Frequency Modulation - Electronic Engineering (MCQ) questions & answers

ANSWER: Both a and b

Explanation:
The guard bands are provided to prevent the interference between adjacent channels in FM signals. Guard bands of 25 KHz are allowed on the either sides so the channel width becomes 2 (75+ 25) = 200KHz where 75KHz is the maximum permissible frequency deviation allowed for commercial FM broadcast. So a much wider channel width is required for FM transmission. FM systems are more complex and therefore costlier than AM transmitters and receivers.

ANSWER: Deviation ratio

Explanation:
In Frequency Modulation, Deviation ratio is the ratio of maximum peak frequency deviation and the maximum modulating signal frequency. Mathematically, the Deviation Ratio (DR) is given by
DR = Δf _(max) /f_{m (max)}

ANSWER: 30 Hz to 3 KHz

Explanation:
Narrow Band FM signal contains a carrier component and a quadrature carrier linearly modulated by the modulating signal. NBFM signal is band limited to f_m i.e., modulating frequency. The bandwidth of NBFM is 2f_m, hence occupies the same bandwidth as an Amplitude Modulated signal. The range of modulating frequency for Narrow Band FM is 30Hz to 3KHz

ANSWER: 5W

Explanation:
A standard FM signal is represented by
v(t) = A_c cos(2πf_ct + k_fsin2πf_mt)
A_c = carrier amplitude
f_c = carrier frequency
k_f = modulation index
f_m = modulating frequency
k_f = frequency deviation/modulating frequency
the power dissipated across 20Ω resistor is given by
V_rms²/R
=(20/√2)²/R
= 5W

ANSWER: All of the above

Explanation:
FM signal is better than AM signal because FM signals are less immune to noise. Guard bands are provided for less adjacent channel interference so it is easy to be recovered. Amplitude limiters are used to avoid amplitude variations that are caused while transmission due to noise.

ANSWER: 50

Explanation:
Carrier frequency f_c = 100MHz
Modulating frequency f_m = 10 KHz
Frequency deviation Δf = 500 KHz
Modulation index of FM signal is given by
m_f = Δf/f_m
= 500 * 10³/ 10 * 10³
= 50

ANSWER: 10, 2465.9Hz

Explanation:
Standard expression for FM signal is given by
v(t) = A cos ( ω_ct + mf sin ω_mt)
Comparing with the given equation,
Modulation index m_f = 10
Maximum frequency deviation is given by
m_f = Δf/f_m
Δf = m_f * f_m
Here f_m = 1550/2Π = 246.59 Hz
Δf = 10 * 246.59
= 2465.9Hz

ANSWER: 170 KHz

Explanation:
Modulating frequency f_m = 10 KHz
Frequency deviation Δf = 75 KHz
According to Carson s rule, BW = 2(Δf + f_m)
= 2 (75 + 10)
= 170 KHz

ANSWER: Both a and b

Explanation:
The bandwidth of the FM signal depends upon the frequency sensitivity k_f. When k_f is large, the bandwidth of the FM signal becomes wide and this is known as Wide Band FM signal. A large number of sidebands are produced in a FM signal. The bandwidth of a wide band FM signal is very large as compared to that of an AM signal.

ANSWER: Both a and b

Explanation:
The bandwidth of the FM signal depends upon the frequency sensitivity k_f. When k_f is small, the bandwidth of the FM signal becomes narrow and this is known as Narrow Band FM signal. The bandwidth of a narrow band FM signal is almost same as that of an AM signal.