Amplitude Modulation - Electronic Engineering (MCQ) questions & answers

ANSWER: All of the above

Explanation:
Disadvantages of using synchronous detection of AM signal are that it needs an additional system for generation of carrier. The frequency of the locally generated carrier must be identical to that of transmitted carrier. And the phase of the locally generated carrier must be synchronized to that of transmitted carrier otherwise the detected signal would get distorted so it needs an additional system for synchronization of carrier. This makes the receiver complex and costlier.

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: 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: 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: Both a and b are correct

Explanation:
Bandwidth of the AM wave is the difference in the two extreme frequencies of the AM signal. It is given by

B = (ω_c + ω_m)- (ω_c - ω_m)
= ω_m
i.e., the bandwidth of the AM wave is twice the highest frequency present in the modulating signal.

ANSWER: 108 W

Explanation:
The total power in an AM is given by
P_t = Pc (1 + m²/2)
Given: m = 0.6
Therefore DSB power = (m²/2)P_c
= 600* (0.6)²/2
= 108 W

ANSWER: Poor selectivity

Explanation:
Frequency selectivity means to separate out closely lying components of frequency. If the receiver has poor selectivity, it has poor capacity of blocking adjacent channel interference.

ANSWER: 825 Hz

Explanation:
Upper frequency = 850Hz

Bandwidth = 50Hz

Therefore lower Frequency = 850 - 50= 800 Hz

Carrier Frequency = (850-800)/2
= 825 Hz

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: 455 KHz

Explanation:
Standard intermediate frequency used for AM receiver is 455KHz. Intermediate frequency (IF) is obtained by mixing the incoming signal frequency with the locally generated signal frequency. The output is the IF which is a fixed frequency from which original information is detected using filters and amplifiers.