6.2 ANALOG MODULATION

A sinusoidal carrier has three parameters that may be varied by a baseband signal: amplitude, frequency and phase. Consequently, there are three basic forms of analog modulation: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

The general expression for a sinusoidal carrier wave is:

υ_{c} (t) = V_{c} \sin (ω_{c} t + ϕ_{c})

(6.1)

where v_c(t) is the instantaneous carrier voltage, V_c is the peak amplitude of the carrier, ω_c= 2πf_c is the angular frequency, and ϕ_c is the carrier phase. For simplicity in explanation, and because it does not affect the essential description of AM or FM, we set ϕ_c = 0 in those cases.

As illustrated in Equation (6.2), to translate a baseband signal to a higher frequency, the modulating baseband signal is used to vary one of the carrier parameters, either its amplitude (V_c), frequency (f_c), or phase (ϕ_c).

υ_{c} (t) = \underset{variation}{\underset{⏟}{\begin{matrix} V_{c} \sin (2 π f_{c} t + ϕ_{c}) \\ ↑ o r ↑ o r ↑ \end{matrix}}}

(6.2)

The earliest and simplest technique is AM in which the modulating signal is used to vary the amplitude of the carrier and the information is said to be impressed onto the carrier’s amplitude. AM is particularly susceptible to additive amplitude noise, so FM is commonly the preferred analog modulation scheme, owing to its superior resistance to noise and interference.

PM is rarely used as an analog technique in any communication system, but it is of great importance in modern digital modulation schemes. First, let’s look at employing amplitude variation of the carrier—amplitude modulation.