Communication Systems Test - 3

Bandwidth is the portion of the spectrum that a given telecommunications system can use. For example, a system that operates on frequencies between 150 and 200 MHz has a bandwidth of 50 MHz.

Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.

Digital signals are in the form of rectangular waves. This rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies \(ν_{o}, 2ν_{o}, 3ν_{o}, 4ν_{o},..., nν_{o}\) where, n is an integer extending to infinity and \(ν_{o}\) is the fundamental frequency.

It is clear that to reproduce the rectangular wave shape exactly, we need to superimpose all the harmonics \(ν_{o}, 2ν_{o}, 3ν_{o}, 4ν_{o},....\) which implies an infinite bandwidth.

However, for practical purposes, the contribution from higher harmonics can be neglected, thus limiting the bandwidth.

As a result, received waves are a distorted version of the transmitted one.

If the bandwidth is large enough to accommodate a few harmonics, the information is not lost and the rectangular signal is more or less recovered. This is so because the higher the harmonic, the less is its contribution to the waveform.

The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.

Hence, the correct option is (B).

Any device that converts one form of energy into another can be termed as a transducer.

In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form.

An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc) into corresponding variations in the electrical signal at its output.

From the above, it is clear that any device that converts one form of energy into another can be termed as a transducer.

Hence, the correct option is (C).

Channel imperfection is that the reason for distortion when a transmitted signal propagates along the channel. Moreover, the receiver receives a corrupted version of the transmitted signal thanks to the addition of noise to the transmitted signal. Distortion is the alteration of the original shape (or other characteristic) of something. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal representing sound or a video signal representing images, in an electronic device or communication channel.

Hence, the correct option is (A).

Detection of amplitude modulated wave:

• The transmitted message gets attenuated in propagating through the channel.
• The receiving antenna is, therefore, to be followed by an amplifier and a detector.
• In addition, to facilitate further processing, the carrier frequency is usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the detection.
• The detected signal may not be strong enough to be made use of and therefore, is required to be amplified.
• Detection is the process of recovering the modulating signal from the modulated carrier wave.
• An envelope detector is an electronic circuit that takes a (relatively) high-frequency amplitude modulated signal as input and provides an output, which is the demodulated envelope of the original signal.
• A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC).
  
• But amplifier is not a part of this system.
  

Hence, the correct option is (C).

It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

The loss of strength of a signal while propagating through a medium is known as attenuation.

Bandwidth refers to the frequency range over which a piece of equipment operates or the portion of the spectrum occupied by the signal.

We know that the largest distance between a source and a destination up to which the signal is received with sufficient strength is called the range of the communication system.

Hence, the correct option is (B).

Irrespective of the nature of the communication system, every communication system has three essential elements:

1. Transmitter
2. Medium or channel
3. Receiver

In a communication system, the transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter and the channel is the physical medium that connects them.

Depending upon the type of communication system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.

The purpose of the transmitter is to convert the message signal produced by the source of information into a suitable form for transmission through the channel.

If the output of the information source is a non-electrical signal like a voice signal, a transducer converts it to an electrical form before giving it as an input to the transmitter.

When a transmitted signal propagates along the channel it may get distorted due to channel imperfection.

Moreover, noise adds to the transmitted signal and the receiver receives a corrupted version of the transmitted signal.

The receiver has the task of operating on the received signal. It reconstructs a recognizable form of the original message signal for delivering it to the user of information.

Hence, the correct option is (D).

Given: \(f =2\) MHz \(=2 \times 10^{6}\) Hz

We know that the minimum size of the antenna required to send the electromagnetic wave signal is given as,

\(l=\frac{c}{4 f} \quad \cdots\) (1)

Where, \(f=\) frequency and \(c=3 \times 10^{8}\) m/sec \(=\) speed of light

By equation (1) the size of the antenna required to transmit an electromagnetic signal of frequency \(2\) MHz is given as,

\(l=\frac{3 \times 10^{8}}{4 \times 2 \times 10^{6}}\)

\(\Rightarrow l=37.5\) m

Hence, the correct option is (A).

Repeater is a combination of transmitter, amplifier, and receiver.

The receiver receives the signal, which is then amplified using the amplifier. This amplified signal is then transmitted using a transmitter.

A number of repeaters are used in the transmission medium to increase the range of communication. A signal during transmission gets attenuated so repeaters are necessary for effective communication.

A communication satellite is essentially a repeater station in space. A repeater is a combination of a receiver and a transmitter. A repeater picks up the signal from the transmitter, then amplifies and retransmits it to the receiver sometimes with a change in carrier frequency.

Hence, the correct option is (C).

Given: f_m = 20 kHz and f_c = 3 MHz = 3000 kHz

Where, f_c = frequency of the carrier wave and f_m = frequency of the modulating signal

We know that that (f_c + f_m) and (f_c - f_m) are the sidebands of a modulated signal.

Therefore the sidebands is given as,

S₁ = (f_c + f_m)

⇒ S₁ = (3000 + 20)

⇒ S₁ = 3020 kHz     -----(1)

And

S₁ = (f_c - f_m)

⇒ S₁ = (3000 - 20)

⇒ S₁ = 2980 kHz

Hence, the correct option is (C).

Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space, and fiber optic cable.

Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz.

Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.

Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet).

Hence, the correct option is (C).

Effective power radiated by an antenna:

• A theoretical study of radiation from a linear antenna shows that the effective power radiated by an antenna is inversely proportional to the square of the wavelength.
• This implies that for the same antenna length, the power radiated increases with decreasing wavelength or increasing frequency. Thus, the effective power radiated by a long-wavelength baseband signal would be small.
• For a good transmission, we need high powers and hence this also points out the need of using high-frequency transmission.

\(P \propto \frac{1}{\lambda^{2}}\)

Where, \(P=\) effective power radiated by an antenna and \(\lambda=\) wavelength

A theoretical study of radiation from a linear antenna shows that the effective power radiated by an antenna is inversely proportional to the square of the wavelength.

Hence, the correct option is (B).

In general, different kinds of media use different bandwidths of transmission channels to broadcast their message so that it should not interfere with other EM signals broadcasted simultaneously from different stations.

Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.

Hence, the correct option is (A).

In amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal.

Here, we explain the amplitude modulation process using a sinusoidal signal as the modulating signal.

Let \(c(t)=A_{c} \sin \left(\omega_{c} t\right)\) represent carrier wave and \(m(t)=A_{m} \sin \left(\omega_{m} t\right)\) represent the message or the modulating signal.

We know that if \(c(t)=A_{c} \sin \left(\omega_{c} t\right)\) represent carrier wave and \(m(t)=A_{m} \sin \left(\omega_{m} t\right)\) represent message signal, then the modulation index is equal to the,

\(\mu=\frac{A_{m}}{A_{c}}\)

In practice, \(\mu\) is kept less than \(1\) to avoid distortion.

If \(A_{m}>A_{c}\)

Then, \(\mu>1\)

Therefore, distortion will happen with the modulated signal.

Hence, the correct option is (C).

In telecommunications, a point to point connection refers to a communications connection between two communication endpoints or nodes. An example is a telephone call, in which one telephone is connected with one other, and what is said by one caller can only be heard by the other. This is contrasted with a point to multipoint or broadcast connection, in which many nodes can receive information transmitted by one node. Other examples of point to point communications links are leased lines and microwave radio relay.

In point to point communication mode, communication takes place over a link between a single transmitter and a receiver. A telephone call is an example of such a mode of communication.

Hence, the correct option is (A).

Given: f_c + f_m = 1020 kHz and f_c - f_m = 980 kHz

Where, f_c = frequency of the carrier wave and f_m = frequency of the modulating signal

We know that that (f_c + f_m) and (f_c - f_m) are the sidebands of a modulated signal.

Therefore by adding the two sidebands,

(f_c + f_m) + (f_c - f_m) = (1020 + 980) kHz

⇒ 2f_c = 2000 kHz

⇒ f_c = 1000 kHz

⇒ f_c = 1 MHz

Hence, the correct option is (A).

Given: \(f _{1}=4\) MHz and \(f _{2}=6\) MHz

A theoretical study of radiation from a linear antenna shows that the effective power radiated by an antenna is inversely proportional to the square of the wavelength.

\(P \propto \frac{1}{\lambda^{2}} \quad \cdots\) (1)

We know that,

\(f=\frac{c}{\lambda} \quad \cdots\) (2)

Where, \(P=\) effective power radiated by an antenna,

\(f =\) frequency, \(\lambda=\) wavelength and \(c =3 \times 10^{8}\) m/sec \(=\) speed of light

By equation (1) and equation (2),

\(P \propto f^{2}\quad \cdots\) (3)

For case 1: \(( f _{1}=4\) MHz\()\)

\(P_{1} \propto f_{1}^{2}\quad \cdots\) (4)

For case 2: \(f_{2}=6\) MHz

\(P_{2} \propto f_{2}^{2}\quad \cdots\) (5)

By equation (4) and equation (5),

\(\frac{P_{1}}{P_{2}}=\frac{f_{1}^{2}}{f_{2}^{2}}\)

\(\Rightarrow \frac{P_{1}}{P_{2}}=\frac{4^{2}}{6^{2}}\)

\(\Rightarrow \frac{P_{1}}{P_{2}}=\frac{4}{9}\)

Hence, the correct option is (A).

Important argument against transmitting baseband signals directly is more practical in nature.

Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously.

All these signals will get mixed up and there is no simple way to distinguish between them.

This points out a possible solution by using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission.

The above arguments suggest that there is a need for translating the original low-frequency baseband message or information signal into a high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.

In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it.

We know that modulation is a process in which the low-frequency baseband message or information signal is transmitted with the help of a high-frequency wave.

Hence, the correct option is (A).

Given: l=100 m

We know that the minimum size of the antenna required to send the electromagnetic wave signal is given as,

l=λ4⋯ (1)

Where, l= minimum size of the antenna and λ= wavelength

By equation (1) the maximum wavelength of the signal that can be transmitted by an antenna of length 100 m is given as,

λ=4l

⇒λ=4×100

⇒λ=400 m

So, we can say that the antenna of length 100 m can transmit the signal of wavelength 400 m or less.

Hence, the correct option is (B).

The full form of ITU is International Telecommunication Union.

The International Telecommunication Union is a specialized agency of the United Nations responsible for all matters related to information and communication technologies. It was established in 17 May 1865 as the International Telegraph Union, making it among the oldest international organizations still in operation. The ITU promotes the shared global use of the radio spectrum, facilitates international cooperation in assigning satellite orbits, assists in developing and coordinating worldwide technical standards, and works to improve telecommunication infrastructure in the developing world.

Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.

Hence, the correct option is (B).

Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.

This range of frequencies is further subdivided and allocated for various services as indicated in the given table.

Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.

Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.

The commonly used transmission media in communication systems are wire, free space, and fiber optic cable.

Hence, the correct option is (D).

The original low-frequency message or information signal cannot be transmitted to long distances. Therefore, at the transmitter, the information contained in the low-frequency message signal is superimposed on a high-frequency wave, which acts as a carrier of the information. This process is known as modulation. 

Hence, the correct option is (B).

Reciever extracts the message signal from the transmitted carrier signal. It reconstructs actual signals using output transducers. Transducers are devices that convert electromagnetic signals into sound, light or electrical signals. It is present at the output end of the channel. here it segregates the message and unwanted signals.

The receiver operates on the received signal picked up from the transmitted signal at the channel output and changes it to a form corresponding to the original message signal for delivering.

We know that the receiver has the task of operating on the received signal in the communication system. It reconstructs a recognizable form of the original message signal for delivering it to the user of information.

Hence, the correct option is (C).

Given: \(A_{m}=15\) volts and \(A_{c}=30\) volts

Where, \(A_{c}=\) amplitude of the carrier wave and \(A_{m}=\) amplitude of the modulating signal

We know that if \(c(t)=A_{c} \sin \left(\omega_{c} t\right)\) represent carrier wave and \(m(t)=A_{m} \sin \left(\omega_{m} t\right)\) represent message signal, then the modulation index is equal to the,

\(\mu=\frac{A_{m}}{A_{c}}\)

\(\Rightarrow \mu=\frac{15}{30}\)

\(\Rightarrow \mu=0.5\)

Hence, the correct option is (B).

The process of retrieval of information from the carrier wave at the receiver is termed demodulation.

This is the reverse process of modulation. Demodulation is a key process in the reception of any amplitude modulated signals whether used for broadcast or two way radio communication systems.

Demodulation is the process by which the original information bearing signal, i.e. the modulation is extracted from the incoming overall received signal.

The process of demodulation for signals using amplitude modulation can be achieved in a number of different techniques, each of which has its own advantage.

Hence, the correct option is (B).

We can obtain transmission with a reasonable antenna length if the transmission frequency is high (for example, if v is 1 MHz, then \(\lambda\) is 300 m).
Therefore, there is a need of translating the information contained in our original low frequency baseband signal into high or radio frequencies before transmission.
The minimum size of the antenna required to send the electromagnetic wave signal is given as, \(l=\frac{\lambda}{4}=\frac{c}{4 f}\)
Where, \(\lambda=\) wavelength, f \(=\) frequency and c \(=3 \times 10^{8}\) m/sec \(=\) speed of light
We know that the minimum size of the antenna required to send the electromagnetic wave signal is given as,
\(l=\frac{c}{4 f}\)
\(\Rightarrow l\propto \frac{1}{\mathrm{f}}\)
Where, f \(=\) frequency and c \(=3 \times 10^{8}\) m/sec \(=\) speed of light
Hence, the correct option is (B).

Important argument against transmitting baseband signals directly is more practical in nature.

Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously.

All these signals will get mixed up and there is no simple way to distinguish between them.

This points out a possible solution by using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission.

The above arguments suggest that there is a need for translating the original low-frequency baseband message or information signal into a high-frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal.

In doing so, we take the help of a high-frequency signal, known as the carrier wave, and a process known as modulation which attaches information to it.

We know that the significant characteristics of a pulse are pulse amplitude, pulse duration or pulse width, and pulse position (denoting the time of rise or fall of the pulse amplitude).

Hence, the correct option is (D).

In telecommunication and information theory, broadcasting is a method of transferring a message to all recipients simultaneously. Broadcasting can be performed as a high-level operation in a program, for example, broadcasting in Message Passing Interface, or it may be a low-level networking operation, for example broadcasting on Ethernet.

In the broadcast mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are examples of broadcast modes of communication.

Hence, the correct option is (A).

In amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal.

Here, we explain the amplitude modulation process using a sinusoidal signal as the modulating signal.

Let \(c(t)=A_{c} \sin \left(\omega_{c} t\right)\) represent carrier wave and \(m(t)=A_{m} \sin \left(\omega_{m} t\right)\) represent the message or the modulating signal.

We know that if \(c(t)=A_{c} \sin \left(\omega_{c} t\right)\) represent carrier wave and \(m(t)=A_{m} \sin \left(\omega_{m} t\right)\) represent message signal, then the modulated signal is represented as,

\(c _{ m }( t )=\left[ A _{ c }+ A _{ m } \cdot \sin \left(\omega_{ m } t \right)\right] \cdot \sin \left(\omega_{ c } t \right)\)

Where, \(A_{c}=\) amplitude of the carrier wave, \(A_{m}=\) amplitude of the modulating signal, \(\omega_{c}=\) angular frequency of the carrier wave, and \(\omega_{ m }=\) angular frequency of the modulating signal.

Hence, the correct option is (B).

Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.

Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.

Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.

Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.

Hence, the correct option is (A).

The purpose of most transmitters is radio communication of information over a distance. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is called the carrier signal.

A transmitter is an electronic device used in telecommunications to produce radio waves in order to transmit or send data with the aid of an antenna. The transmitter is able to generate a radio frequency alternating current that is then applied to the antenna, which, in turn, radiates this as radio waves.

The purpose of the transmitter is to convert the message signal produced by the source of information into a suitable form for transmission through the channel.

Hence, the correct option is (B).