This set of Analog Communications Questions and Answers for Aptitude test focuses on “Spectrum of AM SC Waves”.
Explanation: Modulation index describes the variation of carrier signal modulation around its unmodulated level. It is defined as where Emax and Emin is respectively the maximum and minimum voltage.
2. Which frequencies is produced if Va sin(ωa t) amplitude modulates the carrier Vc sin(ωc t)?
a) (ωc + ωa) and (ωc – ωa)
b) (ωc) and (ωa)
c) (ωc + ωa) and (2ωc + 2ωa)
d) (2ωc) and (2ωa)
Explanation: If we modulate the carrier having frequency ωc with wave having frequency ωa then the resultant wave has frequency (ωc + ωa) and (ωc – ωa).
3. What is the total sideband power if there is 100% modulation?
a) half the carrier wave
b) twice the carrier wave
c) equal to the carrier wave
d) one-fourth of the carrier wave
Explanation: For 100% modulation, value of modulation index should be equal to 1. If modulation index, m=1 then total sideband power is equal to the half of carrier wave.
4. If AM radio station increases its modulation index then there is an increase in receiving RF signal.
Explanation: Modulation index describes the variation of carrier signal modulation around its unmodulated level. Increase in it also increase the ability of increase reception of RF signals.
5. How an AM radio station increase its signal-to-noise ratio?
a) by increasing its modulation index
b) by decreasing its modulation index
c) by maintaining a constant modulation index
d) by increasing the frequency
Explanation: Signal to noise ratio is the ratio of signal power to the noise power. It is often described in decibels. It is directly proportional with modulation index and thus it increases with modulation index.
6. What is the main problem in using quadrature amplitude modulation?
a) high bandwidth is required
b) high power is required
c) incompability with ordinary AM radio waves
d) bandwidth required is less
Explanation: It is widely used for modulating data signals onto a carrier which is used for radio communication. It is both an analogue and digital modulation technique. It has many advantages over other modulations such as PSK but its disadvantage is its incompability with ordinary radio waves.
7. A single sideband modulation system requires a more complex demodulator circuit?
Explanation: Single side band uses only one of its sideband among two sidebands. The two sidebands carries exactly the same information and thus removing a sideband does not have any effect on our signal. Due to the removal of one sideband total bandwidth required also decreases which makes it more efficient than others. For its demodulation it requires a complex demodulator circuit.
8. A narrow band noise can exist in _________
a) AM only
b) PCM only
c) FM only
d) AM and FM both
Explanation: Narrow band filter is used to isolate a narrow band of frequencies from a wider bandwidth signal. It is a combination of band pass and band reject filter. So it can be used in both AM and FM.
9. To demodulate a USB SSB signal, the receiver must ________
a) be set to USB mode
b) reinsert the carrier
c) both a and b
d) have low-fidelity
Explanation: Single side band uses only one of its sideband among two sidebands. The two sidebands carries exactly the same information and thus removing a sideband does not have any effect on our signal. To demodulate upper sideband of signal it is necessary that the receiver must be set to USB mode and have the ability to reinsert the carrier.
10. The __________ of an AM signal resembles the shape of baseband signal.
Explanation: Envelope is basically a smooth curve that outlines the extremes of any baseband signal. So basically it is message or baseband signal that determines the envelope.
Sanfoundry Global Education & Learning Series – Analog Communications.
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