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Correct OPTION is (d) High frequency AM waves

Explanation: An ENVELOPE detector is USED to DETECT high-frequency AMPLITUDE modulated (AM) waves. It takes high-level AM waves as input and then provides an output that can be considered as an envelope of the original signal. An envelope detector may consist of a simple RC circuit.

The correct choice is (c) Amplifier

For explanation I would say: The amplifier is a PART of the RECEIVER system, of which, the detector is a part of. But, the detector system alone consists of the rectifier, ENVELOPE detector, and OBVIOUSLY the amplitude-modulated wave that passes through it.

Correct option is (a) Microwave SYSTEM

For explanation I WOULD say: Microwave System is not a transmission medium. INSTEAD, it is used for the transmission of data over a cable as WELL as lines. All the others are COMMONLY used transmission media. Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz.

The correct option is (c) 300 Hz to 3100 Hz

The EXPLANATION: For speech signals, the FREQUENCY RANGE of 300 Hz to 3100 Hz is CONSIDERED adequate. Therefore, speech signals REQUIRE a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for commercial telephonic communication.

Correct choice is (B) Rectifier

For explanation: Rectifier is not involved in the PROCESS of PRODUCTION of the amplitude-modulated wave. When the message signal is superimposed with the carrier signal to form the desired signal, that signal is in one phase only. So, to overcome this problem we use a square-law DEVICE placed in the circuit to square the waveform generated. Bandpass filters are also used in the process.

Correct choice is (d) Increasing frequency deviation

Explanation: Noise is a form of amplitude VARIATIONS in the transmitted signal due to several factors such as the atmosphere, INDUSTRIES, etc. This noise can be REDUCED by increasing the deviation of the frequency of the signal.

Correct option is (a) Frequency varies with time

To explain I WOULD SAY: In a frequency MODULATED wave, the frequency of the carrier wave is considered to vary corresponding to the MODULATING SIGNAL. At the resonant frequency, the modulated wave is resistive in nature.

Correct choice is (d) The modulated signal is quite strong

Explanation: No, the modulated signal is quite weak and cannot sustain ATTENUATION over a large DISTANCE. So, the modulator is followed by a power amplifier that provides the necessary power, and then the modulated signal is FED to an antenna of appropriate SIZE for radiation.

The correct OPTION is (a) TRUE

To elaborate: Yes, this is a true statement. A bandpass filter rejects low and high frequencies and allows a BAND of frequencies to pass through. So, when a signal is passed through a bandpass filter, it rejects dc and the sinusoids of frequencies ωm, 2ωm, and 2ωc and RETAINS the frequencies ωc, ωc – ωm, and ωc + ωm.The output of the bandpass filter, therefore, is of the same form is hence an AM WAVE.

Correct answer is (b) False

Easiest EXPLANATION: PAM stands for PULSE amplitude MODULATION. It is defined as the process in which amplitudes of the regularly spaced rectangular pulses vary in DIRECT proportion to the instantaneous values of the continuous message SIGNAL.

Right option is (b) It is more noise immune than other modulation systems

The EXPLANATION is: In the communication systems, amplitude modification is USED for broadcasting because its use avoids receiver complexity. Only a diode and a capacitor are sufficient to SEPARATE the audio SIGNAL from the amplitude modified wave.

The correct choice is (d) The transmitting ANTENNA length is very large and impracticable

The BEST I can explain: Audio signals require a transmitting antenna of very large and impracticable length. This is because of the high wavelength of the sound waves PRODUCED. THEREFORE, audio signals cannot be TRANSMITTED directly.

Right option is (d) Skywave propagation is USEFUL in the range of 30 to 40 MHz

The best explanation: Skywave propagation is useful for radio-waves of frequencies RANGING from 2 MHz to 30 MHz. Higher frequencies than this value cannot be reflected by the IONOSPHERE. All the other statements are CORRECT.

The correct option is (b) 3.00

The BEST explanation: Modulation INDEX = \(\frac {\TRIANGLE f}{f}\)

Modulation index = \(\frac {15 kHz}{5 kHz}\)

Modulation index = 3.

The correct ANSWER is (a) √2r

Explanation: Initial range, r = √2Rh.

When the HEIGHT of the antenna is DOUBLED,

r’ = √2R × 2H = √2r.

Right option is (c) Transducer

Best explanation: Any DEVICE that CONVERTS energy from ONE FORM to another is CALLED a transducer. Transducers convert the corresponding variations in the electrical signal at its output.

Right choice is (a) h^1/2

Explanation: The maximum distance upto which TV TRANSMISSION from a TV TOWER of height h can be received is proportional to h^1/2.

d = √2Rh.

d ∝ h^1/2.

Right CHOICE is (b) Electric signal

The EXPLANATION is: If the output of the information source is a non-electrical signal such as a voice signal, then that’s where the transducer comes into PLAY. The transducer CONVERTS this voice signal into an electric form before GIVING it as an input to the transmitter.

The correct CHOICE is (C) Signal

Easy EXPLANATION: Signal is the electrical analogue of the INFORMATION produced by the source. It may be defined as a single-valued function of time (that conveys the information) and which, at every instant of time, takes a unique VALUE.

The correct answer is (b) FALSE

Explanation: No, this STATEMENT is false. The receiver is a DEVICE which recovers the original message signal from the signal received at the output of the COMMUNICATION channel. Whereas, a transmitter transmits it to the receiving end through a transmission medium.

Correct ANSWER is (b) False

Easy explanation: No, this is a false STATEMENT. Modulation refers to the superposition of low-frequency audio SIGNALS on a high-frequency radiowave. The frequency of the modulated wave is considered to vary with the AMPLITUDE of the modulating wave.

Right answer is (C) Operating on the received signal

For explanation I would say: The RECEIVER does the FUNCTION of operating on the received signal. The receiver reconstructs a recognizable form of the ORIGINAL message signal in order to DELIVER the signal to the user of information.

The CORRECT CHOICE is (d) Channel imperfection

For explanation: 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.

Correct OPTION is (b) Demodulation

Explanation: The PROCESS of recovering the original information signal from the modulated wave at the receiver END is called demodulation or detection. It is the reverse process of the modulation process that OCCURS at the TRANSMITTING end.

Right OPTION is (a) Reduce the BANDWIDTH used

Explanation: Modulation is used to reduce the bandwidth used. The use of high-frequency carrier WAVES reduces bandwidth. Modulation allows US to send a SIGNAL over a bandpass frequency range.

The correct choice is (a) Stops ultraviolet radiation

Easiest explanation: The OZONE layer stops ultraviolet radiation. It acts as a FILTER against the harmful Ultraviolet radiation from the SUN from reaching the LIVING being on earth, THEREBY protecting the life on earth. This is the significance of the ozone layer.

Correct option is (a) 3

Easiest EXPLANATION: Communication is the very ACT of transmission of information. Irrespective of its nature, EVERY communication system has three essential elements, namely a transmitter, a medium or CHANNEL, and a receiver.

Correct answer is (B) From 2 MHZ to 30 MHz

For explanation I would say: Skywave propagation is USEFUL for radiowaves of frequency from 2 MHz to 30 MHz. Frequencies that are higher than 30 MHz cannot be reflected by the IONOSPHERE.

Right ANSWER is (a) Pulse code MODULATION

The explanation is: Pulse code modulation is the one preferred for digital communication. Pulse codes are used to convert an ANALOG signal to a digital one. It is the standard form of digital audio in COMPUTERS, compact discs, etc.

Right CHOICE is (a) True

To explain I would say: Yes, this is a true statement. A rectifier CONVERTS ALTERNATING current (AC), which periodically reverses its direction, to direct current, which travels only in one direction. In a communication system, the modulated SIGNAL coming from the intermediate frequency (IF) stage is passed through the rectifier, located in the detector system, and the rectifier, in turn, produces the OUTPUT required.

The correct choice is (B) AMPLITUDE modulation

Easiest explanation: In Amplitude modulation, the MODULATING wave is superimposed on a high-FREQUENCY carrier wave such that the amplitude of the modulated wave varies as the amplitude of the modulating wave. The frequency of the modulated wave is equal to the frequency of the carrier wave.

Right option is (C) Carrier SIGNALS have high frequency

Easiest explanation: Carrier signals have high frequencies and hence are used in the transmission of radio waves. This is because SINCE SPACE is an insulator, for having electric current in order to magnetize it is only possible if the source contains a rate of change of VOLTAGE or a varying electric field. All the other statements are not valid.

The CORRECT ANSWER is (B) Amplitude modulation

To explain: In Amplitude modulation, the MODULATING wave is SUPERIMPOSED on a high-frequency carrier wave such that the amplitude of the modulated wave varies as the amplitude of the modulating wave.

Right choice is (c) 28.2 km

For explanation: d = √2hR.

d = √(2 × 50 × 8 × 10^6)

d = 28284.27 m ≈ 28284 m

d = 28.2 km.

Therefore, the maximum distance up to which the TV transmission can be RECEIVED is 28.2 km.

Correct answer is (d) FM radio

To explain: The communication CHANNEL for the transmission of a message SIGNAL that has a bandwidth of 200KHZ is FM radio. AM radio is the channel REQUIRED for a message signal having a bandwidth of 5 MHZ. Other options are not valid.

The correct answer is (C) Less than one

Best EXPLANATION: Ionosphere is the uppermost layer of the earth’s atmosphere. The ionosphere has a very low DENSITY. The refractive INDEX of the ionosphere is less than one.

Right ANSWER is (b) Detection

Explanation: Detection is the process of recovering the MODULATING signal from the modulated carrier 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. The detected signal may not be strong enough to be made use of and hence is required to be amplified.

Correct choice is (a) True

Explanation: YES, this is a true statement. 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 6MHz of bandwidth for transmission.

Right ANSWER is (b) False

The best I can EXPLAIN: No, this statement is false. The ozone layer stops the harmful ultraviolet radiation. It acts as a filter for harmful Ultraviolet radiation from the sun, thereby PROTECTING the life on earth. It is a greenhouse gas.

Right OPTION is (B) Telephone

Explanation: The telephone is the odd one out. TELEPHONES belong to the category of point-to-point communication, whereas all the others belong to the broadcast mode of communication. In point-to-point communication mode, communication TAKES place over a link between a single transmitter and a receiver. In broadcast mode, there are a large number of receivers corresponding to a single transmitter.

Right choice is (b) MICROWAVES to ultraviolet

Easiest EXPLANATION: OPTICAL communication which is also called optical telecommunication is referring to a type of communication at a distance USING light to carry information. Optical communication using fibers is PERFORMED in the frequency range of 1 THz to 1000 THz, i.e. microwaves to ultraviolet.

Correct choice is (a) Superimposition with a carrier signal

The best I can EXPLAIN: Any signal that is generated from a SOURCE and is required to be sent over large DISTANCES from the source to the receiver, needs to be MODIFIED. This can be achieved by superimposition with a carrier signal in order to ensure that the signal is transmitted in a suitable bandwidth.

Right answer is (a) Automatic gain control

Best EXPLANATION: AGC STANDS for automatic gain control. Automatic gain control changes (AGC) the overall gain of a receiver automatically so that the strength of the received signal REMAINS ALMOST CONSTANT.

Correct option is (c) Converting the signal into a suitable form

The best I can explain: The PURPOSE of a transmitter is to convert the MESSAGE signal produced by the SOURCE of information into a suitable form that can be TRANSMITTED through the channel that connects the transmitter and the receiver.