Analog Communication Test Questions - Set 5

ANSWER: All of the above

Explanation:
The automatic gain control (AGC) circuit is used to adjust the gain of the receiver depending upon the strength of the received signal at the receiver. The AGC gives a DC voltage as an output that is proportional to the amplitude of the received signal. In AGC, the output is a linear function of the input.

ANSWER: All of the above

Explanation:
Sensitivity of a receiver is defined as the ability of the receiver to amplify weak signals received by the receiver. It is the voltage that must be applied at the input terminals of the receiver to achieve a minimum standard output at the output of the receiver. The factors that determine the sensitivity of super heterodyne receiver are -

- Gain of the IF amplifier
- Noise figure of the receiver
- Gain of RF amplifier

ANSWER: All of the above

Explanation:
The ability of the receiver to select the wanted signals among the various incoming signals is termed as Selectivity. It rejects the other signals at closely lying frequencies. Selectivity is determines performance of a radio receiver that how much it responds only to the radio signal that is required to receive. Selectivity of a receiver changes with incoming signal frequency and is poorer at high frequencies

ANSWER: All of the above

Explanation:
The RF amplifiers have greater gain that is they have better sensitivity. They have better ability to amplify weak signals received by the receiver. The RF amplifiers have better selectivity i.e., better ability to select the wanted signals among the various incoming signals. The incoming weak signals are raised to a higher level by the RF amplifiers and therefore they improve signal to noise ratio.

ANSWER: All of the above

Explanation:
Intermediate frequency (IF) should be carefully chosen as

- High IF results in poor selectivity and therefore poor rejection of adjacent channels.
- High IF results in problems in tracking of signals in the receivers.
- Image frequency rejection becomes poor at low IF or if very high.

ANSWER: AM

Explanation:
Analog communication is a type of communication in which the message to be transmitted is analog in nature. Here the message signal modulates a high frequency carrier signal. A continuous wave signal is defined continuously in time domain. However if the signal is defined at discrete times, the signal is a discrete signal.

ANSWER: 455 KHz

Explanation:
Intermediate frequency (IF) is a frequency at which the received carrier frequency is shifted for detection of message signal. The IF is generated by mixing the received carrier with the locally generated signal. The IF is generated and is shifted to further amplifiers and detectors for amplification and detection of message signal respectively. In super heterodyne AM receivers, an intermediate frequency of 455 KHz is used.

ANSWER: All of the above

Explanation:
The radio receivers receive the incoming modulated carrier by the receiving antennas. Then they select the wanted signal from the carrier and reject the unwanted signals and noise. The received signal is then amplified by the RF amplifiers. Detection of the information signal is done from the received carrier and the information or the modulating signal is then filtered and amplified.

ANSWER: Multiplication of incoming signal and the locally generated carrier

Explanation:
The function of a frequency mixer in a super heterodyne receiver is that it mixes or multiplies the incoming modulated carrier with the locally generated carrier. It then produces a number of frequencies. The produced frequencies are either the sum or the differences of the frequencies of the received signals. The output of the mixer is used to detect the modulating or information signal from the received modulated carrier.

ANSWER: All of the above

Explanation:
The RF amplifiers are used in super heterodyne receivers. The advantages of using RF receivers are
- They have better sensitivity i.e. they have improved gain to the signal
- They have better signal to noise ration than other amplifiers
- The selectivity is better as they have better rejection to the adjacent undesired signals.

ANSWER: DSB-SC signal

Explanation:
Costas receiver is a synchronous receiver system used for demodulating DSB-SC waves. The incoming modulated carrier is applied at two inputs of the coherent detectors of the receiver. The locally generated carrier applied to the two inputs is however in phase quadrature with each other. The frequencies of the local carriers are adjusted as same as the carrier frequency of the received signal.

ANSWER: The disturbance caused in the nearby channel or circuit due to transmitted signal

Explanation:
Cross talk is the disturbance caused in the nearby channel or circuit due to transmitted signal. Cross talk is usually caused by undesired coupling of capacitors or inductors from one channel to another. Electromagnetic interference (EMI) causes cross talk in the circuits or the channels. In crosstalk, the signals traveling closer to each other interfere with each other and cause disturbance in the signal.

ANSWER: f_s + 2f_i

Explanation:
The image frequency is defined as the signal frequency added to twice the value of intermediate frequency. It is the undesirable frequency generated at the receiver which also gets amplified with the message signal. The image frequency causes interference and therefore two or more stations or sources may be received simultaneously at the receiver output.

ANSWER: All of the above

Explanation:
In Frequency Modulation amplitude of the carrier remains same where as frequency of the carrier varies in accordance with the modulating signal. The variation of the instantaneous carrier frequency is proportional to the modulating signal. The number of side bands are infinite in FM signal.

ANSWER: All of the above

Explanation:
In Frequency Modulation, frequency of the carrier varies in accordance with the modulating signal. The variation of the instantaneous carrier frequency is proportional to the modulating signal. A number of side bands are formed. There is deviation of carrier frequency above and below the carrier frequency. This is known as frequency deviation.

ANSWER: The total variation in frequency from the lowest to the highest point

Explanation:
Carrier swing is defined as the total variation in frequency from the lowest to the highest point. The carrier swing = 2 * frequency deviation of the FM signal
= 2 * Δω

ANSWER: Amplitude of the modulating signal

Explanation:
The amount of frequency deviation or variation in FM signal depends on amplitude of the modulating signal. So in case of audio signals, louder is the sound, more is the frequency deviation in the carrier signal and vice versa. The frequency deviation helps in determining the bandwidth of the FM signal.

ANSWER: Both a and b

Explanation:
In direct method of FM generation, as the carrier generation is directly affected by the modulating signal, it is not easy to get high order stability in carrier frequency. The non linear characteristics of varactor diode produce a distorted FM signal due to harmonics in the modulating signal.

ANSWER: High power FM generation is possible

Explanation:
The direct method for FM generation is advantageous as it may be used for high power FM generation in several applications. Since there is direct generation of FM carrier in accordance with the modulating signal, the direct method is a cheaper method for generation of FM.

ANSWER: Both a and b

Explanation:
The linear characteristics of the detector are limited to only small deviations in the frequency. So the distortions occur due to non linear characteristics of the detector at higher deviations in frequency. The low pass RC filter of the detector is not purely band limited so it introduces distortions. But the circuit may be designed to keep the distortions minimum possible with adequate operation.