Frequency Modulation Overview

Frequency modulation is accomplished by varying the frequency of a carrier waveform according to the amplitude of a modulating waveform. The general equation for a frequency modulated waveform is:

FM(t) = C[t + (M(t))],

where C(t) is the carrier waveform, M(t) is the modulating waveform, and FM(t) is the frequency modulated signal.

This class driver provides modulating waveform property definitions that must be followed when developing specific instrument drivers. The carrier waveform is defined as the waveform the function generator produces without any modulation. The modulating waveform is defined by the following properties:

• Waveform Type

—The overall "shape" of one period of the modulating waveform. This class driver defines five modulation waveform types: Sine, Square, Triangle, Ramp Up, and Ramp Down.

• Frequency

—The number of modulating waveform cycles generated in one second.

• Peak Frequency Deviation

—The variation of frequency the modulating waveform applies to the carrier waveform. This value is expressed in hertz. At 0 hertz deviation, the modulating waveform has no effect on the carrier waveform. As frequency deviation increases, the frequency variation in the modulated waveform increases.

At the maximum peak of the modulating signal, the frequency of the output signal is equal to the frequency of the carrier signal plus the frequency of the modulating signal. At the minimum peak of the modulating signal, the frequency of the output signal is equal to the frequency of the carrier signal minus the frequency of the modulating signal.

The following diagram illustrates the effect of frequency modulation on a carrier signal, and the effect on the output signal of varying the peak frequency deviation.

One kHz Carrier Sine Wave and a Frequency Modulated Wave at 20 Hz Peak Deviation