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

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1 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 (Compulsory Question) ***** 1 Answer the following: (10 X 02 = 20 Marks) (a) Define modulation. (b) State the difference between single side band and vestigial side band transmission system. (c) Differentiate phase and frequency modulation. (d) What are the applications of phase locked loop? (e) Define signal to noise ratio. (f) What is thermal noise? (g) What is meant by figure of merit of a receiver? (h) What is the purpose of preemphasis and deemphasis in FM? (i) Define channel capacity of the discrete memory less channel. (j) What is entropy? PART B (Answer all five units, 5 X 10 = 50 Marks) UNIT I 2 Analyze the concepts of AM modulation and derive the equation of an AM wave. Also draw the modulated AM wave for various modulation index. OR 3 Explain about balanced modulator to generate DSB-SC signal. UNIT II 4 Derive the expression for the frequency modulated signal. Explain what is meant by narrowband FM and wideband FM. OR 5 An angle modulated signal with carrier frequency is described by the equation. (a) Find the power of the modulated signal. (b) Find the frequency deviation f. (c) Find the deviation ratio β. (d) Estimate the bandwidth of. UNIT III 6 Explain noise in FM and PM systems. OR 7 Explain the following: (a) Time domain representation of narrow band noise. (b) Quadrature representation of narrow band noise. UNIT IV 8 Explain pulse position modulation in detail. OR 9 Write short notes on: (i) Sensitivity. (ii) Selectivity. (iii) Fidelity in radio receiver measurements. UNIT V 10 (a) A discrete source emits one of five symbols once every millisecond. The symbol probabilities are respectively. Find the source entropy and information rate? (b) A binary source is emitting an independent sequence of 0 s and 1 s with probabilities p and 1-p respectively. Plot the entropy of the source versus p (0<p<1). OR 11 Discuss source coding theorem, give the advantages and disadvantages of channel coding in detail. *****

2 Set No. 1 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) With necessary expressions, waveforms and spectrums, Explain AM for an arbitrary baseband signal m(t). (b) The output power of an AM transmitter is 1KW when sinusoidally modulated to a depth of 100%. Calculate the power in each side band when the modulation depth is reduced to 50%. [10+6] 2. (a) Draw the circuit diagram for balanced ring modulator and explain its operation indicating all the waveforms and spectrums. (b) In an AM-SC system, modulating signal is a single tone sinusoid 4 cos 2Π 10 3 t, which modulates a carrier signal 6 cos 2Π10 6 t. Write the equation of modulated wave. Plot the two sided spectrum of the modulated wave. Calculate the amount of power transmitted. [8+8] 3. (a) Describe the time domain band-pass representation of SSB with necessary sketches. (b) Find the percentage of power saved in SSB when compared with AM system. [12+4] 4. (a) 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? (b) Compute the bandwidth requirement for the transmission of FM signal having a frequency deviation 75 KHz and an audio bandwidth of 10KHz. [12+4] 5. Explain how S/N ratio is a figure of merit incase of performance of a communication channel. [16] 6. (a) Draw the block diagram of an SSB - SC transmitter employing sideband suppression filter and explain. (b) Why are limiters and preemphasis filters used in FM radio. [16] 7. (a) With the aid of the block diagram explain TRF receiver. Also explain the basic superheterodyne principle. (b) List out the advantages and disadvantages of TRF receiver. [16] 8. (a) Describe the synchronization procedure for PAM, PWM and PPM signals. 1 of 2

3 Set No. 1 (b) Discuss about the spectra of PWM and PDM signals. [16] 2 of 2

4 Set No. 2 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) What are the main objectives of a communication system design? What are the primary resources of any communication system. (b) The RC load for a diode envelope detector consists of a 1000 pf capacitor in parallel with a 10-KΩ resistor. Calculate the maximum modulation depth that can be handled for sinusoidal modulation at a frequency of 10 KHz if diagonal peak clipping is to be avoided. (c) A broadcast AM transmitter radiates 50 KW of carrier power. What will be the radiated power at 85 % of modulation and what is the side band power? [6+6+4] 2. Considering the modulating and carrier waves as sinusoids, Explain the single tone modulation and demodulation of DSB-SC wave with necessary expressions, waveforms and spectrums and explain how only one side band is necessary for transmission of information. [16] 3. A synchronous detection of SSB signal shows phase and frequency discrepancy. N ConsiderS(t) = P cos [(ω c t) cos(ω i t + Φ i ) sin(ω c t) sin(ω i t + Φ i )]is an SSB signal. i=1 The signal is multiplied by the locally generated carrier cosω c t and then passed through a low-pass filter. (a) Prove that the modulating signal can be completely recovered if the cut-off frequency of the filter is f N < f o < 2f c. (b) Determine the recovered signal when the multiplying signal is cos[ω c + Φ]. (c) Determine the recovered signal when the multiplying signal is cos[(ω c t+ ω)t]. Give Φ << Φ i where ω c = 2Πf c, ω= 2 Q f. [16] 4. Explain demodulation of FM signal with the help of PLL. [16] 5. Compare noise performance of PM and FM system. [16] 6. (a) Draw the block diagram of an SSB - SC transmitter employing sideband suppression filter and explain. (b) Why are limiters and preemphasis filters used in FM radio. [16] 7. (a) Explain the working of TRF receiver with its block diagram. 1 of 2

5 Set No. 2 (b) A TRF receiver is turned to 1000 KHz AM radio broadcast signal by a variable tuned circuit with 1 KHz bandwidth. Find the bandwidth when receiver is returned to 1550 KHz and 550 KHz. Determined the recovered baseband. [16] 8. (a) Describe the generation and demodulation of PPM with the help of block diagram and hence discuss its spectral characteristics. (b) Define and distinguish between PTM and PAM schemes. Sketch and explain their waveform for a single tone sinusoidal input signal. [16] 2 of 2

6 Set No. 3 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) Draw the one cycle of AM wave and calculate the modulation index of it in terms of V max and V min voltages. (b) A modulating signal consists of a symmetrical triangular wave having zero dc component and peak to peak voltage of 12V. It is used to amplitude modulate a carrier of peak voltage 10V. Calculate the modulation index and the ratio of the side lengths L 1 /L 2 of the corresponding trapezoidal pattern. (c) The rms antenna current of an AM transmitter is 10 A when un-modulated and 12 A when sinusoidally modulated. Calculate the modulation index. [6+6+4] 2. (a) Explain about the quadrature null effect of coherent detector. (b) In DSB-SC, suppression of carrier so as to save transmitter power results in receiver complexity - Justify this statement. [8+8] 3. (a) Why SSB transmission is the preferred than DSB-SC? N (b) i. Prove that the signal s(t) = P cos [(ω c t) cos (ω i t + Φ i ) sin (ω c t) sin (ω i t + Φ i )] i=1 is an SSB signal (f c >> f N ), where ω c = 2f c, carrier angular frequency and ω i = 2Πf i is modulating angular frequency. Identify the side band. ii. Obtain an expression for missing side band. iii. Obtain an expression of the total DSB-SC signal. [4+12] 4. (a) Describe generation of FM carrier by Transistor reactance modulator with necessary diagrams. (b) Compare the phasor diagram of narrow band FM signal and AM signal and discuss about the similarities and differences of the two signals. [8+8] 5. (a) Derive the expression for figure of merit of AM system for large case. (b) Given an analog base band communication system with additive white noise having power spectral density η 2 and a distorting channel having the frequency response. The distortion is equalized by a receiver filter having the frequency response 1 H c (w) = 1+J w/w 1 H eq (w) = H c 0 w w Obtain an expression for the output SNR. (w) 0 otherwise [16] 1 of 2

7 Set No (a) Classify radio transmitters based on the type of modulation and Service involved. (b) Explain AM transmitters with modulation at high carrier power level with the help of its block diagram. [16] 7. (a) With the aid of the block diagram explain TRF receiver. Also explain the basic superheterodyne principle. (b) List out the advantages and disadvantages of TRF receiver. [16] 8. (a) How is PDM wave converted into PPM system. (b) Explain why a single channel PPM of system requires the transmission of synchronization signal, where as a single channel PAM or PDM system does not it. [16] 2 of 2

8 Set No. 4 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) Draw the one cycle of AM wave and calculate the modulation index of it in terms of V max and V min voltages. (b) A modulating signal consists of a symmetrical triangular wave having zero dc component and peak to peak voltage of 12V. It is used to amplitude modulate a carrier of peak voltage 10V. Calculate the modulation index and the ratio of the side lengths L 1 /L 2 of the corresponding trapezoidal pattern. (c) The rms antenna current of an AM transmitter is 10 A when un-modulated and 12 A when sinusoidally modulated. Calculate the modulation index. [6+6+4] 2. (a) Explain the DSB-SC generation by balanced modulator using diodes. (b) The modulating signal in an AM-SC system is a multiple-tone signal given by m(t) = A 1 cos ω 1 t + A 2 cos ω 2 t + A 3 cos ω 3 t. The signal m(t) modulates a carrier A c cos ω c t. Plot the single-sided spectrum and find the bandwidth of the modulated signal. Assume that ω 3 > ω 2 > ω 1 and A 1 > A 2 > A 3. [8+8] 3. (a) Why VSB system is widely used for TV broadcasting - Explain? (b) An AM transmitter of 1KW power is fully modulated. Calculate the power transmitted if it is transmitted as SSB. (c) Calculate the filter requirement to convert DSB signal to SSB Signal, given that the two side bands are separated by 200HZ. The suppressed carrier is 29 MHZ. [6+4+6] 4. (a) Give the procedure to determine the effective bandwidth of an FM signal. (b) Which method of FM signal generation is the preferred choice, when the stability of the carrier frequency is of major concern? Discuss about the method in detail. [8+8] 5. (a) Find the output SNR in a PM system for tone modulation. (b) A phase modulation (PM) system, with the modulated wave defined by S (t) = A c C os [2π f c t + k p m (t)] where k p is a constant and m(t) is the message signal. The additive noise n(t) at the phase detector input is n(t) = n I (t) cos (2πf c t) n Q (t) sin 2πf c t Assuming that the carrier-tonoise ratio at the detector input is high compared with unity, determine i. the output signal-to-noise ratio and 1 of 2

9 Set No. 4 ii. the figure of merit of the system. [16] 6. (a) Classify radio transmitters in detail. (b) Compare low level modulation and high level modulation of radio transmitters. [16] 7. (a) Describe the circuit of an FET amplitude limiter, and with the aid of the transfer characteristic explain the operation of the circuit. (b) What can be done to improve the overall limiting performance of an FM receiver? Explain the operation of the double limiter and also AGC in addition to a limier. [16] 8. (a) What is the fundamental difference between pulse modulation, on the one hand, and frequency and amplitude modulation on the other? (b) What is pulse width modulation? What other names does it have? How is it demodulated? [16] 2 of 2

10 4/8/ Code No: R22041 R10 SET - 1 II B. Tech II Semester Regular Examinations April/May 2013 (Electronics and Communications Engineering) Time: 3 hours Max. Marks: 75 All Questions carry Equal Marks ~~~~~~~~~~~~~~~~~~~~~~~~ 1. a) Explain the need for modulator. b) Describe an expression for AM wave and sketch its frequency spectrum. c) Explain the square law detection of AM signals. 2. a) Explain the generation of double side band suppressed carrier (DSB-SC) modulator. Write the necessary equations, b) Discuss the effect of frequency and phase error in demodulation of DSB-SC wave using synchronous detector. 3. a) Draw the block diagram of phase cancellation SSB generation and explain how the carrier and the unwanted-side band are suppressed. b) Describe the principle of VSB transmission. What are its advantages over SSB? 4. a) Explain the terms of Narrow band FM and wide band FM b) Describe with a neat block diagram Armstrong method of generating FM signals for transmission. c) Compare AM and FM 5. a) Derive an expression for SNR for DSB system b) Explain about FM threshold effect in brief 6. a) Draw a neat block diagram of an AM transmitter and explain the function of each block b) Discuss the effect of feedback on the performance of AM transmitter. 7. a) Draw the block diagram of superhetrodyme receiver and the function of each block. b) Discuss the factors influencing the choice of intermediate frequency (IF) for a radio receiver. 8. a) Write short notes on Time division multiplexing. b) Explain the demodulator of PWM with necessary circuit diagram. 1 of 1 Like Us Fo ' r ' ' G ' e '' t ' D ' '' a '' ily Genuine Fast Updates Directly Your Facebook:

11 4/8/ Code No: R22041 R10 SET - 2 II B. Tech II Semester Regular Examinations April/May 2013 (Electronics and Communications Engineering) Time: 3 hours Max. Marks: 75 All Questions carry Equal Marks ~~~~~~~~~~~~~~~~~~~~~~~~ 1. a) A sinusoidal carrier frequency 1 MHz and amplitude 100 V is modulated by a sinusoidal voltage of frequency 5 KHz producing 50% modulation. Calculate the amplitude and frequency of USB and LSB b) Explain the generation of AM modulated signal using switching modulator with necessary circuit and frequencies 2. a) Prove that the balanced modulator produces an output consisting of sidebands only with the carrier removed. b) Explain the principle of coherent detector of DSB-SC modulated more with a neat block diagram. 3. a) Explain the frequency discrimination method for generating an SSB modulated wave b) Discuss the effect of frequency and phase error in demodulation of SSB-SC wave using synchronous detector 4. a) An angle modulated signal has the forum v(t ) = 100 cos[2π f e t + 4 Sin 2000 πt ] where f c =10 MHz i) Determine the average transmitted power ii) Determine the peak phase deviation iii) Determine the peak frequency deviation iv) Is this FM or a PM signal? Explain b) Explain the detection of FM wave using balanced frequency discrimination. 5. a) Derive an expression for the deviator SNR of a SSB-SC system in terms of that a base band system. b) Explain the need for pre-emphasis and de-emphasis in the case of FM systems. 6. a) Draw the variable reactance type of FM transmitter and explain its operation. b) Discuss the frequency stability in FM translator 7. Explain the following i) Image frequency rejection ii) Tracking in superhetrodyne receiver. 8. a) Explain the PPM generation from PWM with a neat block diagram and necessary figures. b) Compare TDM with FDM 1 of 1 Like Us Fo ' r ' ' G ' e '' t ' D ' '' a '' ily Genuine Fast Updates Directly Your Facebook:

12 4/8/ Code No: R22041 R10 SET - 3 II B. Tech II Semester Regular Examinations April/May 2013 (Electronics and Communications Engineering) Time: 3 hours Max. Marks: 75 All Questions carry Equal Marks ~~~~~~~~~~~~~~~~~~~~~~~~ 1. a) A transmitter radiates 10 kw with the carrier un modulated and 11.5 kw when it is sinusoidally modulated. Calculate the modulation index. If the sine wave resulting in 50% modulation is transmitted simultaneously, find the total radiated power. b) Explain the generator of AM waves using square law modulator and derive an expression for the output. 2. a) Explain in detail the generation of DSB-SC waves wing ring modulator with a neat circuit diagram. b) Draw the block diagram of Costas loop for the detection of original signal from DSB-SC wave and explain its working. 3. a) Explain how a SSB signal is generated by phase shift method with a neat block diagram. Mention the merits and demerits of this method. b) Explain the envelope detection of a VSB wave pulse carrier. 4. a) For an FM modulator with a modulating signal m(t)= V m sin300wt, the carrier signal Vc(t)=8 sin( )t and the modulator index β = 2. Find out the significant side frequencies and their amplitudes. b) Write short notes on PLL FM detector 5. a) Derive an expression for the S/N ratio for an FM System. b) Discuss the threshold effect in Angle modulation 6. a) Explain the working of FM transmitter with a neat block diagram. b) Discuss the frequency stability in FM transmitter. 7. a) Mentron the advantages of superhetrodyme receiver over TRF receiver b) Distinguish between simple AGC and delayed AGC c) Draw the block Schematic for FM broad cast receiver and explain the function of each unit. 8. Write short notes on i) Single polarity PAM ii) Generation of PWM 1 of 1 Like Us Fo ' r ' ' G ' e '' t ' D ' '' a '' ily Genuine Fast Updates Directly Your Facebook:

13 4/8/ Code No: R22041 R10 SET - 4 II B. Tech II Semester Regular Examinations April/May 2013 (Electronics and Communications Engineering) Time: 3 hours Max. Marks: 75 All Questions carry Equal Marks ~~~~~~~~~~~~~~~~~~~~~~~~ 1. a) Derive an expression for AM Wave and draw its frequency spectrum. b) The signal V (t) = (1+m cos w m t) cos w c t is detected using a diode envelope detection. Sketch the detector output when m=2. 2. a) For the balanced ring modulator circuit, the carrier input frequency f c =500 KHz and the modulator input signal frequency range from 0 to 5 KHz Determine the output frequency range and output frequency for a single modulating signal input frequency of 3.4KHz. b) Discuss the effect of phase error of locally generated carrier in the coherent detection of DSB- SC wave. 3. a) Draw the block diagram of SSB generator using the filter system and explain its working b) Compare AM, AM-DSB & AM SSB-SC systems. 4. a) Bring out the merits and demerits of FM over AM. b) Discuss the Transmission Bandwidth of FM wave. c) Write short notes on zero crossing FM detector. 5. a) Prove that the figure of merit for SSB-SC is 1. b) Discuss the threshold effect for AM with envelope detector. 6. a) Explain the working of an SSB-SC transmitter with a neat block diagram. b) Discuss the effect of feedback on the performance of AM transmitter. 7. a) Using the circuit diagram, Explain the operation of a self exited transistor mixer by the three frequency approaches. b) Bring out the differences between AM and FM receivers. 8. Write short notes on the following i) Double polarity PAM ii) Demodulation of PWM 1 of 1 Like Us Fo ' r ' ' G ' e '' t ' D ' '' a '' ily Genuine Fast Updates Directly Your Facebook:

14 Set No. 1 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) With necessary expressions, waveforms and spectrums, Explain AM for an arbitrary baseband signal m(t). (b) The output power of an AM transmitter is 1KW when sinusoidally modulated to a depth of 100%. Calculate the power in each side band when the modulation depth is reduced to 50%. [10+6] 2. (a) Draw the circuit diagram for balanced ring modulator and explain its operation indicating all the waveforms and spectrums. (b) In an AM-SC system, modulating signal is a single tone sinusoid 4 cos 2Π 10 3 t, which modulates a carrier signal 6 cos 2Π10 6 t. Write the equation of modulated wave. Plot the two sided spectrum of the modulated wave. Calculate the amount of power transmitted. [8+8] 3. (a) Describe the time domain band-pass representation of SSB with necessary sketches. (b) Find the percentage of power saved in SSB when compared with AM system. [12+4] 4. (a) 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? (b) Compute the bandwidth requirement for the transmission of FM signal having a frequency deviation 75 KHz and an audio bandwidth of 10KHz. [12+4] 5. Explain how S/N ratio is a figure of merit incase of performance of a communication channel. [16] 6. (a) Draw the block diagram of an SSB - SC transmitter employing sideband suppression filter and explain. (b) Why are limiters and preemphasis filters used in FM radio. [16] 7. (a) With the aid of the block diagram explain TRF receiver. Also explain the basic superheterodyne principle. (b) List out the advantages and disadvantages of TRF receiver. [16] 8. (a) Describe the synchronization procedure for PAM, PWM and PPM signals. 1 of 2

15 Set No. 1 (b) Discuss about the spectra of PWM and PDM signals. [16] 2 of 2

16 Set No. 2 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) What are the main objectives of a communication system design? What are the primary resources of any communication system. (b) The RC load for a diode envelope detector consists of a 1000 pf capacitor in parallel with a 10-KΩ resistor. Calculate the maximum modulation depth that can be handled for sinusoidal modulation at a frequency of 10 KHz if diagonal peak clipping is to be avoided. (c) A broadcast AM transmitter radiates 50 KW of carrier power. What will be the radiated power at 85 % of modulation and what is the side band power? [6+6+4] 2. Considering the modulating and carrier waves as sinusoids, Explain the single tone modulation and demodulation of DSB-SC wave with necessary expressions, waveforms and spectrums and explain how only one side band is necessary for transmission of information. [16] 3. A synchronous detection of SSB signal shows phase and frequency discrepancy. N ConsiderS(t) = P cos [(ω c t) cos(ω i t + Φ i ) sin(ω c t) sin(ω i t + Φ i )]is an SSB signal. i=1 The signal is multiplied by the locally generated carrier cosω c t and then passed through a low-pass filter. (a) Prove that the modulating signal can be completely recovered if the cut-off frequency of the filter is f N < f o < 2f c. (b) Determine the recovered signal when the multiplying signal is cos[ω c + Φ]. (c) Determine the recovered signal when the multiplying signal is cos[(ω c t+ ω)t]. Give Φ << Φ i where ω c = 2Πf c, ω= 2 Q f. [16] 4. Explain demodulation of FM signal with the help of PLL. [16] 5. Compare noise performance of PM and FM system. [16] 6. (a) Draw the block diagram of an SSB - SC transmitter employing sideband suppression filter and explain. (b) Why are limiters and preemphasis filters used in FM radio. [16] 7. (a) Explain the working of TRF receiver with its block diagram. 1 of 2

17 Set No. 2 (b) A TRF receiver is turned to 1000 KHz AM radio broadcast signal by a variable tuned circuit with 1 KHz bandwidth. Find the bandwidth when receiver is returned to 1550 KHz and 550 KHz. Determined the recovered baseband. [16] 8. (a) Describe the generation and demodulation of PPM with the help of block diagram and hence discuss its spectral characteristics. (b) Define and distinguish between PTM and PAM schemes. Sketch and explain their waveform for a single tone sinusoidal input signal. [16] 2 of 2

18 Set No. 3 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) Draw the one cycle of AM wave and calculate the modulation index of it in terms of V max and V min voltages. (b) A modulating signal consists of a symmetrical triangular wave having zero dc component and peak to peak voltage of 12V. It is used to amplitude modulate a carrier of peak voltage 10V. Calculate the modulation index and the ratio of the side lengths L 1 /L 2 of the corresponding trapezoidal pattern. (c) The rms antenna current of an AM transmitter is 10 A when un-modulated and 12 A when sinusoidally modulated. Calculate the modulation index. [6+6+4] 2. (a) Explain about the quadrature null effect of coherent detector. (b) In DSB-SC, suppression of carrier so as to save transmitter power results in receiver complexity - Justify this statement. [8+8] 3. (a) Why SSB transmission is the preferred than DSB-SC? N (b) i. Prove that the signal s(t) = P cos [(ω c t) cos (ω i t + Φ i ) sin (ω c t) sin (ω i t + Φ i )] i=1 is an SSB signal (f c >> f N ), where ω c = 2f c, carrier angular frequency and ω i = 2Πf i is modulating angular frequency. Identify the side band. ii. Obtain an expression for missing side band. iii. Obtain an expression of the total DSB-SC signal. [4+12] 4. (a) Describe generation of FM carrier by Transistor reactance modulator with necessary diagrams. (b) Compare the phasor diagram of narrow band FM signal and AM signal and discuss about the similarities and differences of the two signals. [8+8] 5. (a) Derive the expression for figure of merit of AM system for large case. (b) Given an analog base band communication system with additive white noise having power spectral density η 2 and a distorting channel having the frequency response. The distortion is equalized by a receiver filter having the frequency response 1 H c (w) = 1+J w/w 1 H eq (w) = H c 0 w w Obtain an expression for the output SNR. (w) 0 otherwise [16] 1 of 2

19 Set No (a) Classify radio transmitters based on the type of modulation and Service involved. (b) Explain AM transmitters with modulation at high carrier power level with the help of its block diagram. [16] 7. (a) With the aid of the block diagram explain TRF receiver. Also explain the basic superheterodyne principle. (b) List out the advantages and disadvantages of TRF receiver. [16] 8. (a) How is PDM wave converted into PPM system. (b) Explain why a single channel PPM of system requires the transmission of synchronization signal, where as a single channel PAM or PDM system does not it. [16] 2 of 2

20 Set No. 4 II B.Tech II Semester Regular Examinations, Apr/May 2008 ( Common to Electronics & Communication Engineering and Electronics & Telematics) Time: 3 hours Max Marks: 80 All Questions carry equal marks 1. (a) Draw the one cycle of AM wave and calculate the modulation index of it in terms of V max and V min voltages. (b) A modulating signal consists of a symmetrical triangular wave having zero dc component and peak to peak voltage of 12V. It is used to amplitude modulate a carrier of peak voltage 10V. Calculate the modulation index and the ratio of the side lengths L 1 /L 2 of the corresponding trapezoidal pattern. (c) The rms antenna current of an AM transmitter is 10 A when un-modulated and 12 A when sinusoidally modulated. Calculate the modulation index. [6+6+4] 2. (a) Explain the DSB-SC generation by balanced modulator using diodes. (b) The modulating signal in an AM-SC system is a multiple-tone signal given by m(t) = A 1 cos ω 1 t + A 2 cos ω 2 t + A 3 cos ω 3 t. The signal m(t) modulates a carrier A c cos ω c t. Plot the single-sided spectrum and find the bandwidth of the modulated signal. Assume that ω 3 > ω 2 > ω 1 and A 1 > A 2 > A 3. [8+8] 3. (a) Why VSB system is widely used for TV broadcasting - Explain? (b) An AM transmitter of 1KW power is fully modulated. Calculate the power transmitted if it is transmitted as SSB. (c) Calculate the filter requirement to convert DSB signal to SSB Signal, given that the two side bands are separated by 200HZ. The suppressed carrier is 29 MHZ. [6+4+6] 4. (a) Give the procedure to determine the effective bandwidth of an FM signal. (b) Which method of FM signal generation is the preferred choice, when the stability of the carrier frequency is of major concern? Discuss about the method in detail. [8+8] 5. (a) Find the output SNR in a PM system for tone modulation. (b) A phase modulation (PM) system, with the modulated wave defined by S (t) = A c C os [2π f c t + k p m (t)] where k p is a constant and m(t) is the message signal. The additive noise n(t) at the phase detector input is n(t) = n I (t) cos (2πf c t) n Q (t) sin 2πf c t Assuming that the carrier-tonoise ratio at the detector input is high compared with unity, determine i. the output signal-to-noise ratio and 1 of 2

21 Set No. 4 ii. the figure of merit of the system. [16] 6. (a) Classify radio transmitters in detail. (b) Compare low level modulation and high level modulation of radio transmitters. [16] 7. (a) Describe the circuit of an FET amplitude limiter, and with the aid of the transfer characteristic explain the operation of the circuit. (b) What can be done to improve the overall limiting performance of an FM receiver? Explain the operation of the double limiter and also AGC in addition to a limier. [16] 8. (a) What is the fundamental difference between pulse modulation, on the one hand, and frequency and amplitude modulation on the other? (b) What is pulse width modulation? What other names does it have? How is it demodulated? [16] 2 of 2