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An angle modulated signal is given by s(t) = cos 2
p
(2 x 106t + 30 sin 150t + 40 cos 150t). The maximum frequency and phase deviations of s(t) are
  • a)
    10.5 kHz, 140
    p
    rad
  • b)
    6 kHz and 80
    p
    rad
  • c)
    10.5 kHz, 100
    p
    rad
  • d)
    7.5 kHz, 100
    p
    rad
Correct answer is option 'D'. Can you explain this answer?
Verified Answer
An angle modulated signal is given by s(t) = cos 2p (2 x 106t + 30 sin...
s(t) = cos 2
p
(2 x 10+16t + 30 sin 150t + 40 cos 150t)
Phase deviation = 100
p
Frequency deviation = 7.5 kHz.
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Most Upvoted Answer
An angle modulated signal is given by s(t) = cos 2p (2 x 106t + 30 sin...
Given: s(t) = cos 2π (2 × 106t 30 sin 150t 40 cos 150t)

To find: Maximum frequency and phase deviations

Solution:

Frequency Modulation (FM) equation:

s(t) = Ac cos [2πfc t + kf ∫ m(t) dt]

where,

Ac = carrier amplitude

fc = carrier frequency

kf = frequency sensitivity

m(t) = modulation signal

Given signal:

s(t) = cos 2π (2 × 106t 30 sin 150t 40 cos 150t)

This signal can be written as:

s(t) = cos 2π (2 × 106t 60 sin (2π × 25t) 80 cos (2π × 25t))

Carrier frequency fc = 2 × 106 Hz

Modulation index:

μ = (Maximum frequency deviation) / (Modulation frequency)

Here, modulation frequency = 25 Hz

Maximum frequency deviation = 80 Hz

μ = 80 / 25 = 3.2

Maximum frequency deviation:

Δf = kf × m

Here, maximum value of m(t) = 80

kf = Δf / m

kf = 80 / 80 = 1 Hz/V

Maximum frequency deviation = Δf = kf × m = 1 × 80 = 80 Hz

Phase deviation:

Δθ = kp × m

where,

kp = phase sensitivity

For FM modulation,

kp = 2π × Δf / fm

kp = 2π × 80 / 25 = 2π × 3.2

kp = 20π rad/V

Maximum phase deviation = Δθ = kp × m = 20π × 80 = 1600π rad

Maximum phase deviation in radians = 1600π ≈ 5026 rad

Maximum phase deviation in degrees = (Maximum phase deviation in radians) × (180 / π) ≈ 287.5°

Hence, the maximum frequency deviation is 80 Hz and the maximum phase deviation is 100π rad (or 287.5°). Therefore, option D is the correct answer.
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