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The CORRECT choice is (a) because of its circular polarization

Explanation: A HELICAL antenna generally consists of a conducting wire which is wound in the form of a helix. It generally EXHIBITS circular polarization DUE to which it is mostly used for SATELLITE tracking.

Right option is (d) transit-time noise

To explain I WOULD say: If the time taken by ELECTRONS to travel from emitter to collector BECOMES comparable to the PERIOD of the signal which is amplified which is only possible at high frequencies CALLED transit time. At transit time noise input admittance of the transmitter increases.

Right option is (b) Voltage of impulse noise is INDEPENDENT of bandwidth

The best I can explain: Impulse noise includes unwanted noise which is almost INSTANTANEOUS and has a sharp sound. High LEVEL of such noise may damage internal ORGANS. It depends on bandwidth of the signal.

Right CHOICE is (a) NOISE generated in the receiver

Explanation: Noise which is generated INSIDE the receiver is very MUCH difficult to HANDLE.

The correct choice is (d) calculations are based on RMS VALUES

To explain I would SAY: All calculations of any random signals are done by using their RMS (ROOT MEAN Square) values.

Correct answer is (c) Noise MIXERS are caused by inadequate IMAGE frequency rejection

The EXPLANATION is: Image frequency rejection is the ratio of intermediate frequency (IF) PRODUCED by desired input frequency to that which is produced by the image frequency.

Correct answer is (a) encoding

The best I can explain: Communication process is the set of some SEQUENTIAL steps which is involved in transferring messages as well as feedback. The steps INCLUDED in process of reception are decode – to decode the message signal, interpretation – to interpret the message carefully, and STORING – to store the message carefully so that it can USE in FUTURE if needed.

Right option is (a) Modulation is USED to reduce the bandwidth

To explain I would say: Modulation is the process of varying one or more PROPERTIES of a PERIODIC waveform that contains the information to be transmitted. It is used to SEPARATE different transmission signal so they do not get mixed, it ensure that the wave travels a long distance. But it is not used to reduce bandwidth.

The correct choice is (d) to AVOID flicker

To explain: Interlacing is used for doubling the perceived frame RATE of a video display without consuming the EXTRA bandwidth. It is mostly used in TELEVISION to avoid flicker.

Right option is (d) useful as VHF and UHF receiving antenna

For explanation I would say: A DISCONE antenna is almost same as a biconical antenna in which ONE of the cones is replaced by a disc. It is mounted VERTICALLY, with the disc at top and cone BENEATH. It is useful as UHF and VHF receiving antenna.

The correct choice is (c) Log-periodic

Explanation: OMNIDIRECTIONAL antenna radiates RADIO wave power UNIFORMLY in all directions in one plane. From above given antennas, log-periodic antenna is not an omnidirectional antenna.

Correct option is (a) unchanged

For explanation: If the VALUE of RESISTOR CREATING THERMAL noise is doubled then ALSO the noise power generator remains unaffected, since the two are independent of each other.

The correct OPTION is (a) INPUT noise voltage

The best explanation: Noise performance of different RECEIVERS cannot be compared on the BASIS of input noise voltage. It can be calculated on the basis of figure of merit, or noise figure, or on the basis of equivalent noise resistance.

The correct choice is (c) noise factor

For explanation: Noise factor measures degradation of signal to noise ratio which is caused by components in radio FREQUENCY (RF) signal CHAIN. It is a number by which the performance of an amplifier can be SPECIFIED, with lower values INDICATING better performance.

Correct ANSWER is (a) 262.5

For explanation I WOULD SAY: The NUMBER of LINES per field in the United States TV system is 262.5.

Right option is (d) 30

The EXPLANATION: The number of frames PER second in the United States TV SYSTEM is 30.

Right choice is (b) False

The best explanation: The signals sent by the TV transmitters to ensure correct SCANNING in the RECEIVER are called sync. Luminance refers to the COMPONENT of television which carries INFORMATION on the brightness of the image. It refers to the INTENSITY of the screen.

The correct CHOICE is (b) pulse width

For explanation I would say: The IF bandwidth of a radar receiver is inversely PROPORTIONAL to the pulse width.

The correct answer is (c) 3m

Explanation: Frequency multiplier is an ELECTRONIC circuit that generates an output signal whose output frequency is a MULTIPLE of its INPUT frequency. So after PASSING through frequency tripler, new MODULATION index will be thrice the input which is 3m.

Correct option is (a) CARRIER frequency (ωc) with amplitude A

Explanation: ACTUALLY the frequency components of AM WAVES are – Carrier frequency (ωc) with amplitude A, Lower side band (ωc – ωm) having amplitude ^mA⁄2and UPPER side band (ωc + ωm) having amplitude ^mA⁄2 (where m represents modulation index).

Right answer is (b) the AMPLITUDE of any SIDEBAND is totally depends on modulation INDEX

The EXPLANATION: In FREQUENCY modulation, frequency of wave is modulated with respect to the wave being propagated. In frequency modulation, the amplitude of any side band depends on modulation index.

Right option is (a) An audio amplifier when the CARRIER is absent

The explanation: The use of SQUELCH circuit is to suppress the audio or video output of a receiver in the absence of a sufficiently STRONG desired input SIGNAL.

Right choice is (b) Higher bandwidth is required

Explanation: Frequency Modulation has many ADVANTAGES over amplitude modulation such as it has better NOISE IMMUNE, it requires LESS modulating power. But it requires higher bandwidth is not an advantage of FM when compared to AM.

Correct CHOICE is (a) The circuit is always biased in class C amplifier

The explanation is: Amplitude Limiter REMOVES the amplitude of output SIGNALS to the desired margin irrespective of VARIATIONS in the INPUT signal. It is always biased in class C amplifier by virtue of the leak-type bias. Amplitude limiter is present in FM receiver and not AM receiver.

Correct option is (c) is not rejected by the IF TUNED circuits

Explanation: To eliminate the image frequency of a superheterodyne receiver, adequate attenuation on the incoming signal by the RF AMPLIFIER filter is needed. It is not rejected by the IF tuned circuits and THUS, it PRODUCES image frequency.

Right answer is (a) squelch

To elaborate: Squelch is a CIRCUIT that is PREFERABLY USED to PREVENT overloading of the IF amplifier in a receiver.

Right answer is (d) Phase DISCRIMINATOR

For explanation I would say: Phase discriminator cannot be USED to demodulate SSB. It is used to discriminate between DIFFERENT phases.

Correct CHOICE is (b) Input of MIXER must be tuned to the signal frequency

The best I can explain: For a superheterodyne receiver, one of the inputs of the mixer must be tuned to the signal frequency and ANOTHER input is of the LOCAL OSCILLATOR frequency. Tuning means that the circuit works for that one signal frequency only.

Right answer is (a) the LINEARITY is worse than in phase discriminator

The explanation is: Detector, ALSO known as a demodulator, in a radio receiver extracts the original information that is contained in a modulated radio wave. The linearity of a radio detector is worse than in phase discriminator. HOWEVER, the radio detector does not get AFFECTED by amplitude variations and minimizes the limiting required.

The correct option is (d) Equalization is necessary

Explanation: The ARMSTRONG METHOD GENERATES a double sideband carrier signal, phase SHIFTS this signal and then reinserts the carrier to produce a frequency modulated signal. In Armstrong modulation SYSTEM equalization is not necessary.

Correct ANSWER is (c) conical

BEST EXPLANATION: Conical is the best scanning system for tracking. But it is only preferred for tracking after a target has been ACQUIRED.

Right choice is (B) Gaussian

To explain: Thermal noise is approximately white, it MEANS that its power spectral DENSITY is nearly equal throughout the frequency spectrum. The AMPLITUDE of the signal has a Gaussian probability density function.

Correct answer is (c) constant

Best explanation: White NOISE consists of SIGNALS of all frequencies. Random signals are CONSIDERED as white noise if they are observed to have a FLAT SPECTRUM. Its spectral density remains constant.

Right option is (d) it exhibits a threshold

For EXPLANATION: Noise performance of WIDEBAND FM SYSTEM exhibits a threshold.

Correct option is (b) 1.26:1

For explanation I would say: POWER RATIO is a METHOD used by MEDIA companies to measure revenue performance compared to the audience share it CONTROLS. Power ratio of one decibel is close to 1.26:1.

The correct answer is (c) Information

To EXPLAIN: BIT is a UNIT of information. Information is the message to be TRANSMITTED. It is used in computing and DIGITAL communications.

The CORRECT option is (d) REMAINS UNCHANGED

To explain I would say: The THERMAL noise power remains unchanged if we double the values of given resistors as both are INDEPENDENT of each other.

Right option is (a) Impulse Function

The EXPLANATION is: White noise is a RANDOM signal having equal intensity at different frequencies, giving it a constant power SPECTRAL DENSITY. Its auto correlation function is an impulse function.

Right ANSWER is (B) Galactic noise

The explanation is: Galactic noise is a RANDOM noise that originates from outside the earth’s atmosphere, WHEREAS others are present in the Earth’s atmosphere.

Right CHOICE is (c) Amount of INFORMATION per SECOND

Explanation: The channel CAPACITY of a COMMUNICATION system is exactly equal to the amount of information per second.

The correct OPTION is (a) Distant stars

The explanation: Cosmic noise a RANDOM noise that originates from outside the EARTH’s atmosphere. It can be DETECTED and heard on radio receivers.

The CORRECT choice is (a) LADDER

To explain I WOULD SAY: It is the ladder filter which is used only at higher frequencies.

The correct choice is (B) PCM

The BEST explanation: Sampling theorem can be seen as a FUNDAMENTAL bridge between continuous time signals and DISCRETE time signals. It finds its application in Pulse Code MODULATION (PCM).

The correct ANSWER is (a) negative charge

For explanation: The IONOSPHERE is the ionized PART of Earth’s upper atmosphere. It consists of a negative layer.

Right answer is (c) Better than that of a synchronous detector

The explanation is: The NOISE performance of a square LAW DEMODULATOR of AM SIGNAL is better than that of a synchronous detector.