Fm Concepts - Keysight X-Series 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 changes 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.
28
Δ f f
β × ⁄ ΔΦ
= =
p m
Keysight N9063C Analog Demod Measurement Guide
p