Rr220401-communication-theory

Set No. 1

Code No: RR220401

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008 COMMUNICATION THEORY (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) The antenna current of an AM transmitter is 10A when only the carrier is sent, but it increases to 12A when the carrier is sinusoidally modulated. Find the percent of modulation. Determine the antenna current when the depth of modulation is 0.75. (b) Compare different AM systems with respect to power required, Bandwidth and applications. [10+6] 2. (a) Explain the modulation of VSB wave using Filter method with neat block diagram and waveforms. (b) What does modulation actually do to a message and carrier?

[10+6]

3. (a) In an Armstrong Modulator the crystal oscillator frequency is 200 KHz. It is desired in order to avoid distortion to limit the maximum angular deviation to φm = 0.2. The system is to accommodate modulation frequencies down to 10Hz. At the output of the modulator the carrier frequency is to be 108 MHz and the frequency deviation be 80KHz. Select multiplier and mixer oscillator frequencies to accomplish this. (b) Derive the suitable condition required for envelope detection.

[10+6]

4. (a) Derive the formula for the instantaneous value of an FM wave and define the modulation index. (b) Theoretically the number of sidebands in FM is infinite, how it is restricted to finite value ? Explain [9+7] 5. A signal A sin ω t is input to a square-law device (Eo = Em )2 . The output of which is given to an FM Modulator as the modulating signal. The frequency deviation characteristics of the FM Modulator is f = f c + KE(t). Where E(t) is the modulation signal and K is a constant. Determine the FM Signal and the frequency components in its spectrum. [16] 6. Draw the practical circuit of a balanced ratio detector, and show how it is derived from the basic circuit. Explain the improvement effected by each of the changes. [16] 7. A noise signal with a power density spectrum Sn (ω) is shown in figure 7 is transmitted through an ideal band pass filter shown in figure 7 . Express the output signal. [16]

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Set No. 1

Code No: RR220401

Figure 7

Figure 7 8. Derive the expression for SNR in the FM reception ⋆⋆⋆⋆⋆

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16]

Set No. 2

Code No: RR220401

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008 COMMUNICATION THEORY (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Derive an expression for AM wave and draw its frequency spectrum. (b) Draw the one cycle of AM wave and calculate the modulation index of it in terms of Vmax and Vmin voltages. (c) A 500-W carrier is modulated to a depth of 80%. Calculate the total power in the modulated wave. [6+6+4] 2. (a) How modulation can be achieved using non-linear characteristics of an active device and hence show the circuit diagram of Modulator. (b) Draw the schematic diagram of chopper type modulator. Hence, explain the generation of DSB-SC using the Ring Modulator. [8+8] 3. (a) Explain the frequency discrimination method for generating an SSB modulated wave. (b) Using block diagram, explain the phase-shift method of suppressing the unwanted sideband. [8+8] 4. (a) In FM, at Eigen values of modulation index, the carrier component will disappear completely ? Explain. (b) What are the advantages and disadvantages of AM over FM?

[8+8]

5. (a) Draw the block diagram of the Armstrong generator for generation and explain its operation. (b) How the FM is generated by PM and vise versa - Explain.

[8+8]

6. (a) Explain how amplitude limiting is achieved in the FM Ratio detector. (b) What are the merits of ratio detector over the foster seeley detector?

[8+8]

7. A channel with power density spectrum Sn (ω)= 0.5×10−3 is ;used to transmit an AM signal. The modulating signal f (t) is band limited to 5KHz and the carrier frequency is 100KHz. The sideband power is given to be 10KW and the carrier amplitude is 300Volts (Carrier power=45KW). At the receiver, this signal is demodulated by an envelope detector after being passed through an appropriate ideal band pass filter. (a) What is the signal-to-noise power ratio at the demodulator input? (b) What is the signal-to-noise power ratio at the demodulator output?

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[8+8]

Set No. 2

Code No: RR220401

8. What is pre-emphasis? Why it is used? Sketch a typical pre-emphasis circuit and explain why de-emphasis must be used also. Explain with one example. [16] ⋆⋆⋆⋆⋆

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Set No. 3

Code No: RR220401

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008 COMMUNICATION THEORY (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Derive the power relations in the AM wave (b) In AM wave, prove that the total power is the sum of carrier power and sidebands power. [8+8] 2. (a) Explain the operation of high level transistor collector modulator circuit. (b) Draw the circuit diagram of Balanced Modulator and explain how DSB-SC waveform is generated. [8+8] 3. (a) A signal ν(t) = [1 + m(t) cos(ωc t)] is detected using a square law detector having the characteristic v0 =V 2 . If the Fourier transform of m(t) is constant,M0 is extending from - fm to + fm . Sketch the Fourier transform of v0 (t) in the frequency range - fm < f<+ fm . (b) An SSB signal is demodulated by using a synchronous demodulator. However, the locally arranged carrier has phase error ?. Determine the effect of the error on demodulation. What will be the effect of this error if the input is DSB-SC in place of SSB?. [8+8] 4. (a) Derive the formula for the instantaneous value of an FM wave and define the modulation index. (b) Theoretically the number of sidebands in FM is infinite, how it is restricted to finite value ? Explain [9+7] 5. (a) A carrier of frequency 106 Hz and amplitude 3 volts is frequency modulated by a sinusoidal modulating waveform of frequency 500Hz and peak amplitude 1 volt. As a consequence, the frequency deviation is 1 KHz . The level of the modulating signal is changed to 5volts peak, and the modulating frequency is changed to 2 KHz. Write the expression for the new modulated waveform. (b) Explain the method of generation of NBFM.

[10+6]

6. Explain detection of FM signal using the balanced slope detector. Mention its advantages and disadvantages. Compare this with other demodulators. [16] 7. A channel with power density spectrum Sn (ω)= 0.5×10−3 is ;used to transmit an AM signal. The modulating signal f (t) is band limited to 5KHz and the carrier frequency is 100KHz. The sideband power is given to be 10KW and the carrier amplitude is 300Volts (Carrier power=45KW). At the receiver, this signal is demodulated by an envelope detector after being passed through an appropriate ideal band pass filter. 1 of 2

Set No. 3

Code No: RR220401

(a) What is the signal-to-noise power ratio at the demodulator input? (b) What is the signal-to-noise power ratio at the demodulator output?

[8+8]

8. (a) In FM, the effect of noise is higher at higher frequencies - explain and prove. (b) Prove that the FM provides a high level of noise immunity. ⋆⋆⋆⋆⋆

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[8+8]

Set No. 4

Code No: RR220401

II B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008 COMMUNICATION THEORY (Electronics & Communication Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) The antenna current of an AM transmitter is 10A when only the carrier is sent, but it increases to 12A when the carrier is sinusoidally modulated. Find the percent of modulation. Determine the antenna current when the depth of modulation is 0.75. (b) Compare different AM systems with respect to power required, Bandwidth and applications. [10+6] 2. (a) Draw the block diagram of the square law modulator and explain its operation with the help of waveforms. (b) Write short notes on VSB generation.

[8+8]

3. (a) Explain the demodulation of AM with carrier using Envelope detector. Compare Envelope and Rectifier detectors. (b) Explain the operation of the balanced slope detector using a circuit diagram and draw its response characteristics. Discuss in particular the method of combining the outputs of the individual diodes. In what way is this circuit an improvement on the slope detector and in turn what are the advantages? [8+8] 4. (a) If v(t) = cos ωc t + 0.2 cos ωm t sin ω c t. i. Show that v(t) is a combination AM-FM signal. ii. Sketch the phasor diagram at t=0. (b) Give the phasor comparison of narrow band FM and AM waves for sinusoidal modulation and write the conclusions from the comparison. [8+8] 5. (a) Illustrate the principle of the Armstrong system of generating FM signal with suitable sketches and expressions. (b) Describe the balanced slope detection of FM demodulator.

[8+8]

6. (a) The baseband signal m(t) is the frequency-translated signal v(t)= m(t) cos2Π fc t is recovered by multiplying v(t) by the waveform cos 2Π (fc + ∆f)t. The product wave- form is transmitted through a low-pass filter, which rejects the double-frequency signal. Find the output signal of the filter? (b) Compare and contrast various FM detection methods.

[9+7]

7. Show that for AM, when the noise is small compared to the signal the performance of the envelope detector is identical to that of synchronous detector. [16] 1 of 2

Set No. 4

Code No: RR220401

8. (a) In FM, the effect of noise is higher at higher frequencies - explain and prove. (b) Prove that the FM provides a high level of noise immunity. ⋆⋆⋆⋆⋆

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[8+8]