(b) What are the differences between FM and PM? (c) What are the differences between NBFM and WBFM? [9+4+3]

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Cameron Burns
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1 Code No: RR Set No (a) The antenna current of an AM Broadcast transmitter is 10A, if modulated to a depth of 50% by an audio sine wave. It increases to 12A as a result of simultaneous modulation by another audio sine wave. What is the modulation index due to this second wave? (b) Find the % power saved in DSB-SC system while compared to AM system. [12+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) A signal ν(t) = [1+m(t) cos(ω c t)] is detected using a square law detector having the characteristic v 0 =V 2. If the Fourier transform of m(t) is constant,m0 is extending from - f m to + f m. Sketch the Fourier transform of v 0 (t) in the frequency range - f m < f<+ f m. (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) In FM, the side bands at equal distance from f c, have equal amplitude- Explain. (b) What are the differences between FM and PM? (c) What are the differences between NBFM and WBFM? [9+4+3] 5. (a) Describe variable reactance method of generation of FM carrier with necessary diagrams. (b) What are the modifications required to generate a wideband FM signal from a narrowband FM signal generator- 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) Derive an expression for effect of noise on conventional AM system. (b) Derive an expression for effect of noise on DSB-SC AM system. [8+8] 1 of 2

2 Code No: RR Set No (a) Write short notes on Threshold in FM and threshold improvement through De-emphasis. (b) Explain Pre-emphasis and De-emphasis. [8+8] 2 of 2

3 Code No: RR Set No (a) Why VSB system is widely used for TV broadcasting- Explain? (b) In a DSB system the carrier is c(t) = A cos2π f c t and the message signal is given by m(t) = sin c(t) + sin c 2 (t). Find the frequency domain representation and the bandwidth of the modulated signal. [5+11] 2. (a) Explain with a circuit diagram the principle of square-law modulation. (b) Write short notes on Balanced Modulators. [8+8] 3. (a) Explain the operation of a non-linear detector with relevant circuit diagram and waveforms. (b) Explain the operation of a linear detector with relevant circuit diagram and waveforms. [8+8] 4. (a) Derive the equation for wideband FM wave and hence draw its spectrum. (b) What is the bandwidth required for an FM signal in which the modulating frequency is 3 KHz and the maximum deviation is 10 KHz. [12+4] 5. Draw the circuit diagram of varactor diode modulator and derive the expression for frequency deviation and modulation index in terms of the diode parameters. [16] 6. (a) Explain balanced slope detector for detecting FM signal. (b) Compare the merits and demerits of various methods of FM demodulators and which one of them is mostly used and why. [7+9] 7. A channel with power density spectrum S n (ω)= 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? [8+8] 8. (a) With the help of vector diagrams explain what happens when the carrier is modulated by a single noise frequency. (b) With the help of the phasor diagram explain the effect of noise in an anglemodulated system. [8+8] 1 of 1

4 Code No: RR Set No (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 V max and V min 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) Draw the circuit diagram of a balanced modulator and show that it produces DSB-SC wave. (b) An audio signal 10 sin 1000 πt is used to modulate a carrier of 50 sin 2π 10 5 t. Calculate bandwidth and power delivered to a load of 600 ohms in case of [8+8] i. AM ii. DSB-SC and iii. SSB. 3. (a) What is the effect of frequency and phase error in demodulation of DSB-SC wave using synchronous detector. (b) What is the effect of frequency and phase error in demodulation of SSB-SC wave using synchronous detector. [8+8] 4. (a) Define Modulation Index in FM. Discuss the spectra of NBFM and WBFM for various Modulation Indices. (b) Derive equation for FM carrier from fundamentals. Also differentiate FM and PM. [7+9] 5. A carrier 100 cos 2 π 1 0t volts is frequency modulated by a signal 5 cos 2 π 10 2 t volts. The modulation index β is 0.3. The modulated wave is passed through a band pass filter whose transfer function is: H(f) = (f-9) when 9<f<11 = 0 otherwise and frequency is in KHz. The filtered signal; is passed throughout an ideal envelope detector which is followed by an ideal low pass filter with cut off at 1.5 KHz. Find the signal power at the output of the low pass filter. Use the following values of Bessel functions wherever required. J 0 (0.3)=0.99, J 1 (0.3)=0.1, J 2 (0.3)=0.01, J 3 (0.3)=0.001, J 4 (0.3)= [16] 1 of 2

5 Code No: RR Set No Using the circuit diagrams, show how the Foster-seeley discriminator is derived from the balanced slope detector, and how, in turn, the ratio detector is derived from the discriminator. In each step stress the common characteristics, and how what it is that makes each circuit different from the previous one. [16] 7. Explain the SNR s at Input and output of demodulators of DSB-SC and SSB-SC and compare them. [16] 8. (a) What is Threshold in FM systems? Explain a mechanism by which threshold can be improved. (b) Explain the band pass nature of noise representation in communication systems. [8+8] 2 of 2

6 Code No: RR Set No (a) How frequency translation is obtained- Explain using the spectrum of DSB-SC wave. (b) Compare various AM systems. [8+8] 2. (a) For the balanced ring modulator circuit, the carrier input frequency f c = 500 khz and modulating input signal frequency ranges from 0 to 5 khz. Determine output frequency range and output frequency for a single input frequency of 3.4 khz. (b) Why single sideband (SSB) modulation is not used in the transmission of video signals? Explain. Discuss clearly about the modulation technique used for transmission of video signals in commercial television broadcasting. [8+8] 3. (a) Explain the principle and operation of slope Detector. (b) Give the block diagram for coherent detection of DSB-SC modulated wave and explain the operation of it. Explain the process of frequency translation. [7+9] 4. (a) A carrier is angle modulated by two sinusoidal modulating waveforms simultaneously so that v(t) = A cos(ω c t + β 1 sin ω 1 t + β 2 sin ω 2 t). Show that this waveform has sidebands separated from the carrier not only at multiples of ω 1 and of ω 2 but also has sidebands as well as separations of multiples of ω 1 + ω 2 and of ω 1 - ω 2. (b) Explain clearly the difference between AM, FM, and PM, beginning with the definition of each type and the meaning of the modulation index in each case. [9+7] 5. A signal A sin ω t is input to a square-law device (E o = E m ) 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. (a) Explain balanced slope detector for detecting FM signal. (b) Compare the merits and demerits of various methods of FM demodulators and which one of them is mostly used and why. [7+9] 7. (a) Why VSB system is widely used for TV broadcasting- Explain? 1 of 2

7 Code No: RR Set No. 4 (b) Show that the figure of merit of an SSB system using coherent detection is 1. [8+8] 8. Write notes on: Threshold in FM and Threshold Improvement Technique. [16] 2 of 2

B.Tech II Year II Semester (R13) Supplementary Examinations May/June 2017 ANALOG COMMUNICATION SYSTEMS (Electronics and Communication Engineering)

Code: 13A04404 R13 B.Tech II Year II Semester (R13) Supplementary Examinations May/June 2017 ANALOG COMMUNICATION SYSTEMS (Electronics and Communication Engineering) Time: 3 hours Max. Marks: 70 PART A