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Basic Electronics Communication Systems

Shrishail Bhat, Dept. of ECE, AITM Bhatkal 1

Communication Systems

Syllabus: Communication Systems: Introduction, Elements of Communication Systems,

Modulation: Amplitude Modulation, Spectrum Power, AM Detection (Demodulation), Frequency

and Phase Modulation. Amplitude and Frequency Modulation: A comparison. (6 Hours)

Introduction

One of the greatest applications of electrical technology is communication systems.

Communication is the process of transferring information from one point to the other.

Information may be in the form of voice, text, picture or a combination of these.

Elements of Communication System

Fig. 1 shows a block diagram of a communication system.

Fig. 1 Block diagram of a communication system

Source: The aim of a communication system is to convey a message and this message

originates from a source. Common examples of source are analog audio, video or some

digital data.

Modulator and Transmitter: It processes the message signal from the source and makes it

suitable for transmission over the channel. The transmitter consists of encoders, decoders,

transducers, amplifiers, etc.

Channel: It is the physical medium that connects transmitter and receiver. Communication

channels can be a pair of conductors, optical fiber or just free space.

Noise: Noise is random, unwanted energy that gets added to the message signal during

transmission over the channel.

Demodulator and Receiver: It performs the reverse process of modulation and transmission.

The receiver processes the signal and gets back the actual message that is transmitted. It

performs demodulation and extracts the message signal from the carrier wave. The receiver

consists of amplifier, detector, mixer, oscillator, transducer, etc.

Modulation

Baseband Communication

A signal in its original frequency is called a baseband signal and transfer of these

signals directly over the channel is called baseband communication.

Source Destination

Modulator

and

Transmitter

Channel

Noise

Demodulator and

Receiver

Communication Systems Basic Electronics

2 Shrishail Bhat, Dept. of ECE, AITM Bhatkal

However, the baseband signals are not suitable for transmission, as they get

attenuated and cannot travel longer distances. Hence modulation is used.

Modulation

Modulation is a process in which some characteristic or property of a high frequency

signal called carrier signal is varied in accordance with the instantaneous amplitude of the

message signal. The message signal is called the modulating signal.

The carrier signal is a sinusoidal signal that can be represented as

𝒗𝒄(𝒕) = 𝑽𝒄 𝐬𝐢𝐧(𝝎𝒄𝒕 + 𝜽) (1)

where 𝑣𝑐(𝑡) is instantaneous voltage as a function of time

𝑉𝑐 is peak amplitude

𝜔𝑐 is angular frequency (rad/s), 𝜔𝑐 = 2𝜋𝑓𝑐 where 𝑓𝑐 is carrier frequency in Hz

𝑡 is time in seconds

𝜃 is phase angle in radians

The characteristic of the carrier wave that is modified may be amplitude 𝑉𝑐, frequency

𝑓𝑐 or phase angle 𝜃. Accordingly, we have three types of modulation:

1. Amplitude Modulation

2. Frequency Modulation

3. Phase Modulation

The modulated signal is not a single frequency signal and it occupies a great

bandwidth. The bandwidth of the modulating signal also depends on the modulating signal

frequency range and the modulating scheme in use. Table 1 gives the commonly used

frequency ranges and their applications.

Table 1 Commonly used frequency ranges and applications

Frequency Range Applications

Super high frequencies (3 GHz – 30 GHz) Radar

Ultra high frequencies (300 MHz – 3 GHz) Communication satellites, cellular phones,

personal communication systems

Very high frequencies (30 MHz – 300 MHz) TV and FM broadcast

High frequencies (3 MHz – 30 MHz) Short-wave broadcast commercial

Medium frequencies (300 kHz – 3 MHz) AM broadcast

Low frequencies (30 kHz – 300 kHz) Navigation, submarine communications

Very low frequencies (3 kHz – 30 kHz) Submarine communications, navigation

Voice frequencies (300 Hz – 3 kHz) Audio, submarine communications,

navigation

Extremely low frequencies (30 Hz – 300 Hz) Power transmission



Need for Modulation

Since the baseband signals are incompatible for direct transmission over the channel,

modulation technique is used. The advantages of using modulation are as listed below:

1. Reduces the height of antenna: The minimum height of antenna required is given as 𝜆/4.

The wavelength 𝜆 is given as 𝜆 =𝑐

𝑓 where 𝑐 is the velocity of light and 𝑓 is the frequency.

Modulation increases the frequency of the signal to be radiated and thus reduces the

wavelength, which reduces the size of the antenna required.

2. Avoids mixing of signals: Different signals from different sources can be sent over the

same channel by using different carrier frequencies for these signals. This avoids mixing

of signals.

3. Increases the range of communication: Modulation increases the frequency of the signal

to be radiated and thus increases the distance over which the signals can be transmitted.

4. Allows multiplexing of signals: Multiplexing means transmission of two or more signals

simultaneously over the same channel. Different signals from different sources can be sent

over the same channel by using different carrier frequencies for these signals.

5. Allows adjustments in the bandwidth: Bandwidth of a modulated signal can be made

smaller or larger than the original signal. Signal to noise ratio (SNR), which is a function

of bandwidth, can thus be improved.

6. Improves quality of reception: Modulation reduces the effect of noise to great extent and

thus improves the quality of reception.

Amplitude Modulation

Amplitude Modulation is a process in which the amplitude of the carrier signal is

varied in accordance with the instantaneous amplitude of the message signal.

Fig. 2 shows a modulating signal, a higher frequency carrier and the amplitude

modulated signal.

The instantaneous value of the message signal (modulating signal) is

𝒗𝒎(𝒕) = 𝑽𝒎 𝐬𝐢𝐧 𝝎𝒎𝒕 (2)

where 𝑣𝑚(𝑡) is instantaneous amplitude of modulating signal

𝑉𝑚 is peak amplitude of modulating signal

𝜔𝑚 is angular frequency (rad/s), 𝜔𝑚 = 2𝜋𝑓𝑚 where 𝑓𝑚 is modulating frequency in Hz

The instantaneous value of the carrier signal is

𝒗𝒄(𝒕) = 𝑽𝒄 𝐬𝐢𝐧 𝝎𝒄𝒕 (3)

where 𝑣𝑐(𝑡) is instantaneous voltage of carrier signal

𝑉𝑐 is peak amplitude of carrier signal

𝜔𝑐 is angular frequency (rad/s), 𝜔𝑐 = 2𝜋𝑓𝑐 where 𝑓𝑐 is carrier frequency in Hz



Fig. 2 Amplitude modulation

The amplitude of amplitude modulated signal is then given by

𝑉𝐴𝑀 = 𝑉𝑐 + 𝑣𝑚(𝑡) (4)

Using Eqn. (2) in (4) 𝑉𝐴𝑀 = 𝑉𝑐 + 𝑉𝑚 sin 𝜔𝑚𝑡 (5)

The instantaneous value of amplitude modulated signal is then given by

𝑣𝐴𝑀(𝑡) = 𝑉𝐴𝑀 sin 𝜔𝑐𝑡 (6)

Using Eqn. (4) in (6) 𝒗𝑨𝑴(𝒕) = (𝑽𝒄 + 𝑽𝒎 𝐬𝐢𝐧 𝝎𝒎𝒕) 𝐬𝐢𝐧 𝝎𝒄𝒕 (7)

Eqn. (7) is the equation of the AM wave.

Modulation Index

Modulation index is defined as the amount by which the carrier amplitude gets

modified by the modulating signal. It is also called modulation factor, modulation coefficient

or the degree of modulation.

For amplitude modulation, the modulation index is given by

𝒎 =𝑽𝒎

𝑽𝒄 (8)

where 𝑉𝑚 is peak amplitude of modulating signal

𝑉𝑚

𝑉𝑐

𝑉𝑐 + 𝑉𝑚 𝑉𝑚

𝒗𝒎(𝒕)

𝒗𝒄(𝒕)

𝒗𝑨𝑴(𝒕)



𝑉𝑐 is peak amplitude of carrier signal

The modulation index of AM is a number between 0 and 1 and is often expressed as a

percentage and called the percentage modulation.

Modulation Index in terms of 𝑽𝒎𝒂𝒙 and 𝑽𝒎𝒊𝒏

Fig. 3 shows amplitude modulated wave in time domain.

Fig. 3 Amplitude modulated wave

From Fig. 3,

𝑽𝒎 =𝑽𝒎𝒂𝒙−𝑽𝒎𝒊𝒏

𝟐 (9)

and 𝑉𝑐 = 𝑉𝑚𝑎𝑥 − 𝑉𝑚 (10)

Substituting Eqn. (9) in Eqn. (10),

𝑉𝑐 = 𝑉𝑚𝑎𝑥 − (𝑉𝑚𝑎𝑥−𝑉𝑚𝑖𝑛

2)

𝑉𝑐 =2𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛

2

𝑽𝒄 =𝑽𝒎𝒂𝒙+𝑽𝒎𝒊𝒏

𝟐 (11)

Now we have modulation index

𝑚 =𝑉𝑚

𝑉𝑐=

𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛

2𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛

2

𝒎 =𝑽𝒎𝒂𝒙−𝑽𝒎𝒊𝒏

𝑽𝒎𝒂𝒙+𝑽𝒎𝒊𝒏 (12)



Overmodulation

In AM wave, overmodulation takes place when modulation index 𝑚 > 1, i.e. when

𝑉𝑚 > 𝑉𝑐. In overmodulated AM wave, loss of information takes place and hence it must be

avoided. Fig. 4 shows an overmodulated wave when 𝑚 = 1.25.

Fig. 4 Overmodulated AM wave

Fig. 5 shows an AM wave when 𝑚 = 1.

Fig. 5 AM wave when 𝑚 = 1

Frequency Spectrum

We know that the amplitude modulated signal is

𝒗𝑨𝑴(𝒕) = (𝑽𝒄 + 𝑽𝒎 𝐬𝐢𝐧 𝝎𝒎𝒕) 𝐬𝐢𝐧 𝝎𝒄𝒕



We also know that modulation index is given by

𝒎 =𝑽𝒎

𝑽𝒄

∴ 𝑽𝒎 = 𝒎𝑽𝒄 (13)

Using Eqn. (13) in equation for 𝑣𝐴𝑀(𝑡), we get

𝑣𝐴𝑀(𝑡) = (𝑉𝑐 + 𝑚𝑉𝑐 sin 𝜔𝑚𝑡) sin 𝜔𝑐𝑡

𝒗𝑨𝑴(𝒕) = 𝑽𝒄(𝟏 + 𝒎 𝐬𝐢𝐧 𝝎𝒎𝒕) 𝐬𝐢𝐧 𝝎𝒄𝒕 (14)

𝒗𝑨𝑴(𝒕) = 𝑽𝒄 𝐬𝐢𝐧 𝝎𝒄𝒕 + 𝒎𝑽𝒄 𝐬𝐢𝐧 𝝎𝒎𝒕 𝐬𝐢𝐧 𝝎𝒄𝒕 (15)

Using the trigonometric relation sin 𝐴 sin 𝐵 =1

2[cos(𝐴 − 𝐵) − cos(𝐴 + 𝐵)], we get

𝒗𝑨𝑴(𝒕) = 𝑽𝒄 𝐬𝐢𝐧 𝝎𝒄𝒕 +𝒎𝑽𝒄

𝟐𝐜𝐨𝐬(𝝎𝒄 − 𝝎𝒎)𝒕 −

𝒎𝑽𝒄

𝟐𝐜𝐨𝐬(𝝎𝒄 + 𝝎𝒎)𝒕 (16)

Similarly, if 𝒗𝒎(𝒕) = 𝑽𝒎 𝐜𝐨𝐬 𝝎𝒎𝒕 and 𝒗𝒄(𝒕) = 𝑽𝒄 𝐜𝐨𝐬 𝝎𝒄𝒕, then

𝑣𝐴𝑀(𝑡) = (𝑉𝑐 + 𝑚𝑉𝑐 cos 𝜔𝑚𝑡) cos 𝜔𝑐𝑡

𝑣𝐴𝑀(𝑡) = 𝑉𝑐 cos 𝜔𝑐𝑡 + 𝑚𝑉𝑐 cos 𝜔𝑚𝑡 cos 𝜔𝑐𝑡

Using the trigonometric relation cos 𝐴 cos 𝐵 =1

2[cos(𝐴 − 𝐵) + cos(𝐴 + 𝐵)], we get

𝒗𝑨𝑴(𝒕) = 𝑽𝒄 𝐜𝐨𝐬 𝝎𝒄𝒕 +𝒎𝑽𝒄

𝟐𝐜𝐨𝐬(𝝎𝒄 − 𝝎𝒎)𝒕 +

𝒎𝑽𝒄

𝟐𝐜𝐨𝐬(𝝎𝒄 + 𝝎𝒎)𝒕 (17)

From Eqn. (16) and (17), we can say that the first term represents unmodulated carrier

and two additional terms represent two sidebands. The frequency of lower sideband is

𝑓𝐿𝑆𝐵 = 𝑓𝑐 − 𝑓𝑚 and the frequency of upper sideband is 𝑓𝑈𝑆𝐵 = 𝑓𝑐 + 𝑓𝑚.

Fig. 6 represents the frequency spectrum of AM wave.

Fig. 6 Frequency spectrum of an AM wave

Carrier Lower side band Upper side band

Carrier Lower side band Upper side band



Bandwidth of AM Wave

The bandwidth of an AM wave is given by

𝐵𝑊 = 𝑓𝑈𝑆𝐵 − 𝑓𝐿𝑆𝐵

𝐵𝑊 = (𝑓𝑐 + 𝑓𝑚) − (𝑓𝑐 − 𝑓𝑚)

𝑩𝑾 = 𝟐𝒇𝒎

Spectrum Power

The AM wave has three components: unmodulated carrier, lower sideband and upper

sideband. Therefore, the power of an AM wave is the sum of carrier power 𝑃𝑐, power in lower

sideband 𝑃𝐿𝑆𝐵 and power in upper sideband 𝑃𝑈𝑆𝐵 .

The total transmitted power is given as

𝑷𝑻𝒐𝒕𝒂𝒍 = 𝑷𝒄 + 𝑷𝑳𝑺𝑩 + 𝑷𝑼𝑺𝑩 (18)

If average carrier voltage is (𝑉𝑐

√2), average carrier power is given by

𝑃𝑐 =

(𝑉𝑐

√2)

2

𝑅

𝑷𝒄 =𝑽𝒄

𝟐

𝟐𝑹 (19)

Similarly, if average sideband voltage is (𝑚𝑉𝑐

2

√2) , average power in lower sideband and upper

sideband,

𝑃𝐿𝑆𝐵 = 𝑃𝑈𝑆𝐵 =

(

𝑚𝑉𝑐

2√2

)

2

𝑅

𝑷𝑳𝑺𝑩 = 𝑷𝑼𝑺𝑩 =𝒎𝟐𝑽𝒄

𝟐

𝟖𝑹 (20)

We can also write

𝑃𝐿𝑆𝐵 = 𝑃𝑈𝑆𝐵 =𝑚2𝑉𝑐

2

8𝑅=

𝑚2

4×

𝑉𝑐2

2𝑅

But 𝑉𝑐

2

2𝑅= 𝑃𝑐. Therefore

𝑷𝑳𝑺𝑩 = 𝑷𝑼𝑺𝑩 =𝒎𝟐

𝟒𝑷𝒄 (21)

By using Eqn. (19) and (20) in Eqn. (18) , the average total transmitted power is then

given by,

𝑃𝑇𝑜𝑡𝑎𝑙 =𝑉𝑐

2

2𝑅+

𝑚2𝑉𝑐2

8𝑅+

𝑚2𝑉𝑐2

8𝑅




2

2𝑅(1 +

𝑚2

4+

𝑚2

4)


2

2𝑅(1 +

𝑚2

2)

∴ 𝑷𝑻𝒐𝒕𝒂𝒍 = 𝑷𝒄 (𝟏 +𝒎𝟐

𝟐) (22)

Modulation Index in terms of 𝑷𝑻 and 𝑷𝒄

We know that total transmitted power

𝑃𝑇𝑜𝑡𝑎𝑙 = 𝑃𝑐 (1 +𝑚2

2)

𝑃𝑇𝑜𝑡𝑎𝑙

𝑃𝑐= 1 +

𝑚2

2

𝑚2

2=

𝑃𝑇𝑜𝑡𝑎𝑙

𝑃𝑐− 1

𝑚2 = 2 (𝑃𝑇𝑜𝑡𝑎𝑙

𝑃𝑐− 1)

𝒎 = √𝟐 (𝑷𝑻𝒐𝒕𝒂𝒍

𝑷𝒄− 𝟏) (23)

Transmission Efficiency

The transmission efficiency of AM wave is defined as the ratio of the transmitted

power which contains the information to the total transmitted power. In an AM wave, the

information is contained in the sidebands.

The transmission efficiency is then given by,

𝜼 =𝑷𝑳𝑺𝑩+𝑷𝑼𝑺𝑩

𝑷𝑻𝒐𝒕𝒂𝒍 (24)

Using Eqn. (21) and (22) in Eqn. (24), we get

𝜂 =

𝑚2

4 𝑃𝑐 +𝑚2

4 𝑃𝑐

𝑃𝑐 (1 +𝑚2

2 )

𝜂 =𝑃𝑐 (

𝑚2

4 +𝑚2

4 )

𝑃𝑐 (1 +𝑚2

2 )

𝜂 =

𝑚2

2

1 +𝑚2

2

=

𝑚2

22 + 𝑚2

2

𝜼 =𝒎𝟐

𝟐+𝒎𝟐 (25)



The percentage transmission efficiency is given as

% 𝜼 =𝒎𝟐

𝟐 + 𝒎𝟐× 𝟏𝟎𝟎 %

Improvement Techniques

The modulating signal is band of frequencies with varying amplitude of ∆𝜔

components. The spectrum of the modulating signal is shown in Fig. 7 (a). The corresponding

spectrum of the modulated signal is shown in Fig. 7 (b).

Fig. 7 AM improvement techniques

We observe that the information is contained only in the sidebands and the carrier

contains no information. Hence to improve the power efficiency, the carrier need not be

transmitted, but only two sidebands are transmitted. This is called double sideband

suppressed carrier (DSB-SC) technique as shown in Fig. 7 (c). DSB-SC requires less

transmission power, but the carrier has to be generated at the receiving end by a high

frequency oscillator.

Furthermore, as upper and lower sidebands are mirror images of each other, it is

sufficient to transmit only the upper sideband. This is called Single Side Band (SSB) as shown

in Fig. 7 (d). The detector at the receiving end becomes complicated.

AM Detection (Demodulation)

Detection or demodulation is the process of recovering the original modulating signal

from the received signal at the receiver. The simplest demodulator for AM is the envelope

detector.

𝑉𝑚 𝑚𝑉𝑐

2

(a) Modulating signal (b) Modulated signal

(c) DSB-SC (d) SSB



Fig. 8 shows a demodulation circuit. It consists of a diode as half wave rectifier and

RC circuit as a low pass filter. The received signal is passed through a diode to cut-off the

lower half and the peaks detected and smoothed out by a parallel RC circuit.

Fig. 8 Demodulation circuit

The time constant RC must meet the following condition:

𝑻𝒄 ≪ 𝑹𝑪 ≪ 𝑻𝒎

where 𝑇𝑐 is carrier time period, 𝑇𝑐 =1

𝑓𝑐=

𝜔𝑐

2𝜋

𝑇𝑚 is time period of modulating signal, 𝑇𝑚 =1

𝑓𝑚=

𝜔𝑚

2𝜋

The condition can also be written as

𝝎𝒄

𝟐𝝅≪ 𝑹𝑪 ≪

𝝎𝒎

𝟐𝝅

Fig. 9 shows the demodulator waveforms.



Fig. 9 Demodulator waveforms

Frequency and Phase Modulation

The frequency and phase of the carrier signals are closely related, since frequency is

the rate of change of phase angle. If either frequency or phase is changed in a modulation

system, the other will change as well. So frequency modulation and phase modulation are

generally known as angle modulation.

Frequency Modulation

Frequency Modulation is a process in which the frequency of the carrier signal is

varied in accordance with the instantaneous amplitude of the message signal.

Fig. 10 shows a sine wave modulating a higher frequency carrier signal with frequency

modulation.

The frequency modulated signal is represented by

𝒗(𝒕) = 𝑨 𝒔𝒊𝒏 [𝝎𝒄𝒕 +∆𝒇

𝒇𝒎𝒔𝒊𝒏 𝝎𝒎𝒕]

𝒗(𝒕) = 𝑨 𝒔𝒊𝒏[𝝎𝒄𝒕 + 𝒎𝒇 𝒔𝒊𝒏 𝝎𝒎𝒕]

where 𝝎𝒄 = 2𝜋𝑓𝑐 is the angular frequency of carrier signal

𝝎𝒎 = 2𝜋𝑓𝑚 is the angular frequency of modulating signal

∆𝑓 is the frequency deviation

𝑚𝑓 is the modulation index of FM



Fig. 10 Frequency modulation

Frequency Deviation

The amount of change in carrier frequency produced by the modulating signal is

known as frequency deviation. Maximum frequency deviation occurs at the maximum

amplitude of the modulating signal.

Modulation Index

Modulation index of FM is the ratio of the frequency deviation to the modulating

frequency.

𝒎𝒇 =∆𝒇

𝒇𝒎

where ∆𝑓 is the frequency deviation

𝑓𝑚 is the modulating frequency

Bandwidth of FM Wave

The bandwidth of an FM signal is given by

𝑩𝑾 = 𝟐[𝒎𝒇 + 𝟏]𝒇𝒎

𝐵𝑊 = 2 [∆𝑓

𝑓𝑚+ 1] 𝑓𝑚

𝑩𝑾 = 𝟐[∆𝒇 + 𝒇𝒎]

𝑉𝑚

𝑉𝑐

𝑉𝑐



Phase Modulation

Phase Modulation is a process in which the phase angle of the carrier signal is varied

in accordance with the instantaneous amplitude of the message signal.

The phase modulated signal is represented by

𝒗(𝒕) = 𝑨 𝒔𝒊𝒏[𝝎𝒄𝒕 + 𝒎𝒑 𝒔𝒊𝒏 𝝎𝒎𝒕]

where 𝝎𝒄 = 2𝜋𝑓𝑐 is the angular frequency of carrier signal

𝝎𝒎 = 2𝜋𝑓𝑚 is the angular frequency of modulating signal

𝑚𝑝 is the modulation index of PM

Fig. 11 shows a modulating sine wave and a phase modulated signal.

Fig. 11 Phase modulation

Here the positive half cycle of modulating signal produces a lagging phase shift and

negative half cycle produces a leading phase shift.

As the modulating signal goes positive, amount of phase lag increases with the

increase of modulating signal. This results in lower frequency of the modulated signal.

As the modulating signal goes negative, amount of phase lead increases with the

increase of modulating signal. This results in higher frequency of the modulated signal.

Amplitude and Frequency Modulation: A Comparison

Table 2 gives a comparison of amplitude modulation and frequency modulation

techniques with reference to different characteristics.



Table 2 Comparison between AM and FM

Characteristics Amplitude Modulation Frequency Modulation

Wave equation 𝑣(𝑡) = 𝑉𝑐[1 + 𝑚 sin 𝜔𝑚𝑡] sin 𝜔𝑐𝑡 𝑣(𝑡) = 𝐴 𝑠𝑖𝑛[𝜔𝑐𝑡 + 𝑚𝑓 𝑠𝑖𝑛 𝜔𝑚𝑡]

Principle

The amplitude of the carrier

varies in accordance with the

message signal. Carrier

frequency remains constant.

The frequency of the carrier

varies in accordance with the

message signal. Carrier

amplitude remains constant.

Modulation index Modulation index is always

between zero and one

Modulation index can be

either less than one or more

than one

No. of sidebands Only two sidebands are

produced

A large number of sidebands

are produced

Channel bandwidth AM has smaller bandwidth,

𝐵𝑊 = 2𝑓𝑚

FM has larger bandwidth

because it produces a larger

number of side bands.

𝐵𝑊 = 2[∆𝑓 + 𝑓𝑚]

Operating carrier

frequency

AM utilizes lower carrier

frequency

FM utilizes higher carrier

frequency (above 30 MHz)

because of its higher

bandwidth

Transmission efficiency AM has lesser transmission

efficiency FM has better efficiency

Noise performance AM has poor noise

performance

FM has better noise

performance

Common Channel

Interference (CCI)

Due to CCI, distortion occurs

in AM

FM is better due to capture

effect

Externally generated

noise pulses

In AM, such tuning is not

essential

FM receiver responds slightly

to noise pulses generated by

external sources, but if it is

slightly mistuned, then its

ability to suppress noise pulses

is highly reduced

Area of reception AM covers more distance than

FM

FM is limited to a small

distance; as distance increases,

signal quality becomes poorer



Questions

1. Define communication. (Dec ’17)

2. With a neat block diagram, explain the elements of communication system.

(Dec ’17 – 6M, Jun ’17 – 6M, Dec ’16 – 5M, Jun ’16 – 6M, MQP ’15 – 5M)

3. What are commonly used frequency ranges in communication system? Mention the

application of each range. (Dec ’17 – 4M, Dec ’14 – 5M, MQP ’14 – 4M)

4. What is modulation? Explain the need for modulation. List the different types of

modulation schemes.

(Jun ’17 – 4M, Jun ’16 – 5M, Dec ’15 – 5M, Jun ’15 - 6M, Dec ’14 – 4M)

5. What is amplitude modulation? Explain with neat waveforms and derive the

expression for the AM wave. Also draw the frequency spectrum.

(Dec ’17 – 8M, Jun ’17 – 8M, Dec ’16 – 6M, Jun ’16 – 5M, Dec ’15 – 8M, Jun ’15 – 8M,

Dec ’14, MQP ’15 – 5M, MQP ‘14)

6. Define amplitude modulation. Draw the AM signal and its spectrum. For an

amplitude modulated wave, prove that total power is given by Pt = Pc [1 +μ2

2], where

μ is the modulation index. (Dec ’17 – 6M)

7. Define modulation index. Obtain the expression for modulation index of AM wave in

terms of 𝑉𝑚𝑎𝑥 and 𝑉𝑚𝑖𝑛. (Dec ‘15)

8. Derive an expression for modulation index in AM. (Dec ’16 – 6M)

9. Derive the expression for the total power transmitted in an AM wave.

(Jun ’17, Dec ’14 – 5M, MQP ’14 – 6M)

10. With a neat diagram, explain demodulation (detection) of an AM wave.

(Jun ’17 – 4M, Jun ’16 – 5M)

11. Explain frequency modulation with neat waveforms.

(Jun ’17 – 6M, Dec ’16 – 5M, Dec ’15 – 8M, MQP ’15 – 5M)

12. Mention the advantages of frequency modulation. (Jun ’16 - 5M)

13. Differentiate between amplitude modulation and frequency modulation.

(Dec ’17 – 6M, Jun ’17 – 4M, Dec ’16 – 5M, Dec ’15 – 4M, Jun ’15 – 8M, Dec ’15 – 5M,

MQP ’15 – 6M, MQP ’14 – 4M)

14. A carrier of 10 V peak and frequency 100 kHz is amplitude modulated by a sine wave

of 4 V and frequency 1000 Hz. Determine the modulation index for the modulated

wave and draw the amplitude spectrum. (Dec ’16 – 6M)

15. An audio frequency signal 5 sin 2π(1000)t is used to amplitude modulate a carrier of

100 sin 2π(106)t. Assume modulation index of 0.4. Find

i) Sideband frequencies

ii) Bandwidth required

iii) Amplitude of each sideband



iv) Total power delivered to a load of 100Ω (Dec ’17 – 6M)

16. An audio frequency signal 10 sin(2π × 500)t is used to amplitude modulate a carrier

of 50 sin(2π × 105)t. Calculate

i) Modulation index

ii) Sideband frequencies

iii) Bandwidth

iv) Amplitude of each sideband

v) Total power delivered to a load of 600Ω

vi) Transmission efficiency (Dec ’16 – 8M, Jun ’15 – 6M)

17. A carrier of 1 MHz, with 400 W of its power is amplitude modulated with a sinusoidal

signal of 2500 Hz. The depth of modulation is 75%. Calculate the sideband frequencies,

the bandwidth, the power in the sidebands and the total power in the modulated

wave. (Jun ’16 – 5M)

18. A 1 MHz carrier is amplitude modulated by a 40 kHz modulating signal with a

modulation index of 0.5. The unmodulated carrier is having a power of 1 kW.

Calculate the power of the amplitude modulated signal. Also find the sideband

frequencies. (Jun ’16 – 5M)

19. A 500 W, 1 MHz carrier is amplitude modulated with a sinusoidal signal of 1 kHz.

The depth of modulation is 60%. Calculate the bandwidth, power in the sidebands

and the total power transmitted. (Dec ’15 - 7M)

20. The total power content of an AM signal is 1000 W. Determine the power being

transmitted at carrier frequency and at each of the sidebands when percentage

modulation is 100%. (Dec ’14 – 5M)

21. A 500 W, 100 kHz carrier is modulated to depth of 60% by modulating signal of

frequency 1 kHz. Calculate the total power transmitted. What are the side band

components of the AM wave? (MQP ’15 - 6M)

22. Calculate the percentage power saving when one side band and carrier is suppressed

in an AM signal with modulation index equal to 1. (MQP ’14 - 5M)

23. If an FM wave is represented by the equation V = 10 sin(8 × 108 + 4 sin 1000t),

calculate

i) Carrier frequency

ii) Modulating frequency

iii) Modulation index

iv) Bandwidth (Dec ’17 – 6M)

24. A 15 kHz audio signal is used to frequency modulate a 100 MHz carrier, causing

deviation of 75 kHz. Determine modulation index and bandwidth of the FM signal.

(Dec ’16 – 4M)



References

1. D.P. Kothari, I. J. Nagrath, “Basic Electronics”, McGraw Hill Education (India) Private

Limited, 2014.

2. Simon Haykins, “Communication Systems”, 5th Edition, John Willey India Pvt. Ltd.,

2009.

3. Simon Haykins, “An Introduction to Analog and Digital Communication”, John

Wiley India Pvt. Ltd., 2008

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