Mumbai University > Electronics Engineering > Sem4 > Fundamentals of Communication Engineering

Marks: 5M

Year: Dec2014

• unwanted deviation of the carrier frequency. Noise signal produces amplitude and phase modulation in FM.
• The magnitude of this unwanted frequency deviation depends on the relative amplitude of the noise with respect to the carrier. Let us consider the noise signal and carrier signal vectorially.
• When noise signal is superimposed on carrier, the amplitude of noise is added with carrier amplitude.
• produces amplitude modulation in FM. The maximum deviation of amplitude carrier = Vn, as shown in Fig1.

• At the same time, noise vector is constantly changing phase angle w.r.t carrier signal Vc which will change the phase deviation (θ) of FM wave i.e. phase deviation θ of FM changes due to noise. Thus, noise modulates the carrier in terms of amplitude as well as phase.

• From noise triangle in the Fig2., it is seen that noise in AM and PM remains constant for entire audio range because the modulation index due to signal are independent on modulating frequency but in FM the effect of noise will be increased with the increase in modulating frequency.

• Thus, noise has more effect on higher frequencies in FM. The triangular distribution of noise in FM is called as FM noise triangle.