JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year ANALOG COMMUNICATIONS Previous Question Paper (November/December - 2020 - R16)
Answer any Five Questions. All Questions Carry Equal Marks.
Main
State and derive Power equation for various AM techniques.
Answer
The power content in an Amplitude Modulation (AM) wave is the sum of the carrier power and the sideband powers. For a standard AM signal defined as s(t) = Ac[1 + μ cos(ωmt)] cos(ωct), where Ac is the carrier amplitude, μ is the modulation index, and ωm, ωc are the angular frequencies of the message and carrier respectively, the power is derived as follows:
To derive this, consider the mean square value of the AM signal. The total power Pt is the sum of the carrier power Pc and the power in the upper and lower sidebands Pusb and Plsb.
- The carrier power is Pc = Ac2 / 2R (assuming R=1 ohm).
- Expanding the AM equation: s(t) = Ac cos(ωct) + (μAc/2) cos(ωc + ωm)t + (μAc/2) cos(ωc - ωm)t.
- The sideband power is Psb = (μAc/2)2 / 2 + (μAc/2)2 / 2 = μ2Ac2 / 4.
- Substituting Pc, we get Psb = μ2Pc / 2.
- Total power Pt = Pc + μ2Pc / 2 = Pc(1 + μ2/2).
For other AM techniques:
- DSB-SC (Double Sideband Suppressed Carrier): Since the carrier is suppressed, the total power is contained entirely in the sidebands. Pt = μ2Pc / 2.
- SSB-SC (Single Sideband Suppressed Carrier): Only one sideband is transmitted, so the power is halved compared to DSB-SC. Pt = μ2Pc / 4.
Express the DSB-SC both in time-domain and frequency domain and then explain the same.
Answer
Double Sideband Suppressed Carrier (DSB-SC) modulation is a technique where the carrier wave is suppressed, and only the sidebands are transmitted, resulting in higher power efficiency compared to standard AM.
In the time domain, the DSB-SC signal s(t) is expressed as the product of the message signal m(t) and the carrier signal c(t) = Ac cos(2πfct):
In the frequency domain, applying the modulation property of the Fourier Transform, the spectrum S(f) is given by:
Explanation: The time-domain expression shows that the amplitude of the carrier is directly controlled by the message signal. Because the carrier term itself is absent (multiplied by zero at the carrier frequency), no power is wasted on the carrier component. In the frequency domain, the message spectrum M(f) is shifted to fc and -fc. The resulting spectrum consists of the Upper Sideband (USB) and Lower Sideband (LSB), with the carrier frequency component at fc being suppressed to zero.
What is meant by single tone modulation? Elaborate.
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Derive the equation for VSB and explain it with neat block diagram.
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Explain SSB generation using phasing method.
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Compare different AM Techniques.
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Derive and explain the Bessel function for FM.
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Compare Direct and Indirect method of FM generation.
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State the Non linear effects of FM.
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The maximum frequency deviation is 10 kHz and signal frequency is 10 kHz. Find bandwidth using Carson’s rule and Modulation index.
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Explain Pre-Emphasis and de-emphasis in detail.
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Explain SNR enhancement for FM method using a numerical Example.
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With neat diagram explain super heterodyne receiver in detail.
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Compare various pulse modulation techniques.
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