: : : :In this equation , is the instantaneous frequency of the oscillator and is the frequency deviation , which represents the maximum shift away from fc in one direction , assuming xm ( t ) is limited to the range �1.While most of the energy of the signal is contained within fc � f ? , it can be shown by Fourier analysis that a wider range of frequencies is required to precisely represent an FM signal .
The maximum shift away from 'fc ' in one direction is represented by the modulation index 'h ' in the frequency modulation equation .