Analog Communication Test Questions - Set 7

ANSWER: All of the above

Explanation:
The amplitude of FM is constant. So transmitter power remains unchanged in FM but it changes in AM. The depth of modulation in FM can be changed to any value by changing the frequency deviation. So the signal is not distorted. There is less possibility of adjacent channel interference due to presence of guard bands so interference of noise is minimum in FM.

ANSWER: Direct FM

Explanation:
Voltage controlled oscillator is used to generate Direct FM signal. In this type of modulation, the frequency of the carrier is varied directly by the instantaneous voltage of the modulating signal i.e., the instantaneous frequency of the output signal is directly proportional to the voltage of the input modulating signal. Direct FM modulation is obtained by directly giving the modulating signal as an input to the VCO.

ANSWER: Indirect FM

Explanation:
Indirect FM is obtained by modulation of the phase of the carrier i.e., the instantaneous phase of the carrier varies with the amplitude of the modulating signal. Crystal controlled oscillators are used to generate Indirect FM and then the result is passed through a frequency multiplier to convert the signal into wideband FM signal.

ANSWER: FM demodulator

Explanation:
PLL or Phase Locked Loop is used as a FM demodulator. It is a high performance FM detector due to its key advantages such as Linearity and low manufacturing costs.

ANSWER: Frequency deviation

Explanation:
In FM, as the amplitude of the modulating signal changes, the frequency of the carrier changes from its central frequency. This is known as Frequency Deviation.

ANSWER: Bandwidth of FM signal

Explanation:
According to Carson s rule, the bandwidth required is twice the sum of the maximum frequency deviation and the maximum modulating signal frequency. Or,
B=2(Δf +f_m) Hz.

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: Phase modulated signal

Explanation:
A phase modulated wave is given by
v(t) = A cos [ω_ct + k_p Φ(t)]
A = amplitude
Φ(t) = baseband signal
ω_c = carrier frequency
k_p = phase sensitivity of the modulator.

Where, A is a constant that represents Amplitude and ω_c represents carrier frequency and k_p is the phase sensitivity of the modulator.

ANSWER: 1750 Hz

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 = 2200/2π = 350 Hz
k_f = frequency deviation/modulating frequency
5 = freq deviation/ 350
Therefore, deviation = 5 * 350
= 1750Hz

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: 1050 Hz

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
therefore, f_c = 6600/2π
= 1050Hz

ANSWER: 20

Explanation:
Modulation index is the measure of how much the modulation parameter changes from its un modulated value. The modulation index of FM is given by
μ = frequency deviation/ modulating frequency
= Δf/ f_m
Where Δf is the peak frequency deviation i.e. the deviation in the instantaneous value of the frequency with modulating signal.
f_m is the value of modulating frequency
μ = 5000/250
= 20

ANSWER: Remains unchanged

Explanation:
When a carrier is Frequency modulated, the output signal has the frequency deviated above and below the carrier frequency, this is known as Frequency Deviation. When the FM signal is passed through a mixer, the mixer changes the carrier frequency but the deviation remains unchanged.

ANSWER: All of the above

Explanation:
Armstrong method is used for the generation of FM signal. The FM signal has a high quality of audio signal and reduced noise as compared to the AM wave. In FM, multiple side bands are generated therefore has infinite bandwidth though only the first few sets of sidebands are useful for detection of signal.

ANSWER: 75KHz, 200Khz

Explanation:
Maximum frequency deviation allowed for commercial FM broadcast is 75KHz. Therefore approximate bandwidth allowed is 2 * 75 = 150KHz. Guard bands of 25KHz are allowed on the either sides so the channel width becomes 2 (75 + 25) = 200KHz. The guard bands are added to prevent the interference between adjacent channels.

ANSWER: Prevent interference from adjacent channels

Explanation:
The guard bands are provided to prevent the interference between adjacent channels in FM signals. Guard bands of 25KHz 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.

ANSWER: 159.1MHz, 194.1Hz

Explanation:
Standard expression for FM signal is given by
v(t) = A cos ( ω_ct + m_f sin ω_mt)
comparing with the given equation,
Carrier frequency is given by f_c = 10 * 10⁸/2Π
= 159.1MHz
Modulating frequency is given by f_m = 1220/2Π
= 194.1Hz

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: 10 Watts

Explanation:
Standard expression for FM signal is given by
v(t) = A cos ( Ω_ct + m_f sin Ω_mt)
Comparing with the given equation,
A = 20
The dissipated power is given by P = V²_rms/R
= (20/√2)²/ 20
= 10Watts

ANSWER: 200 KHz

Explanation:
Carrier frequency f_c = 100MHz
Modulating frequency f_m = 5 KHz
Frequency deviation Δf = 100 KHz
Carrier swing of the FM signal = 2 * Δf
= 2 * 100
= 200 KHz