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Correct choice is (b) False

Easiest explanation: CRYSTAL oscillator is USED in frequency modulation to MAINTAIN a specific frequency with high STABILITY overall.

Correct ANSWER is (b) False

The best I can explain: In PHASE modulation, the phase of the carrier signal is VARIED with RESPECT to the amplitude of the message signal. For a phase modulated signal, the frequency deviation is PROPORTIONAL to amplitude only.

Right option is (b) lies in the different definition of modulation INDEX

Easiest EXPLANATION: It is the difference in definition of modulation index and also the general expression of the MODULATED WAVE that makes it different between phase and frequency modulated SYSTEMS.

Correct option is (B) noise gets decrease if we increase deviation

To elaborate: In frequency MODULATED SYSTEM, noise is inversely PROPORTIONAL to the frequency deviation. THUS, if we increase deviation then the noise in system gets decreases.

The correct choice is (b) False

To EXPLAIN I would SAY: De-emphasis means attenuating those frequencies by the amount by which they are BOOSTED. De-emphasis is DONE at the receiver end i.e. it is used after detection. Detection refers to demodulation through which the original message signal is RETRIEVED.

Correct choice is (a) remains constant

Best explanation: In a frequency modulated system, if we INCREASE the modulation index the POWER remains constant as the power DEPENDS only on the amplitude of the MESSAGE SIGNAL.

Right choice is (c) before encoding

For explanation: Pre-emphasis is USED in frequency MODULATED or phase modulated transmitters to equalize the drive power of transmitting signal in TERMS of deviation ratio. It is DONE at the transmitter. Its CIRCUIT is used before modulation.

The correct option is (b) Fourier transform PAIR

The best I can explain: Fourier transform breaks a function into an ALTERNATE representation, characterized by sine and cosines. If a function is periodic, the SPECTRAL density and auto-correlation FUNCTIONS are Fourier transform pair.

The correct ANSWER is (B) 1

To explain I would say: For getting maximum UNDISTORTED power in any modulated system, then the modulation index of that system should be 1. Thus, the SIGNAL should be 100% modulated.

The correct answer is (b) ISB SYSTEMS

Best EXPLANATION: ISB systems refers to Independent Sideband MODE and is used to PROVIDE the same carrier to two or more voice circuits.

The correct answer is (a) Grid modulated class C amplifier

Explanation: The output stage of TELEVISION TRANSMITTERS are basically class C amplifiers. It is grid modulated.

Right ANSWER is (a) True

Explanation: Modulation is the property of varying one or more PROPERTIES, LIKE amplitude, frequency or phase of a periodic WAVEFORM, known as a carrier wave, with respect to the amplitude of the wave to be transmitted. It is used for separating DIFFERENT transmission.

Right answer is (b) False

The EXPLANATION is: NOTCH FILTER is a band stop filter SINCE it allows most frequencies to pass through it, except a specific range. It is also known as band-reject filter. It is just opposite of a band-pass filter.

Correct OPTION is (a) f(x) = -f(x)

To explain I WOULD SAY: Geometrically a FUNCTION f(x) is even, if PLOT of the function is symmetric over y-axis. Algebraically, for any function f(x) to be even, f(x) = f(-x).

While for a function f(x) to be odd, f(x) = -f(-x).

The CORRECT CHOICE is (B) 100%

The explanation is: For getting maximum undistorted POWER output, value of modulation index should be 1. If the value of modulation index is 1 then we can say that it is 100% MODULATED.

Correct OPTION is (a) True

The EXPLANATION: Auto-correlation FUNCTION is also known as serial-correlation. It is the correlation of a function with a delayed copy of itself. So it is non-unique description of signals. It is used for processing signals BELONGING to series of values.

The correct OPTION is (a) True

To explain: Modulation is the process of VARYING one or more properties, like AMPLITUDE, carrier or phase, of the carrier with RESPECT to the amplitude of the wave being propagated. It is used to separate different transmission signals and also to ensure that a message is transmitted over a long path. It is also used to ALLOW the use of practical antennas.

Right ANSWER is (c) It avoids receiver complexity

Best EXPLANATION: AM broadcasting EMPLOYS amplitude MODULATION (AM) transmissions. It is highly used as compared to others for broadcasting purpose since it avoids complexity in receiver.

The CORRECT OPTION is (d) it is defined only for PERIODIC signals

To explain: Auto-CORRELATION function is also known as serial-correlation. It is the correlation of a function with a delayed copy of itself. So it is a non-unique description of signals. It is used for PROCESSING signals belonging to a series of values.

The CORRECT ANSWER is (d) even function

Easiest EXPLANATION: Auto-correlation is an even function. SOMETIMES it is also known as serial-correlation. It is the correlation of a function with a delayed COPY of itself.

The CORRECT choice is (b) False

Easy explanation: A random signal is ERGODIC if time averages equals ENSEMBLE averages. Thus, if all TYPE of ensemble averages can be INTERCHANGED.

The correct OPTION is (a) all type of ensemble averages are interchangeable

The best explanation: A stochastic process is said to be ergodic if its PROPERTIES can be reduced from a single. A non ergodic process changes erratically at an inconsistent rate. Ergodic process exists if MANY random signals have identical TIME and ensemble averages. Thus, if all type of ensemble averages can be interchanged.

Correct choice is (c) ensemble and TIME AVERAGES are identical

For explanation I would say: A stochastic process is said to be ergodic if its properties can be reduced from a SINGLE. A non ergodic process changes erratically at an inconsistent rate. Ergodic process exists if many RANDOM signals have identical time and ensemble averages.

The CORRECT choice is (b) False

Best EXPLANATION: Communication consists of means of SENDING, receiving and processing information VIA electromagnetic waves.

The CORRECT option is (d) Helps to detect and correct error and also REDUCES efficiency

For explanation I would say: Redundancy helps in many ways in communication. It not only helps to detect an error but also helps in correcting it. It sometimes reduces efficiency also. It is basically the duplication or can say that it is the backup of some important components, so as to efficiently detect the code, with the INTENTION of increasing RELIABILITY of the SYSTEM.

The correct answer is (a) OPERATES in CLASS A mode

To explain: Emitter modulator amplifier for AM operates in class A mode and also has very low efficiency. The OUTPUT of this modulator is very small and therefore it is not SUITABLE for modulation at a HIGH level.

The correct answer is (a) True

Best explanation: Super HETERODYNE RECEIVER mixes the INCOMING signal FREQUENCY with the locally generated signal frequency, using a mixer, in order to convert the incoming RF signal to a low frequency signal which can be PROCESSED easily. It has better sensitivity and selectivity but it needs an extra circuitry for frequency conversion.

The correct CHOICE is (b) larger than

To EXPLAIN: Neper is a UNIT used to EXPRESS ratios, such as gain, loss and relative values. One neper is equal to 8.685 dB. Hence it is greater than dB.

The CORRECT answer is (c) it is used for amplitude modulation

Explanation: In amplitude modulation, the amplitude of the carrier wave is VARIED with respect to the INSTANTANEOUS amplitude of the wave to be transmitted. Square Law modulators are generally used for GENERATION of amplitude modulation. They have nonlinear current-voltage characteristics. They are highly nonlinear in LOW voltage region.

The correct option is (b) True

Explanation: In ERGODIC PROCESS, ENSEMBLE and time AVERAGES are identical to each other.

The correct OPTION is (B) infinite

For explanation I would SAY: In any continuous SIGNAL, the AMOUNT of information that comes out is always infinite.

Right choice is (a) fm

For explanation: In an AM modulated system, total bandwidth required is from fc + fm to fc – fm i.e. bandwidth is equal to 2FM. In SSB-SC TRANSMISSION, the CARRIER and one of the sideband GETS suppressed, so the bandwidth becomes fm only.

The correct answer is (b) 20 Hz to 20 KHz

The BEST I can explain: As per standards audio frequency range lies in between 20 Hz to 20 KHz. This is the human audible range.

The correct choice is (a) True

The best explanation: The main advantage of using high frequency SIGNALS is that the signal gets transmitted over very long distances and THUS dissipates very LESS power. The antenna height required for transmission also gets reduced at high frequencies. And also it allows less noise interference and enables multiplexing. This is the REASON for sending the audio signals at high frequency carrier signals for communication purpose.

Correct OPTION is (C) 28.5%

BEST EXPLANATION: which gives m = 0.285 or 28.5%.

Correct option is (d) 900W

For EXPLANATION: PT=PC (1+^u^2⁄2), ACCORDING to the PROBLEM PC = 800W and m = 0.5. On substituting VALUES in the equation we get PT=800(1+ ^0.5^2⁄2) = 900W.

Right option is (a) Sampling of SIGNALS LESS than at Nyquist rate

The explanation is: ALIASING REFERS to the sampling of signals less than at Nyquist rate. Nyquist rate STATES that the rate of sampling of signals should be greater than or equal to twice the bandwidth of modulating signal. It gets reduced if sampling is done at a higher rate than nyquist rate of sampling. Aliasing can be avoided by using anti-aliasing filters.

The correct choice is (b) 455 KHz

To elaborate: STANDARD intermediate frequency (IF) used for AM receiver is 455KHz. IF is obtained by mixing the incoming siganl frequency with the LOCALLY generated signal frequency in order to CONVERT the incoming RF signal frequency to a low frequency which can be processed easily.

The correct answer is (a) 19.8 KHz

The best I can explain: According to GIVEN problem, fm = fmax-fmin = 10KHZ – 100Hz = 10000 – 100

= 9900Hz. Therefore bandwidth is EQUAL to 2fm = 2 X 9900 = 19.8 KHz.