

To create even more spectral variety, the modulating waveform may consist of the sum of several sinusoids.

If the carrier frequency is $f_c$ and the modulating frequencies are $f_{m1}$ and $f_{m2}$, then the resulting spectrum will contain components at the frequency given by $f_c \pm if_{m1}\pm kf_{m2}$, where $i$ and $k$ are integers greater than or equal to 0.

For example, when $f_c = 100$ Hz, $f_{m1} = 100$ Hz, and $f_{m2} = 300$ Hz, the spectral component present in the sound at 400 Hz is the combination of sidebands given by the pairs: $i=3, k=0; i=0, k=1; i=3^{-},k=2$; and so on (see Figure 18).

Figure 18: The FM spectrum produced by a modultor with two frequency components.