SRI VENKATESWARA COLLEGE OF ENGINEERINGDepartment of Electronics and Communication Engineering

EC2252– Communication TheoryQUESTION BANK

UNIT – I AMPLITUDE MODULATION SYSTEMS

PART A

1. What is the need for modulation?2. An AM transmitter radiates 9KW without modulation & 10.125KW after modulation.

Determine the depth of modulation.3. If a 10KW AM transmitter is modulated sinusoidally by 50% what is the total RF power

delivered?4. A carrier of 20MHz is amplitude modulated with a signal frequency of 3 KHz & amplitude

of 5V. If the ma=0.5, sketch the spectra of the waveform..5. Define modulation index of AM signal.6. Define transmission efficiency of AM signal.7. How many AM broadcast stations can be accommodated in a 100KHz bandwidth if the

highest frequency modulating a carrier is 5KHz?8. A carrier is amplitude modulated to a depth of 75 percent. Calculate the total power in the

modulated wave, if the carrier power is 40 watts.9. Calculate the percentage of power saving when the carrier and one of the sidebands are

suppressed in an AM wave modulated to a depth of 100 percent.10. Distinguish between low level modulation and high level modulation.11. A 2 MHz carrier having amplitude of 5V is modulated by a 4KHz audio signal having an

amplitude of 2V. Determine the modulation index and draw the frequency spectrum of amplitude modulated wave.

12. What is meant by coherent detection?13. Why VSB modulation is suited for TV signal transmission14. A message signal given by m(t)=2cos(100πt)+3sin(240πt) modulates a carrier of frequency

12MHz.give the expression of DSB-SC and plot its spectrum.

PART B1. (a) Draw the filtering scheme for the generation of VSB modulated wave & explain.

(6)(b) Explain the double sideband suppressed carrier modulation technique. (10)

2. (a) Explain Frequency Translation. (8)(b) Discuss Frequency Division Multiplexing. (8)

3. (a) In AM system the transmitter gives an output power of 5 KW when modulated to a depth of 95%. If after modulation by a data signal which produces an average modulation depth of 20%, the carrier & one side band are suppressed, determine the average power in the remaining output. (5)

(b) Explain the generation of SSB using Balanced Modulator. (11)4. (a) Explain the generation of SSB signal. (10)

(b) Show that an AM can be recovered, irrespective of vale of % modulation by using synchronous detection technique. (6)5. (a) A carrier Acosωct is modulated by a modulating signal

f(t)=E1cosω1t+ E2cosω2t+ E3cosω3tDerive expressions for total modulated power and net modulation index of the AM wave. (8)(b) Compare various Amplitude Modulation Systems. (4)(c) Write notes on FDM. (4)

6. (a) Write the expression that could be used for SSB generation. (8)(b) Give the spectra of SSB, DSBSC and standard AM for the message spectrum and carrier frequency of your choice. (3)© Justify the statement – SSB is suitable for speech signal and unsuitable for image.

(3)(d) Name the scheme that is preferred over SSB for image and justify the same. (2)

7. (a) Discuss the generation and detection scheme for standard AM. (10)(b) Compare standard AM with DSBSC and SSB in terms of various parameters of

relevance in communication. (6)8. Explain DSB-SC generation using Ring modulator (12)9. Explain the demodulation of DSB-SC using Costas loop (10)10. For periodic train of rectangular pulse. Find the fourier coefficient and draw the amplitude and phase spectrum.11. The input to square law modulator is given by x(t) = 0.5cos2π70t + .5cos2π120tWith carrier frequency of 10Hz find the output

UNIT – II ANGLE MODULATION SYSTEMS

PART A

1. Define angle modulation.2. Compare Frequency and Phase modulation.3. Determine the bandwidth of wideband FM, given the carrier signal of 100MHz, frequency

modulates a signal of 5 KHz as frequency deviation.4. Determine & draw the instantaneous frequency of a wave having a total phase angle given

by φ(t)=2000t+sin 10t5. Write Carson’s Rule applied for FM bandwidth calculation.6. Differentiate NBFM and WBFM.7. Draw a phase locked loop.8. List the properties of Bessel function of order n.9. What do you mean by narrowband and wideband FM?10. Give the frequency spectrum of narrowband FM?11. Why Armstrong method is superior to reactance modulator.12. Define frequency deviation in FM?13. What are the advantages of FM.?14. Define PM.15. What is meant by indirect FM generation?16. Draw the phasor diagram of narrow band FM.

17. Write the expression for the spectrum of a single tone FM signal.18. How to obtain a FM signal using a PM signal?19. Define modulation index of FM and PM.20. A carrier of frequency 100 MHz is frequency modulated by a signal x(t)=20sin

(200πx103t ). What is the bandwidth of the FM signal if the frequency sensitivity of the modulator is25 KHz per volt?

21. What is the bandwidth required for an FM wave in which the modulating frequency signal is 2 KHz and the maximum frequency deviation is 12 KHz?

PART B

1. How FM wave is generated by Indirect Method? (16)2. A 20 MHz is frequency modulated by a sinusoidal signal such that the maximum

frequency deviation is 100KHz. Determine the modulation index and approximate bandwidth of the FM signal for the following modulating signal frequencies (i) 1KHz (ii) 100KHz and (iii) 500KHz.

3. Explain in detail on Stereo Multiplexing. (16)4. (a) Draw the circuit of reactance modulator & explain its principle of operation. Derive the

expression for the equivalent capacitance. (10)(b) A modulating signal 5cos 2π15*1000t angle modulates a carrier Acosωt. Find the modulation index & bandwidth for FM system & determine the change in bandwidth & modulation index for FM if fm is reduced by 5 KHz. (6)

5. (a) Explain any two techniques of demodulation of FM. (10) (b) Explain how varactor diode can be used for frequency modulation. (6)6. (a) Explain the generation & demodulation of FM signal. (11) (b) Draw the frequency spectrum of FM and explain. (5)7. Explain the FM

(a) Generation using VCO (8)(b) Demodulation using PLL. (8)

8. (a) Obtain the expression for single tome FM signal. (6)(b) Explain the Armstrong method for generation of FM signal with block diagram.

(10)9. Derive the expression for the frequency modulated signal. Explain what is meant by

narrow band FM and wide band FM?10. Draw the circuit diagram of Foster-Seeley discriminator and explain its working.(16)

UNIT III – NOISE THEORY

PART A

1. Define noise figure.2. What is white noise?3. When a random process is said to be ergodic?4. What is thermal noise? Give the expression for the thermal noise voltage across a

resistor5. What is shot noise?6. Define noise temperature.

7. Find the thermal noise voltage developed across a resistor of 700ohm. The bandwidth of the measuring instrument is 7MHz and the ambient temperature is 27’C.

8. Define a random variable?9. What is a random process?10. What is Gaussian process?11. What is a stationary random process?12. What is partition noise?13. Draw the Autocorrelation & Power spectral Density of white noise.14. Define noise equation bandwidth 15. Define narrowband noise.

PART B

1. Derive the effective noise temperature of a cascade amplifier. Explain how the various noises are generated and the method of representing them. (16)

2. Explain how the various noises are generated and the method of representing them. (16)3. Give a random process, X(t) = A cos (ωt + θ), where A and ω are constant and θ is a

uniform random variable. Show that X(t) is ergodic in both mean and autocorrelation.4. Write notes on noise temperature and noise figure. (8)5. Derive the noise figure for cascade stages. (8)6. What is narrowband noise? Discuss the properties of inphase and quadrature phase

components of a narrowband noise. (8)7. What is meant by noise equivalent bandwidth? Illustrate it with a diagram (8)8. Discuss the following:

i) noise equivalent bandwidth (4)ii) narrow band noise (4)iii) noise temperature (4)iv) noise spectral density (4)

9. Explaini) shot noise ii) White noise iii) partition noiseiv) Noise figure v) Thermal noise

UNIT IV – NOISE PERFORMANCE OF CW MODULATION SYSTEMS

PART A

1. How to achieve threshold reduction in FM receiver?2. Define improvement factor.3. Define Figure of merit.4. What is extended threshold demodulator?5. Draw the Phasor representation of FM noise.6. Define pre-emphasis and de-emphasis.7. What is the need for performing pre-emphasis and de-emphasis in FM?8. What is the SNR for AM with small noise case?9. What is threshold effect with respect to noise?10. Define SNR.

11. Discuss the factors that influence the choice of intermediate frequency in a radio receiver.

12. What are the advantages of superheterodyne receiver over TRF receivers?13. What is FM threshold effect?14. What is Capture effect in FM?15. What is meant by image frequency rejection?

PART B

1. Explain the working of Superheterodyne receiver with its parameters. (16)2. Discuss the noise performance of AM system using envelope detection. (16)3. Compare the noise performance of AM and FM systems. (16)4. Explain the significance of pre-emphasis and de-emphasis in FM system? (8)5. Derive the noise power spectral density of the FM demodulation and explain its

performance with diagram. (16)6. Draw the block diagram of FM demodulator and explain the effect of noise in detail.

Explain the FM threshold effect? (16)7. Derive the expression for output signal to noise for a DSB-SC receiver using coherent

detection. (16)

UNIT V – INFORMATION THEORY

PART A

1. What is entropy?2. Define information rate?3. Define Channel capacity?4. What is channel capacity of binary synchronous channel with error probability of 0.2?5. State channel coding theorem.6. Define entropy for a discrete memory less source.7. Define channel efficiency and give its mathematical expression.8. Give the mathematical expression for entropy of a continuous system.9. What is channel redundancy?10. Write down the formula for the mutual information.11. When is the average information delivered by a source of alphabet size 2, maximum?12. Name the source coding techniques.13. Define mutual information.14. Write down the formula for mutual information.15. State Shannon’s theorem.16. Give the relation between average mutual information and joint/conditional entropy.17. Write the expression for code efficiency in terms of entropy.18. Is the information of a continuous system non negative? If so, why?19. Explain the significance of the entropy H(X/Y) of a communication system where X is

the transmitter and Y is the receiver.20. An event has six possible outcomes with probabilities ½.1/4,1/8,1/16,1/32,1/32. Find

the entropy of the system.

PART B

1. Discuss Source coding theorem, give the advantage and disadvantage of channel coding in detail, and discuss the data compaction. (16)

2. Explain in detail Huffman coding algorithm and compare this with the other types of coding. (8)

3. Explain the properties of entropy and with suitable example, explain the entropy of binary memory less source. (8)

4. What is entropy? Explain the important properties of entropy. (8)5. Five symbols of the alphabet of discrete memory less source and their probabilities

are given below. (8)S=[S0,S1,S2,S3,S4]P[S]=[0.4,0.2,0.2,0.1,0.1]

Code the symbols using Huffman coding.6. Five symbols of the alphabet of discrete memory less source and their probabilities

are given below. (8)S=[S0,S1,S2,S3,S4]P[S]=[0.4,0.2,0.2,0.1,0.1]

Code the symbols using Shannon fano coding.8. Derive the channel capacity theorem and discuss the implications of the information

capacity theorem. (16)9. Construct binary optimal code for the following probability symbols using Huffman

procedure and calculate entropy of the source, average code Length, efficiency, redundancy and variance 0.2, 0.18, 0.12, 0.1, 0.1, 0.08, 0.06, 0.06, 0.06, 0.04 (16)

10. Define mutual information. Find the relation between the mutual information and the joint entropy of the channel input and channel output. Explain the important properties of mutual information. . (16)

11. Derive the expression for channel capacity of a continuous channel. Find also the expression for channel capacity of continuous channel of a infinite bandwidth. Comment on the results. (16)