Pulse Modulation MCQ Quiz in বাংলা - Objective Question with Answer for Pulse Modulation - বিনামূল্যে ডাউনলোড করুন [PDF]

• The Noise affects the amplitude of the modulated signal
• In the Amplitude Modulated system, the message is contained in the amplitude modulations of the carrier; hence the introduction of noise distorts the amplitude and hence the message contained in the signal
• In a frequency modulated system the message is contained in the frequency variations of the carrier, therefore, the introduction of noise does not affect the message contained in the message signal
• Since the amplitude of FM remains constant, the noise can be eliminated using an amplitude limiter at the demodulator.

Features:	AM	FM
Noise immunity	In AM, the message is stored in the form of variation in amplitude. Noise affects the amplitude of signal most so AM is less noise immune.	In FM, the message is stored in the form of variation in frequency so it has better noise immunity.
Bandwidth	B.W. required in AM is = 2fm. Hence, less bandwidth is required in case of AM.	B.W. required in FM is = 2(β+1)fm. Hence, more bandwidth is required in the case of FM.
Transmitted power	Power transmitted in AM is given by: \({P_T} = {P_c}\left( {1 + \frac{{{\mu ^2}}}{2}} \right)\) As the modulation index ‘μ’ increases power in AM increases.	In FM, power transmitted is always equal to the total power of carrier before modulation. Hence, FM requires less power than AM.

• Analog modulation is a process in which analog low-frequency baseband signals (like an audio or TV signal) are transmitted over a larger distance without getting faded away, by superimposing over a higher frequency carrier signal such as a radio frequency band.
• Different modulation techniques are explained with the help of the following block diagram:

• If the amplitude of a pulse or duration of a pulse is varied according to the instantaneous values of the baseband modulating signal, then such a technique is called as Pulse Amplitude Modulation (PAM).

The bit rate for a PCM system is given by

R_b = nf_s

Where

R_b = Bit rate

n = number of bits in encoder

f_s = sampling frequency

From Nyquist criteria

f_s = 2f_m

Where f_m = message signal frequency

R_b = nf_s

60 MHz = 6 × 2 × f_m

f_m = 5 MHz

Concept:

The total transmitted power for an AM system is given by:

\({P_t} = {P_c}\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

Pc = Carrier Power

μ = Modulation Index

Calculation:

Given P_t = 600 W, P_c = 400 W

Putting these values in the total power of AM wave expression, we get:

\(600 = 400\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

\(\frac{6}{4}=\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

\(\frac{6}{4}=\left( {1 + \frac{{{μ^2}}}{2}} \right)\)

\(\frac{μ^2}{2}=\frac{1}{2}\)

μ = 1.00

In Amplitude modulated system

Power in side band is given by

\({P_{SB}} = {P_c}\left( {\frac{{{\mu ^2}}}{2}} \right)\)

Hence

\(\frac{{{P_{SB}}}}{{{P_c}}} = \left( {\frac{{{\mu ^2}}}{2}} \right)\)

\(\frac{{50}}{{400}} = \left( {\frac{{{\mu ^2}}}{2}} \right)\)

μ = 0.5

Pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission.

Concept:

\(B.W = \frac{{{R_b}}}{{{{\log }_2}M}}\)

Where,

Rb = nf_s

n = no. of bits

M = no. of levels of PAM

Calculation:

f_m = 10 kHz

Nyquist sampling rate = 2f­_m = 20 kHz.

The signal is sampled at 20% above Nyquist rate:

∴ f_s = (1.2 × 20) = 24 kHz.

No. of Quantization levels

= 1024

No. of bits required = log₂ (1024)

= log₂ 2¹⁰ = 10 bits.

∴ \(B.W = \frac{{{R_b}}}{{{{\log }_2}M}}\)

\(= \frac{{10\; \times \;24}}{{ \;{{\log }_2}8}}\)

\(= \frac{{240}}{3}\)

= 80 kHz.

For Baseband	For Passband
Binary: 1) B.W. = Rb	Binary: 1) BW = 2 Rb
Raised cosine (α) : 2)  \(BW = \frac{{{R_b}}}{2}\left( {1 + \alpha } \right)\)	Raised cosine (α) : \(2)\;BW = \frac{{2{R_b}}}{2}\left( {1 + \alpha } \right)\) = Rb (1 + α)
M-ary: 1)  \(B.W. = \frac{{{R_b}}}{{{{\log }_2}M}}\)	M-ary: 1)  \(B.W = \frac{{2{R_b}}}{{{{\log }_2}M}}\)
Raised cosine (α): 2)  \(B.W. = \frac{{{R_b}\left( {1 + \alpha } \right)}}{{2{{\log }_2}M}}\)	Raised cosine (α) : 2)  \(B.W = \frac{{{R_b}\left( {1 + \alpha } \right)}}{{{{\log }_2}M}}\)

• In PCM an analog signal is sampled and encoded into different levels before transmission
• The bandwidth of PCM depends on the number of levels If each sample is encoded into n bits, then the bandwidth of PCM is nfs
• However, in the case of Delta modulation, each sample is sent using only 1 bit which is +Δ or -Δ Hence there is bandwidth saving in Delta modulation​
• DM has a simple hardware requirement in comparison to PCM.

A comparison of different modulation schemes is as shown in the table below:

Parameter	PCM	Delta Modulation (DM)	DPCM
Number of bits	It can use 4, 8 or 16 bits per sample	It uses only one bit for one sample	Bits can be more than one but are less than PCM
Level/Step size	Step size is fixed	Step size is fixed and cannot be varied	Fixed number of levels are used.
Quantization error or Distortion	Quantization error depends on the number of levels used	Slope overload distortion and granular noise is present	Slope overload distortion and quantization noise is present
Bandwidth of the transmission channel	Highest bandwidth is required since the number of bits are high	Lowest bandwidth is required	The bandwidth required is lower than PCM
Signal to Noise ratio	Good	Poor	Fair
Area of Application	Audio and Video Telephony	Speech and images	Speech and video

Analysis:

A general expression for a phase-modulated wave is:

xPM (t) = A cos [2πfct + kpm(t)]

The instantaneous angle is given as:

ϕi(t) = 2πfct + kp m(t)

Phase deviation is, therefore:

\(\Delta [ϕ_i(t)] = k_p m(t)\)

Conclusion: The phase deviation is proportional to the message signal.

  

In phase modulation, if the question was asking about the frequency deviation, then it will be proportional to the frequency of the message signal as well. But the phase deviation is proportional to the amplitude of the message signal only as derived above.

Important Points

A wave has 3 parameters Amplitude, Phase, and Frequency. Thus there are 3 types of modulation techniques.

Amplitude Modulation:

The amplitude of the carrier is varied according to the amplitude of the message signal.

Frequency Modulation:

The frequency of the carrier is varied according to the amplitude of the message signal.

Phase Modulation:

The Phase of the carrier is varied according to the amplitude of the message signal.

To reduce the Quantization noise two things are done.

Oversampling and Noise shaping.

Explanation:

• An analog signal first undergoes the process of sampling before it is applied to ADC for conversion into a digital signal.
• Oversampling is a process in which an analog signal is sampled at a sampling frequency that is much greater than the Nyquist rate.

• Oversampling is:

  fs' = K × fs , ( K > 1)

• A Sigma-Delta ADC is an example of an ADC that employs oversampling.
• It is the same as “Delta modulation”. That is 1 sample is represented with 1 bit.
• In oversampling correlation increases and Quantization noise decreases.

 

Delta Modulation: 

A signal is Delta modulated to convert its analog information into a binary sequence, i.e., 1s and 0s. Here only one bit is required to represent the analog input. An over-sampled input is taken to make full use of the signal correlation. A stair-case approximated waveform will be the output of the delta modulator with the step-size as delta (Δ).

Delta Modulator has two main drawbacks:

1. Slope overload distortion: When the rate of rising of the input signal x(t) is so high that the staircase signal can not approximate it. We will see consecutive pulses of the same polarity in the output for such type of distortion. 

2. Granular noise: When the step size is too large compared to a small variation in the input signal (input is almost constant). Here, the output of the Delta modulator will be a sequence of alternate +ve and -ve pulses. 

∴ We can conclude that if the consecutive pulses are of opposite polarity during any time interval then the signal is almost constant during that interval.