III. Gm-C Filtering - Epublications - Université de Limoges

I.1.b.i Analog modulations
Most common analog modulations are AM, PM and FM, which respectively stand for
amplitude, phase and frequency modulation. Assuming a high frequency signal S(t) of the
form
( t ( ) ) +
S( t)
A(
t)
cos c t Φ
= ω , (I.1)
amplitude modulation actually consists in replacing A(t) by the original signal
( 1 M cos(
ω t)
)
A( t)
= A +
m . (I.2)
Time and frequency representations of AM modulations are depicted in Figure 3:
Figure 3. Time representation of an AM modulation
Phase and frequency modulations consist in modulating respectively the phase Φ(t)
and ωc in S(t) shown in Equation (I.1). The time representation of an FM modulation is
illustrated in Figure 4. It is worth noticing that the PM modulation of a sine wave, which is
the particular case observed in Figure 4, gives the same results as the FM modulation.
Figure 4. Time representations of an FM modulation
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