Frequency Modulation | Advantages & Disadvantages

Frequency Modulation

It is the process in which the frequency of the carrier signal is varied by the modulating signal while the amplitude remains same. It is different from Amplitude modulation as there is amplitude variation.

Let carrier wave be

e_c = A Sin (ω_ct + φ) = A Sin (f_ct + φ)

f_c = carrier wave frequency

The modulated carrier wave frequency is given by

f_cm = f_c (1 + kE_s Cos ω_st)

where, e_s  = E_s Cos ω_st    is the input signal

The maximum value of modulated carrier wave frequency is given by

f_cm/max = f_c  + k f_c  E_s

where δ = k f_c  E_s = maximum deviation

Let e_c = A Sinθ

where, θ = ω_ct and taking φ = 0

For frequency modulated wave, θ is given by

θ = ∫ ω dt = ∫ ω_c {1 + kE_s Cos ω_st} dt

Therefore, the modulated carrier frequency in given by

e_cm = A Sin (ω_ct + m_f Sin ω_st)

where, m_f = modulation index

Excercise

Q. What is the frequency deviation?

Answer. The change or shift above or below the mean or centre frequency is called the frequency deviation.

Q. What is carrier swing?

Answer. The total variation in frequency from minimum to maximum value is called the carrier swing and it is twice the frequency deviation i.e CS = 2Δf.

Q. What is FM index?

Answer. The ratio of frequency deviation to modulation frequency is called the FM Index.

Q. What is deviation ratio?

Answer. The ratio of maximum permitted frequency deviation to the maximum permitted audio frequency is known as the deviation ratio.

Q. What is percent modulation in FM

Answer. The percent modulation when applied to FM, has slightly different meaning than when applied to amplitude modulation. In FM it is given as the ratio of actual frequency deviation to maximum permitted frequency deviation and multiplied by 100.

Q. What is wideband FM?

Answer. When the value of modulation index m_f is quite large, then in FM a large number of sidebands are produced and help the bandwidth of FM is sufficiently large. This type of FM system is known as wideband FM.

Q. What are the disadvantages of frequency modulation?

Answer. A much wider channel typically 200 kHz is required in FM against only in 10 kHz in Amplitude Modulation broadcast. This is serious drawback of FM. FM transmitting and receiving equipment particularly used for modulation and demodulation tend to be more Complex and hence more costly.