JNTU
JNTU B.Tech III ANALOG COMMUNICATIONS Previous Question Paper (May/June - 2019 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
Part A
Give introduction to communication system.
Answer
A communication system is an arrangement of hardware and software components designed to transfer information from one point (the source) to another (the destination) over a physical medium. The fundamental process involves converting information into an electrical signal, modulating it for efficient transmission, and recovering the original message at the receiver end.
The basic block diagram of a communication system consists of an information source, input transducer, transmitter, communication channel, receiver, and output transducer, with noise affecting the signal during transmission through the channel.
What is the need for modulation?
Answer
Modulation is essential in communication systems for several practical reasons:
- Antenna size reduction: For efficient radiation, an antenna must be comparable in size to the wavelength of the signal. Baseband signals (like audio) have very low frequencies, resulting in extremely large, impractical antenna sizes. Modulation shifts these signals to higher frequencies, significantly reducing the required antenna dimensions.
- Multiplexing: Modulation allows multiple signals to be transmitted simultaneously over the same communication channel by assigning each signal to a different frequency band (Frequency Division Multiplexing), preventing interference between them.
- Improved signal-to-noise ratio and range: High-frequency modulated signals are less susceptible to noise and interference during propagation, and they can be transmitted over much longer distances compared to baseband signals, which suffer from high attenuation.
Describe frequency domain description.
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Explain frequency discrimination method for generation of AM SSB modulated wave.
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What is frequency modulation?
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Explain single tone frequency modulation.
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What are the types of noise?
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Describe resistive (thermal) noise source.
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Define sensitivity and selectivity.
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Draw tuned radio frequency receiver.
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Part B
What is amplitude modulation?
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The modulating signal f(t) is a single sinusoid given by
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Explain square law modulator.
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A certain station uses DSB – SC signal of power (mean square value) P watts to cater to a certain area. If it is decided to use SSB – SC instead, what must be the power required to cater to the same area with the same strength? Assume the synchronous demodulation with a locally generated carrier of the same strength in both cases.
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Explain phase discrimination method for generating AM SSB modulated waves.
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The modulating signal f(t) is given by
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Explain modulation and demodulation of SSB waves.
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Describe spectrum analysis of sinusoidal FM wave.
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An FM signal with modulation index β=1 is transmitted through an ideal band – pass filter with midband frequency fc and bandwidth 5 fm, where fc is the carrier frequency and fm is the frequency of the sinusoidal modulating wave. Determine the magnitude spectrum of the filter output.
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Describe narrow band FM.
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In angle modulation (FM or PM), if the modulating signal is a single frequency sinusoid (
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Explain narrowband noise – in phase and quadrature phase components and its properties.
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Calculate the equivalent noise bandwidth of the circuits referred to zero frequency if it is given that
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Explain modeling of noise sources.
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Evaluate the auto correlation functions and cross – correlation functions of the in – phase and quadrature components of the narrowband noise at the coherent detector input for i) the DSB – SC system, ii) an SSB system using the lower sideband, and iii) An SSB system using the upper sideband
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Explain FM receiver
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An 8 – level PAM signal is defined by
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Compare FM receiver with AM receiver.
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i) A pair of signals si(t) and sk(t) have a common duration, T. Show that the inner product of this pair of signals is given by
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