This set of Analog Communications Questions and Answers for Campus interviews focuses on “Power Relations in AM Waves”.
1. Calculate power in each sideband, if power of carrier wave is 176W and there is 60% modulation in amplitude modulated signal?
Explanation: Modulation index = 0.6 and Pc = 176W. Power in sidebands may be calculated as
2. For 100% modulation, power in each sideband is ________ of that of carrier?
3. Overmodulation results in?
b) Weakens signal
c) Strengthens the signal
d) provides immunity to noise
Explanation: When instantaneous level of modulating signal exceeds the value necessary to provide 100% modulation. Overmodulation results in distortion of the modulating signal.
4. Maximum power efficiency of an AM modulator is?
Explanation: Efficiency, ή = μ2⁄2. For maximum efficiency µ = 1 so ή = 12⁄2 = 1⁄2 and ή% =1⁄2 x100 =50%.
5. Noise performance of a square law demodulator of AM signal is?
a) Better than that of synchronous detector
b) Weaker than that of synchronous detector
c) Better than that of envelope detector
d) Weaker than that of envelope detector
Explanation: Process of recovering message signal from received modulated signal is called modulation. It is exactly opposite to modulation. There are two types of AM demodulators, Square Law Demodulator and Envelope Demodulator. Noise performance of it is far better than synchronous detector.
6. For getting 100% modulation, carrier amplitude should ________
a) exceed signal amplitude
b) be equal to signal amplitude
c) be lesser than signal amplitude
d) It is not possible to achieve 100% modulation
Explanation: Modulation, m = Vm/A where Vm is maximum baseband amplitude and A is maximum carrier amplitude. So for getting m = 1, Vm should be equal to A.
7. For 100% modulation, total power is?
a) same as the power of unmodulated signal
b) twice as the power of unmodulated signal
c) four times as the power of unmodulated signal
d) one and half times as the power of unmodulated signal
Explanation: Total power, Pt = Pc (1 +m2⁄2) where m is equal to 1 so Pt = Pc (1 +12⁄2). On solving it we get Pt = 1.5 Pc.
(Q.8 – Q.11) An AM signal is represented by x(t) = (30 + 2Sin(700πt)) Cos(2πt x 10²t)V.
8. The modulation index is
Explanation: Given equation can be written as 30(1 + 0.066 Sin(700πt)). So modulation index is 0.066.
9. Carrier power of the wave is?
10. Find the total power of amplitude modulated wave?
11. What is its sideband power?
Explanation: Sideband power is (Pt – Pc) i.e. 451 – 450 = 1W.
Sanfoundry Global Education & Learning Series – Analog Communications.
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