This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Communication Systems – Production of Amplitude Modulated Wave”.
1. Which one of the following is a way to produce a signal?
a) Superimposition with a carrier signal
b) Superimposition with a received signal
c) Superimposition with a transmitted signal
d) No superimposition is required
Explanation: 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.
2. Which of the following is true about carrier signal?
a) Carrier signals have low frequency
b) Carrier signals are used to increase the wavelength of the signal
c) Carrier signals have high frequency
d) Carrier signals and message signals are the same
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.
3. Identify the one that is not required in the process of production of amplitude modulated wave?
a) Square law device
c) Bandpass filter
d) Combination of message and carrier signal
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.
4. A bandpass filter rejects direct current.
Explanation: 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.
5. Which of the following is false?
a) Bandpass filters filter out the unwanted frequencies
b) The transmitting antenna transmits the signal over large distances
c) Attenuation should be reduced for a signal
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.
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