Fm Concepts - Keysight N9030A Measurement Manual

Concepts

FM Concepts

FM Concepts

Figure 2-3 FM waveform
FM (Frequency Modulation) and PM (Phase Modulation) belong to angle
modulation. In FM, the instantaneous frequency deviation of the modulated carrier
signal changed in proportion to the instantaneous amplitude of the modulating
signal. And in PM, the instantaneous phase deviation of the modulated carrier with
respect to the phase of the unmodulated carrier is directly proportional to the
instantaneous amplitude of the modulating signal.
The modulation index for angle modulation, β, is expressed by this equation:
Equation 2-3
Where Δfp is the peak frequency deviation, fm is the frequency of the modulating
signal, and Δφp is the peak phase deviation.
This expression tells us that the angle modulation index is really a function of phase
deviation, even in the FM case. Also, the definitions for frequency and phase
modulation do not include the modulating frequency. In each case, the modulated
property of the carrier, frequency or phase, deviates in proportion to the
instantaneous amplitude of the modulating signal, regardless of the rate at which the
amplitude changes. However, the frequency of the modulating signal is important in
FM and is included in the expression for the modulating index because it is the ratio
of peak frequency deviation to modulation frequency that equates to peak phase.
Unlike the modulation index for AM, there is no specific limit to the value of β,
since there is no theoretical limit to the phase deviation; thus there is no equivalent of
100% AM. However, in real world systems there are practical limits.
Unlike AM, which is a linear process, angle modulation is nonlinear. This means that
a single sine wave modulating signal, instead of producing only two sidebands,
yields an infinite number of sidebands spaced by the modulating frequency.
β = Δf ⁄ = Δφ
f
p m p
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