ECE



Velammal Institute of Technology

Department of ECE

|SUBJECT CODE: |EC6 402 |

SUBJECT NAME: COM MUNICATION THEORY

Academic Year :2017-18 Even semester

Regulation: 2013 Year and Semester: II/IV

|Prepared by | |

|Mrs.D.Jeyamani Latha,Assoc.Professor/ECE | |

| | | | | | | |

ANNA UNIVERSITY, CHENNAI-25

SYLLABUS COPY

REGULATION 2017

|EC8491 |COMMUNICATION THEORY |L |T |P |C |

| | |3 |0 |0 |3 |

UNIT I AMPLITUDE MODULATION 9

Amplitude Modulation- DSBSC, DSBFC, SSB, VSB - Modulation index, Spectra, Power relations and Bandwidth – AM Generation – Square law and Switching modulator, DSBSC Generation – Balanced and Ring Modulator, SSB Generation – Filter, Phase Shift and Third Methods, VSB Generation – Filter Method, Hilbert Transform, Pre-envelope & complex envelope –comparison of different AM techniques, Super heterodyne Receiver

UNIT II ANGLE MODULATION 9

Phase and frequency modulation, Narrow Band and Wide band FM – Modulation index, Spectra, Power relations and Transmission Bandwidth - FM modulation –Direct and Indirect methods, FM Demodulation – FM to AM conversion, FM Discriminator - PLL as FM Demodulator.

UNIT III RANDOM PROCESS 9

Random variables, Random Process, Stationary Processes, Mean, Correlation & Covariance functions, Power Spectral Density, Ergodic Processes, Gaussian Process, Transmission of a Random Process Through a LTI filter.

UNIT IV NOISE CHARACTERIZATION 9

Noise sources – Noise figure, noise temperature and noise bandwidth – Noise in cascaded systems. Representation of Narrow band noise –In-phase and quadrature, Envelope and Phase – Noise performance analysis in AM & FM systems – Threshold effect, Pre-emphasis and de-emphasis for FM.

UNIT V SAMPLING & QUANTIZATION 9

Low pass sampling – Aliasing- Signal Reconstruction-Quantization - Uniform & non-uniform quantization - quantization noise - Logarithmic Companding –PAM, PPM, PWM, PCM – TDM, FDM.

TOTAL: 45 PERIODS

TEXT BOOKS:

1. J.G.Proakis, M.Salehi, “Fundamentals of Communication Systems”, Pearson Education 2006.

2. S. Haykin, “Digital Communications”, John Wiley, 2005.

REFERENCES:

1. B.P.Lathi, “Modern Digital and Analog Communication Systems”, 3rd Edition, Oxford

University Press, 2007.

2. B.Sklar, “Digital Communications Fundamentals and Applications”, 2nd Edition Pearson

Education 2007.

3. H P Hsu, Schaum Outline Series - “Analog and Digital Communications” TMH 2006.

4. Couch.L., "Modern Communication Systems", Pearson, 2001.

|EC6402COMMUNICATION THEORY |L |T | |P C |

| |3 |0 |0 |3 |

Aim and Objective of the Subject Aim:

To study the fundamental concepts in Communication Theory and also discuss the operation of Wireless/Wired communication and its components required to build different operation.

OBJECTIVES:

✓ To introduce the concepts of various analog modulations and their spectral characteristics.

✓ To understand the properties of random process.

✓ To know the effect of noise on communication systems.

✓ To study the limits set by Information Theory.

2. Need and Importance for Study of the Subject

➢ Makes it possible to design the simple circuits that are: – wired communication – Modulation Circuit – Analyse the performance of AM and FM systems.

➢ Allows students to upgrade their knowledge in Communication system fields.

➢ Helps students/engineers in touch with the latest technologies.( New revolution in 5G, 6G in all the smart phones, smart televisions, etc.)

Importance for Study of the Subject:

At the end of the course, the student should be able to:

✓ Design AM communication systems.

✓ Design Angle modulated communication systems.

✓ Apply the concepts of Random Process to the design of Communication systems.

✓ Analyze the noise performance of AM and FM systems.

2. Industry Connectivity and Latest Developments

Industry Connectivity:

➢ The following companies (Industries) are linked to Communication Theory:

ISRO,TATA COMMUNICATIONS, SIEMENS, NORTEL.

Latest Developments:

• LIFI if fast data transfer in networking.

• New revolution in 5G, 6G in all the smart phones, smart televisions, etc.

• Network on chip.

• AM and FM Modulation Schemes.

• LTE were introduced almost in all the home appliances.

UNIT I AMPLITUDE MODULATION

Amplitude Modulation- DSBSC, DSBFC, SSB, VSB - Modulation index, Spectra, Power relations and Bandwidth – AM Generation – Square law and Switching modulator, DSBSC Generation – Balanced and Ring Modulator, SSB Generation – Filter, Phase Shift and Third Methods, VSB Generation – Filter Method, Hilbert Transform, Pre-envelope & complex envelope –comparison of different AM techniques, Super heterodyne Receiver

TWO MARKS QUESTION WITH ANSWERS

1. Define modulation and what are the types of analog modulation?

Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal.

Types:

➢ Amplitude modulation.

➢ Angle Modulation

➢ Frequency modulation

➢ Phase modulation.

1. What is the need for modulation and what are the degrees of modulation? (or)

What are the advantages of converting low frequency signal in to high frequency signal?

( MAY/JUNE 2013)

Needs for modulation:

➢ Ease of transmission

➢ Multiplexing

➢ Reduced noise

➢ Narrow bandwidth

➢ Frequency assignment

➢ Reduce the equipments limitations

Degrees of modulation

⎫ Under modulation. m1

2. Define Heterodyning. (APRIL/MAY 2015)

Heterodyning means the translating or shifting in frequency. By heterodyning the incoming signal at ωRF with the local oscillator frequency ωLO, the message is translated to an intermediate frequency ωIF, which is equal to either the sum or the difference of ωRF and ωIF.

4. Define Amplitude Modulation?

In amplitude modulation, the amplitude of a carrier signal is varied according to variations in amplitude of modulating signal.

The AM signal can be represented mathematically as, eAM = (Ec + Em sinωmt ) sinωct and the modulation index is given as,m = Em /EC (or) Vm/Vc

5. What is Super Heterodyne Receiver?

The super heterodyne receiver converts all incoming RF frequencies to a fixed lower frequency, called intermediate frequency (IF). This IF is then amplitude and detected to get the original signal.

6. What is single tone and multi tone modulation?

If modulation is performed for a message signal with more than one frequency component then the modulation is called multi tone modulation.

If modulation is performed for a message signal with one frequency component then the 7

modulation is called single tone modulation.

7. Compare bandwidth and power requirement for AM with DSB-SC and SSB-SC? (MAY/JUNE 2013)

|S. No |AM signal |DSB-SC |SSB-SC |

| | | | |

|1 |Bandwidth = 2fm |Bandwidth = 2fm |Bandwidth = fm |

|2 |Contains USB, LSB, Carrier |Contains USB,LSB |USB,LSB |

|3 |More Power is required for |Power required is less than |er required is less than |

| |Transmission |that of AM. |AM &DSB-SC |

8. Define Demodulation.

Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation. The devices used for demodulation or detection are called demodulators or detectors. For amplitude modulation, detectors or demodulators are categorized as,

Square-law detectors

Envelope detectors

9. What are the advantages of VSB-AM? (APRIL/MAY 2011)

➢ It has bandwidth greater than SSB but less than DSB system.

➢ Power transmission greater than DSB but less than SSB system.

➢ No low frequency component lost. Hence it avoids phase distortion.

10. Define SSB-SC and DSB-SC

➢ SSB-SC stands for Single Side Band Suppressed Carrier

➢ When only one sideband is transmitted, the modulation is referred to as Single side band modulation. It is also called as SSB or SSB-SC.

DSB-SC:

After modulation, the process of transmitting the sidebands (USB, LSB) alone and suppressing the carrier is called as Double Side Band-Suppressed Carrier.

11. Define Coherent Detection (APRIL/MAY 2013)

During Demodulation carrier is exactly coherent or synchronized in both the frequency and phase, with the original carrier wave used to generate the DSB-SC wave.This method of detection is called as coherent detection or synchronous detection.

12.What is image frequency? (NOV/DEC-2014)

The local oscillator frequency (fo), input signal frequency (fs) and intermediate frequency (fi) are related as f0 – fs=fi that is f0=fs+fi .If some other frequency fsi=fs+2fi appear at the input of mixer, it produces fi at the output of mixer.This interferes with the desired IF,since it is same as IF.The frequency is called image frequency. Thus image image frequency gets converted to IF range and it is amplified by IF amplifier.

13.Draw the frequency spectrum of VSB.Where it is used?(APRIL/MAY 2015)

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VSB is used for TV transmission, since low frequencies near fc represents picture details. They are unaffected by VSB.

14.A transmitter radiates 9 kW without modulation and 10.125 kW after modulation. Determine depth of modulation. (APR/MAY 2013)

Depth of modulation:

Therefore,

=1.125-1

=0.125

ma2=0.125X2

=0.250

ma=0.50

PART-B

1.Explain With Block Diagram Super Heterodyne Receiver? (April/May 2015)

In a broadcasting system whether it is based on amplitude modulation or frequency modulation, the receiver not only have the task of demodulating the modulated signal, but it is also required to perform some other system functions.

Carrier frequency tuning, the purpose of which is to select the desired signal (i.e.) desired radio or TV station) Filtering, which is required to separate the desire signal from other modulation signals that may be picked up along the way.

Amplification, which is intended to compensate for the loss of signal power incurred in the course of transmission.

R.F Section

The incoming amplitude modulated wave is picked up by the receiving antenna and is fed to the RF section. The RF section consists of a pre selector and an RF Amplifier. The pre selector is a band pass filter with an adjustable centre frequency that is tuned to the desired carrier frequency of

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the incoming signal. The main use of the preselected is to provide sufficient band limiting to prevent undesired ratio in frequency signal or image signal. The effectiveness of suppressing unwanted image signals increases as the number of selective stages in the RF section increases and as the ratio of intermediate to signal frequency increases. R.F amplifiers are used for better selectivity.

Frequency Changer

The combination of mixer and local oscillator provides a heterodyning function whereby the incoming signal is converted to a predetermined fixed intermediate frequency, usually lower than the incoming carrier frequency. This frequency translation is achieved without disturbing the relation of the sidebands to the carrier. The result of heterodyning is to produced an intermediate frequency carrier defined by fIF = flo –fRF

Where fLO is the frequency of the local oscillator and fRF is the carrier frequency of the incoming RF signal. Since the output of the frequency his neither the original input frequency not the final baseband frequency, it is called as intermediate frequency. Sometimes the frequency changer circuits are referred to as the first detector, in which case the demodulator i s called as second detector.

IF Section

The IF section consists of one or more stages of turned amplification with a

bandwidth corresponding to that required for the particular type of modulation that the receiver intended to handle. The IF section provides most of the amplification purpose of which is to recover the baseband or message signal.

If coherent detection is used, then a coherent signal source must be provided in the receiver. Audio Amplifiers

The final stage of the super heterodyne receiver consists of one or more audio amplifiers which is used for the power amplification of the recovered message signal.

Image frequency and its rejection ratio

In a super heterodyne receiver, the mixer will develop and intermediate frequency output when the input signal frequency is greater or less than the local oscillator frequency by an amount equal to intermediate frequency.

Amplitude Limiter

The basic difference between AM and FM super heterodyne receiver lies in the use of an FM

demodulator such as limiter frequency discriminator. In FM system the message signal is

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transmitted by the instantaneous value of carrier signal & its amplitude remain constant. Therefore any variation of the carriers amplitude at the receiver input must result from noise or interference. An amplitude limiter following the IF section is used to remove amplitude variations by clipping the modulated wave is rounded by a band pass filter that suppresses harmonics of the carrier frequency. Thus the filter output is again sinusoidal, with an amplitude that in practically independent of the carrier amplitude of the receiver input. The Fig shows basic block diagram of FM super heterodyne receiver.

Performance Parameters of Receivers

The performance of a Radio receiver is measured on the basic of its selectivity, sensitivity, fidelity and image frequency rejection selectivity.

Selectivity

The selectivity is the ability of the receiver to select a signal of a desired frequency while rejecting all others. The selectivity of the receiver is obtained partially by RF amplifier and mainly by IF amplifiers. The selectivity shows the attenuation that the receiver offers to signals at frequencies near to the one to which it is tuned. Fig. shows the typical selectivity curve of the receiver. The selectivity depends upon tuned LC circuits used in RF and IF stages, fr is the resonating (tuned) frequency and Q is quality factor of these LC Circuits, As shown in Fig. bandwidth should be narrow for better selectivity. Hence Q of the coil should be high.

Fig. selectivity curve

Sensitivity

The ability of the receiver to pick up weak signals and simplify them is called sensitivity. It is often defined in terms of the voltage that must be applied to the receiver input terminals to give the standard output power, measured at the output terminals.

As the gain of the receiver is increased, sensitivity is also increased. The sensitivity is expressed in micro volts or decibels. Fig.shows the typical sensitivity curve of a receiver. As shown in the Fig., the sensitivity is decreased (i.e., voltage is increased) at high frequencies.

Fig sensitivity curve

Fidelity

Fidelity is a measure of the ability of a communication system to produce at the output of the

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receiver, an exact replica of the original source information. This may also be defined as the degree to which the system accurately reproduces at the output, the essential characteristics of signals that are impressed upon the input.

Fig. Fidelity curve

Fig. shows the typical fidelity curve of the receiver. The fidelity curve shown in the Fig.basically represents frequency response of the receiver. A good fidelity requires wide band of frequencies to be amplified. Hence for good fidelity, more bandwidth of RF and IF stages are required.

Signal to noise Ratio

Signal to noise Ratio may be defined as the ratio of signal power to noise power at the receiver output. A good receiver should have high signal to noise ratio (SNR) which indicates negligible noise present at the output.

Image Frequency Rejection

We know that local oscillator frequency is made higher than the signal frequency such that f0

– fs = fi . Here fi is IF. That is f0 = fs + fi. The IF stage passes only fi . If the frequency fi = fs +2fi appears at the input of the mixer, then the mixer will produce different frequency equal to fi . This is equal to IF. The frequency fsi is called image frequency and is defined as the signal frequency

plus twice the IF. The image frequency is converted in the IF stage and it is also amplified by IF amplifiers. This is the effect of two stations being received simultaneously. The image frequency rejection is done by tuned circuit in the RF stage. It depends upon the selectivity of the RF stage. The image rejection should be done before the RF stage.

2.With The Help Of Neat Diagram Explain The Generation Of DSB-SC Using Balanced Modulator And Ring Modulator. (Nov/Dec 2010)

Introduction

In DSB – SC, the transmitted wave consists of only upper and lower sidebands. Transmitted power is saved here through the suppression of the carrier wave because it does not contain any useful information, but the channel bandwidth required is the same as before.

Expression for DSB –SC:

Let the modulating signal,

The carrier signal

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Comparing equation (1.57) and (1.58) the carrier terms Vc sin ωct is missing and only upper and lower sidebands are present.

Fig Frequency spectrum of DSB-SC-AM

In this fig, the carrier term fc is suppressed. It contains only two sideband terms having the frequency of (fc – fm) and (fc + fm).

Generation of DSB – SC – AM

There are two ways of generating DSB – SC – AM such as ,

i) Balanced modulator,

ii) Ring modulator.

Balanced Modulator

The circuit that is very commonly used for DSB – SC generation.

In balanced modulator, two non-linear devices are connected in the balanced mode, so as to suppress the carrier wave. It is assumed that the two transistors are identical and the circuit is symmetrical. Since the operation is confined in non-linear region of its transfer characteristics.

| |Fig Balanced modulator |

|The |modulating voltage across the two windings of a centre-tap transformer is equal, and |

|opposite in phase, |

|i.e., |Vm = V'm |

13

Ring Modulator (or) Diode Balanced Modulator Introduction:

The one of the most popular method of generating a DSB – SC wave is ring modulator.

The circuit employs diodes as non-linear devices and the carrier signal is connected between centre taps of the input and output transformers.

There is no need for a band pass filter at the output. The four diodes are controlled by a carrier Vc(t) of frequency fc

The carrier signal acts as a switching signal to alternate the polarity of the modulating signals at the carrier frequency. For better understanding of the operation, assume that the modulating input is zero. Only carrier signal is present.

Fig.Ring modulator

Positive Half Cycle of Carrier:

Diodes D1 and D2 are form ward biased. AT this time D3 and D4 are reverse biased and act like open circuits. The current divides equally in the upper and lower portions of the primary winding of T12 .

The current in the upper part of the winding produces a magnetic field that is equal and opposite to the magnetic field produced by the current in the lower half of the secondary. Therefore, these magnetic fields cancel each other out and no output is induced in the secondary. Thus the carrier is effectively suppressed.

Negative Half Cycle of Carrier:

When the polarity of the carrier reverses. Diodes D1 and D2 are reverse biased and diodes D3 and D4 conduct. Again the current flows in the secondary winding of Tr1 and the primary winding of Tr2.

The equal and opposite magnetic fields produced in Tr2 cancel each other out and thus result in zero carrier output. The carrier is effectively balanced out.

Principle of Operation:

When both the carrier and the modulating signals are present, during positive half cycle of the carrier, diodes D1 and D2 conduct, while diodes D3 and D4 does not conduct.

During negative half cycle of the carrier voltage diodes D3 and D4 conduct and D1 and D2 does not conduct.

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Fig. Graphical representation of DSB-SC signals

Phase Reversal

When polarity of the modulating signal changes, the result is a 180[pic] phase reversal.

At the time, during the positive half cycle of the carrier, diodes D3 and D4 are in forward bias and the negative half cycle of the carrier, diodes D1 and D2 are in reverse bias. Consider the modulating signal Vm (t) and carrier signal Vc (t) , such that,

The equation (1.76) shows that the output is free from the carrier and other higher order terms, and it contains upper and lower sidebands only.

The ring modulator circuit is also known as double balanced modulator because comparing to balanced modulator here two more diodes are used.

Advantages:

1. DSB –SC is more efficient in transmitted power as compared to DSBFC .

DSB –SC has better signal to noise ratio as compared to single side band (SSB) transmission.

Disadvantage:

Even though the carrier is suppressed the bandwidth of DSBSC remains same as DSBFC.

3.i). Define amplitude modulation.How an amplitude modulated signal can be generated using a non linear modulator circuit.(NOV/DEC 2012)

Amplitude modulation:

Amplitude modulation is the process by which amplitude of the carrier signal is varied in accordance with the instantaneous value of the modulating signal,but frequency and phase remains constant.

15

Non linear modulator circuits:

A simple diode can be used as a non linear modulator by restricting its operation to non linear operation of its characteristics.

The undesired frequency terms are filtered out by a band pass filter.

The methods for generation of AM waves using non linear property are broadly divided into two types

(a)square law modulator

(b)Balance modulator

(a)Square law modulators:

Any device operated in non linear region of its output characteristics is capable of producing amplitude modulated waves when the carrier and modulating signals are fed at the input. Thus a transistor, a triode tube,a diode etc. may be used as the square law modulator. A square law modulator circuit consists of the following:

i) A non linear device

ii) A bandpass filter

(iii)A carrier source and modulating signal

The square law modulator

Consider a non linear device to which a carrier c(t)= A cos(2Π fct) and an information signal m(t) are fed simultaneously as shown in figure1. The total input to the device at any instant is

As the level of the input is small very small, the output can be considered up to square of the input

Taking Fourier transform on both sides we get,

16

Therefore the square law device output Vo consists of the dc component at f=0

The information signal ranging from 0 to W Hz and its second harmonics Signal at fc and 2fc Frequency band centered at fc with a deviation of ± W Hz

The required AM signal with a carrier frequency fc can be separated using a bandpass filter at the output of the square law device. The filter should have a lower cutoff frequency ranging between 2W and (fc - W) and upper cut-off frequency between (fc+W) and 2fc

Therfore the filter output is

The output AM signal is free from distortion and attenuation only when(fc-W)2W or fcωm(μ sin ωmto / 1+ μ cos ωmto). Dec’09

3. (i)Derive an expression for AM wave and draw its frequency spectrum. (ii) Draw the one cycle of AM wave and calculate the modulation index of it in terms of Vmax and Vmin voltages.(iii) A500-W carrier is modulated to a depth of 80%. Calculate the total power .May’08

4. An AM transmitter radiates 8KW with carrier unmodulated and 9KW when the carrier is sinusoidal modulated. Calculate the modulation index. If another sine wave, corresponding to 50% modulation, is transmitted simultaneously, determine the total radiated power. May’09

5. (i)Compare the characteristics of DSBFC, DSBSC, SSBFC, SSBSC, VSB schemes.(AUC DEC 2011)

6. Explain the transmission of television signals usingmodultion. Dec’11

7.The AM signal s(t)=Ac[1+kam(t)]cos (2πfct) is applied to the system shown in fig.3.Assuming

that| kam(t)|2W show that m(t) can be obtained from the square rooter output v3(t)

(15) (APR/MAY 2017)

[pic]

Consider a square law detector, using a non linear device whose transfer characteristics is defined by v2(t)= α1v1(t)+α2v2(t) where α1and α2 are constants,v1(t) is the input and v2(t) is the output.The input consists of the AM wave vi(t)=Ac[1+kα m(t)]cos (2π fct).

(i)Evaluate the output v2(t)

(ii)Find the conditions for which the message signal m(t) may be recovered from v2(t)

8.The discrete Hilbert Transform is a process by which a signal’s negative frequencies are phase advanced by 90 degrees and the positive frequencies are phase –delayed by 90 degrees. Shifting the results of the Hilbert transform (+j) and adding it to the original signal creates a complex signal as mentioned in the equation.If mi[n] is the Hilbert transform of mr[n],then:

mc[n]=mr[n],+jmi[n].

Apply the concept of Hilbert transform to generate and detect SSB-SC signal.(15)(APR/MAY 2017)

UNIT II ANGLE MODULATION

1.Derivetheexpressionforthefrequencymodulatedsignal.Explainwhatismeantby narrowbandFMandwidebandFMusingexpression. (AUCNOV2006)

2.(i)Discuss the effects of non linearities in FM systems. [8]

(ii) How do you demodulate the FM wave using frequency discriminator. [8] May 07

3. (a) Discuss on FM demodulation using PLL. (16) may ‘09

4. Explain the working of the reactance tube modulator and drive an expression to show how the variation of the amplitude of the input signal changes the frequency of the output signal of the modulator. May’08

5.(i).Derive an expression for a single tone FM signal with necessary diagrams and draw its frequency spectrum.(10) (MAY/JUNE 2016)

(ii) An angle modulated wave is described by

V(t)=100 cos(2*106 πt+10 cos200 πt).

Find (i) Power of the modulating signal,(ii)Maximum frequency deviation,(iii)Bandwidth (6) (MAY/JUNE 2016)

6.(i)Obtain the mathematical expression for WBFM.Also compare and contrast its characteristics with NBFM.(6)(APR/MAY 2017)

(ii)Suggest and discuss the method for the generation of FM using direct method.(7)(APR/MAY 2017)

7.(i)Analyse and brief how the ratio detector suppresses the amplitude variation caused by the communication media without using amplitude limiter circuit.(7)(APR/MAY 2017)

(ii)Explain the detection of FM wave using PLL detector.(6)(APR/MAY 2017)

8. Derive the expression for frequency spectrum of FM modulated signal and comment on the transmission bandwidth.(16)(NOV/DEC 2015)

UNIT III RANDOM PROCESS

1.Explainconceptofnoiseequivalentbandwidth (AUCNOV2007)

2. Discussthetype’scausesandeffects ofvariousformsofnoisecreatedwithina receiver? Nov 08

3. AstationaryGaussianprocessX(t) withzeromeanandpower spectral densitySx(f) isappliedtoalinearfilter whoseimpulsesresponseh(t) isshowninfigure.asample Yis takenofthe randomprocessatthefilter outputatthetimeT.

[pic]

i)DeterminethemeanandvarianceofY.

ii)WhatistheprobabilitydensityfunctionofY? (AUC NOV2008)

4. List the different types of random process and give the definitions. (10) Dec’10

5. Show how a narrow band noise can be represented as n(t) = nc(t) coswct - ns(t) sinwct where nc(t) and ns(t) arein-phase and quadrature phase components of noise respectively. Dec’11

6.Consider two linear filters connected in cascoade as shown in Fig1.Let X(t) be a stationary process with a auto correlation function Rx(τ),the random process appearing at the first input filter is V(t) and the second filter output is Y(t). (13)

(i)Find the autocorrelation function of Y(t)

(ii0Find the cross correlation Function Rvy(τ) of V(t) and Y(t)

[pic]

7.The amplitude modulated signal is defined as XAM(t)=A m(t)cos((ωct+θ) where m(t) is the baseband signal and A cos(ωct+θ) is the carrier.The base band signal m(t) is modeled as a zero mean stationary random process with the autocorrelation fuctionRxx(t) and the PSD Gx(f0.The carrier amplitude A and the frequency ωcare assumed to be constant and the initial carrier phase θ is assumed to be a random uniformly distributed in the interval (-π,π).Furthermore ,m(t) and θ are assumed to be independent.(13)(APR/MAY 2017)

(i)Show that XAM(t) is Wide sense stationary

(ii)Find PSD of XAM(t)

8.(i)Two random processes X(t0=A cos(ωt+θ) andY(t)=A sin(ωt+θ) where A and ω are constants and θ is uniformly distributed random variable in (0,2π).Find the cross correlation fuction.(8) (MAY/JUNE 2016)

(ii) Explain in detail about transmission of random process through a linear tiinvariant filter.(8)(MAY/JUNE 2016)

9.(i)When is a random process said to be strict sense stationary (SSS),Wide Sense Stationary (WSS) and Ergodic process.(8)(MAY/JUNE 2016)

(ii)Give a random processes X(t)=A cos (ωt+μ)where A and ω are constants and μ is a uniform random variable.Show that X(t) is ergodic in both mean and auto correlation.(8)(MAY/JUNE 2016)

10. (i) Define the following terms means,correlations.Covariance and ergodicity. (NOV/DEC 2016)

(ii) Explain in detail about the transmission of a random process through a linear time invariant filter. (NOV/DEC 2016)

11.(i) When is random process said to be Strict Sense Stationary (SSS). Wide Sense Stationary (WSS) and Ergodic process. (NOV/DEC 2016)

(ii) What is a Gaussian random process and mention its properties. (NOV/DEC 2016)

12.In a binary communication system ,let the probability os sending a 0 and 1 be 0.3 and 0.7 respectively .Let us assume that a 0 being trannsmitted,the probability of it being received as 1 is 0.01 and the probability of error for a transmission of 1 is 0.01.(16)(NOV/DEC 2015)

(i) What is the probability that the output of this channel is 1?

(ii)If a 1 is received ,then what is the probability that the input to the channel was 1?

8.What is CDF and PDF? State their properties.Also discuss them in detail by giving examples of CDF and PDF for different types of random of random variables.(16)(NOV/DEC 2015)

UNIT IV NOISE CHARACTERIZATION

1.Define Hilbert Transform with a suitable example. May 07

2. Discuss the threshold effect in an envelope detector used in an AM receiver. (10) May ‘09

3. Explain the SNR’s at Input and output of demodulators of DSB-SC and SSB-SC and compare them May’08.

4. ExplainthenoiseinFMreceiverusing PLL with itsnoisymodelblockdiagram. AUCMAY2007,MAY2004

5.(i)Classify the different noise sources and its effect in real time scenario. .(7)

(ii)Discuss the effects of noise in cascaded system.(6)

6.Derive an expression for signal to noise ratio for an AM signal,with assumption that the noise added in the channel is pare its performance with FM system.(13) (APR/MAY 2017)7(i)Define Narrow band noise and explain the representation of Narrow band Noise in terms of In-Phase and Quadrature Components.(8) (MAY/JUNE 2016)

(ii)Explain Pre-emphasis and De-emphasis in FM.(8) (MAY/JUNE 2016)

8.Explain the noise in DSB-SC receiver using synchronous or Coherent detection and calculate the figure of merit for a DSB-SC system>(16)(MAY/JUNE 2016)

9.(i) Define noise and write notes on Shot noise, Thermal noise and White noise. (NOV/DEC 2016)(ii) Derive the figure of merit for AM system. Assume coherent detection. (NOV/DEC 2016)

10. Explain the noise in FM receiver and calculate the figure of merit for a FM system. (NOV/DEC 2016)

11.Consider a message which is a wide- sense stationary random process with the auto correlation function RM(τ))=16 Sinc2(10000 τ) .All the realizations of the message process satisfy the condition max│m(t)│=6.This message needs to be transmitted via a channel with a 50 dB attenuation and additive white noise with the power spectrum density SN(f)=No/2=10-12 W/Hz..The SNR at the modulator output should be at least 50dB.What is the transmitter power and channel bandwidth if the following modulation shemes are employed? (16)(NOV/DEC 2015)

(i)DSB-SC AM

(ii)SSB-SC AM

(iiI)Conventional AM with a modulation index of 0.8

12. Give a detailed account on impact of noise on angle modulation schemes. What is the required received power in an FM system with modulation index ,β=5 if W=15kHz and NO=10-14 W/Hz? The power of the normalized message signal is assumed to be 0.1.Watt and the required SNR after demodulation is 60 dB.(16) (NOV/DEC 2015)

UNIT V SAMPLING AND QUANTIZATIONZATION

1. State the sampling theorem. Explain the ideal sampling process with necessary expressions and diagrams. (16)

2. Explain (i) Natural Sampling and Flat-top Sampling (10)

(ii) Sample and Hold circuit. (6)

3. The signal g(t) = 10 cos(20πt) cos(200πt) is sampled at the rate of 250 samples per second.

(a) Determine the spectrum of the resulting sampled signal.

(b) Specify the cut-off frequency of the ideal reconstruction filter so as to recover g(t) from its sampled version.

(c) What is the Nyquist rate for g(t).

(d) Explain the reconstruction process of a message from its samples. (16)

4. Explain the process of quantization and obtain an expression for signal to Quantization ratio in the case of a uniform quantizer. (16)

5. Explain the characteristics of Non-uniform quantization with diagrams. Also compare uniform and non-uniform quantization methods. (16)

6. Briefly discuss about quadrature sampling of band-pass signals. Derive the expression for SDR. (16)

7. Write short notes on (i) Analog companding and (8) (ii) (ii) Digital companding. (8)

8. With neat diagrams, Pulse Code Modulation and demodulation system.(16)

9. A compact disc (CD) records audio signals digitally using PCM. Assume the audio signal bandwidth to be 15 KHz.

(a) What is the Nyquist rate?

(b) If the Nyquist samples are quantized to L = 65, 536 levels and then binary coded, determine the number of bits required to encode a sample.

(c) Assuming that the signal is sinusoidal and that the maximum signal amplitude is 1 volt; determine the quantization step and the signal-to-quantization noise ratio.

(d) Determine the number of bits per second (bit/s) required to encode the audio signal.

(e) For practical reasons, signals are sampled at above the Nyquist rate, as discussed in class. Practical CDs use 44,000 samples per second. For L = 65, 536 determine the number of bits per second required to encode the signal and the minimum bandwidth required to transmit the encoded signal.

10.What is PAM? Explain TDM process with necessary diagrams. (16)

 

VELAMMAL INSTITUTE OF TECHNOLOGY,

Panchetti

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

EXHAUSTIVE QUESTION BANK

SUBJECTCODE/NAME:EC8491/COMMUNICATION THEORY YEAR/SEM/SEC:II/IV/A

UNITIAMPLITUDEMODULATION

PARTA

1. Computethebandwidth oftheAMPsignalgivenbyS(t)=23(1+0.8(310t)} cos(23000t)(MAY/JUNE 2012)

2. What arethe causes oflineardistortion?(MAY/JUNE2012)

3. An Amplitudemodulationtransmitter radiated1000 watts ofunmodulatedpower.Ifthecarrierismodulatedsimulatanesousl;ybytwotones of 40% and 60%respectively,calculated thetotal power radiated.(NOV/DEC2012)

4. Calculate thelocal oscillator frequencyifincomingfrequencyisF1 andtranslatedcarrierfrequencyisF2(NOV/DEC2012)

5. What aretheadvantageofConvertingthelowfrequencysignal into thehighfrequency signal?(MAY/JUNE 2013)

6. Comparebandwidthand powerrequirementinterms of carrier powerpC.forAM.DSB-SCand

SSB(MAY/JUNE 2013)

7. State thedifferencebetweensinglesidebandand vestigial side band transmission system(MAY/JUNE 2014)

8. ForanAMsystem theinstantaneous values ofcarrierand modulated signal are60sinwct and 40sinwct respectively.Determinethemodulationindex(MAY/JUNE 2014)

10. How manyAMbroadcaststationscan be accommodated in a 100 kHzbandwidthifthehighestfrequencymodulatingacarrieris 5 kHz?(April/May2010)

11. What arethe causes oflineardistortion?(April/May2010)

12. How manyAMbroadcaststationscan be accommodated in a 100 kHzbandwidthifthehighestfrequencymodulatingacarrieri s5kHz?(Nov/Dec2010)

13. Statethe applications of FDM.((NOV/DEC2010)

14. What arethevestiglesideband(April/May2011)

15.Calculatethe local oscillator frequencyifincomingfrequencyisF1 andtranslatedcarrierfrequency isF2(April/May2011)

16. Definemodulation?

17. What arethetypesofanalogmodulation?

18. What arethe advantages ofVSB-AM?

19. Comparelinear and non-linearmodulators.

20. How willyougeneratingDSBSC-AM ?

21. Definedemodulation.

22. drawtheblock diagram of coherent detector.

23. Definemultiplexing.

24. Definesensitivity.

25.Defineselectivity.

26. Draw the AM

27. Draw the AM modulated wave for modulation index = 0.5 and its power spectra (May/June2015)

28. Define Heterodyning.(May/June2015)

29.Do the modulation techniques decide the antenna height? (APR/MAY 2017)

30.Define carrier swing. (APR/MAY 2017)

31.What theorem is used to calculate the average power of a periodic signal gp(t)?State the theorem.(MAY /JUNE 2016)

32.What is pre envelope and complex envelope. .(MAY /JUNE 2016)

33.Suggest a modulation scheme for the broad cast video transmission and justify. .(NOV /DEC 2016)

34.What are the advantages of converting low frequency signal in to high frequency signal? ( NOV /DEC 2016)

35.What is the advantage of conventional DSB-AM over DSB-SC and SSB-SC AM? (NOV/DEC 2015)

36.Draw the block diagram of SSB-AM generator. (NOV/DEC 2015)

pare and contrast DSB-SC and SSB-SC with respect to i)power ii)Badwidth.(Apr/May 2018)

38.Mention the drawbacks of Coherent detection.(Apr/May 2018)

PART B

1(i).Draw an envelopedetectorcircuitusedfordemodulationofAMandexplain its operation(May/June2012)

2.HowSSBcanbegeneratorusingWeaversmethod?Illustratewithaneat block diagram(May/june2012)

3.What is frequencydivisionmultiplexing?Explain((May/June2012)

4. Compar various AmplitudemodulationSystem(May/june2012)

5. DefineAmplitudemodulation .how an amplitudemodulated signal canbegeneratedusinganonlinear modulated circuit(Nov/Dec2012)

6. What is DSB_SC signal?Writetheworkingofasynchronous detector used to detect theDSB_SC signal with thetheoutputamplitudespectrumofeach block(Nov/Dec2012)

7. Discussin details about the frequencytranslationandfrequencydivisionmultiplexingtechniquewith neat diagram(Nov/Dec2012)

8. CompareAmplitudeModulationand frequencyModulation(Nov/Dec2012)

9. Discuss on thefrequencycomponents present in aperiodic and non periodicsignal?Derivethe equation of an

Am wave.Also draw themodulated Am waveforvarious modulation index(May/june2013)

10. The antennacurrentof an Am transmitteris 8amperewhen onlythecarrier is sent the current increaseto8.93

A when thecarrieris modulated byasinglesinewave.Findthepercentagemodulation(May/June2013)

11. Draw theVSBspectrumand explain thesignificance(May/June2013)

12. How doyoudemodulateAMsignal?Explain(May/June2013)

13. A1000KHZcarrierissimultaneouslyAM modulated with 300HZ, 800HZand 1.5 KHzaudio sine wave. What willbethefrequencypresent in theoutput((May/June2013)

14. Explain theneedforcarriersuppression in anAMsystem. Draw andexplain the function ofonesuchsystem

(May/June2014)

15. Explain the workingofaAMtransmitterandthat ofareceiverwithasuitableblock schematic

(May/june2013)

16. With thehelpofneat diagram, explain of an envelopedetector?Whydoes negativeclippingtakeplace

(April/May2011)

17. (i)Explain with suitable diagram the generation of AM using square law method. Also derive its efficiency.

(ii) Explain the demodulation of AM using envelope detection.(May/June2015)

18. (i)Explain with block diagram the super heterodyne receiver.

(ii) Explain the Hilbert Transform with an example.(May/June2015)

19.(i)Using the concept of Hilbert transform Generate SSB-SC usingPhase shift method..(Apr/May 2018)

(ii)using suitable circuit explain the operation of envelop ment the reason for diagonal clipping suggest the necessary condition and expression to overcome the same.

20.(i).defend the need for VSB modulation technique in TV broadcasting.Also sketch its frequency spectra.

(April/May 2018)

(ii)with the neat block diagram,elaborate the working principle of AM superhetrodynereceiver.Also highlight

the drawback of TRP receiver with respect to sensitivity.

UNIT IIANGLEMODULATION

PART A

1.Illustratethe relationship betweenFMand PM, with block diagram(May/June2012)

2. What is meant bydetection?NamethemethodfordetectingFM signal (May/June2012)

3. How is narrow band Fmconverted into thewidebandFM?(Nov/Dec2012)

4. Acarrierisfrequencymodulatedbyasinusoidal modulatingfrequency2KHZ.(Nov/Dec2012)

5. Resultinginfrequencydeviation of5KHZwhatis thebandwidth occupied bythemodulatedwaveform

(Nov/Dec2012)

6. DefinethemodulationindexofFM(May/June2013)

7. What is theneedforpreEmphasis(May/June2013)

8. DefineWhitenoise?(May/June2014)

9.Ifthemaximum phase deviationinaphasemodulationsystemwhenamodulatingsignal of an 10v is applied is0.1 radian determine thevalueofphasedeviation(May /june2014)

10. What is mean bydetection?NamethemethoddetectingFM signal?(April/may2011)

11. What is RayrelayandRicianmethod(April/May2011)

12 draw theblock diagram of coherent detector.

13. Definemultiplexing

14. Definesensitivity.

15. Defineselectivity.

16. Definestability.

17. Definesuperheterodyneprinciple.

18. What arethedrawbacks of emittermodulator?

19. Definefrequencymodulation

20. Definemodulationindexoffrequencymodulation

21. What doyou meant bymultitonemodulation?

22. Definephasemodulation.

23. How FM wave can be converted to PM wave?

24. How PM wave can be converted to FM wave?

25. What arethetypesofFrequencyModulation?

26. Define Lock in Range and dynamic range of a PLL.(May/June2015)

27. A Carrier is frequency modulated with a sinusoidal signal of 2 Khz resulting in a maximum frequency deviation of 5 Khz. Find the bandwidth of the modulated signal.(May/June2015)

28.State the Carso’s rule. (APR/MAY 2017)

29.Distinguish the features of Amplitude modulation (AM) and Narrow band frequency modulation(NBFM) (APR/MAY 2017)

30.A carrier signal is frequency modulated by a sinusoidal signal of 5Vpp and 10 kHz. If the frequency deviation constant is 1 kHz/V,determine the maximum frequency deviation and state whether the scheme is narrow band or wide band FM. .(MAY /JUNE 2016)

31..What is the need for for pre-emphasis? .(MAY /JUNE 2016)

32..Define modulation index of frequency modulation and phase modulation. .(NOV /DEC 2016)

33.What is the need for pre-emphasis? .(NOV /DEC 2016)

pare amplitude and angle modulation schemes. (NOV/DEC 2015)

35.Write the carson’s rule . (NOV/DEC 2015)

36.Differentiate narrowband FM and AM technique. (Apr/May 2018)

37.What is the need for limiter circuits in FM systems. (Apr/May 2018)

PART-B

1.Fig.shows the block diagram of WBFM modulatorused to transmit audiosignalcontainingfrequencyintherange of100HZto 15HZ.thedesired FM signal frequencies in the rangeof100mhzand aminimum frequencies

deviation of75hzAssumethe modulation index,(May/June2012)

2. Draw thecircuitdiagram of foster SeeleyDiscriminatorand explain itsworkingprinciplewithrelevant Phasor

Diagram.(may/june2012)

3. DerivetheexpressionforwidebandFM in terms of Besselfunctions(Nov/Dec2012)

4. How canFMbederivedfrom PM and viceverse?Explain in details(Nov/Dec2012)

5. Explain anytwomethods usedfor FM detection, with neat Sketches(Nov/Dec2012)

6. Derivethemathematical representation ofFMsignal (May/June2013)

7. When thefrequencyinanFM system is400Hzand themodulatingvoltageis2.4V.the modulation indexis60.calculatethe maximum deviation.Whatisthemodulatingindexwhenthemodulatingfrequencyisreduced to250Hzand themodulatingvoltageissimultaneously(May/June2013)

8. Explain the Armstrongmethod to generateFMsignal(May/June2013)

9. How is thephaseandfrequencymodulationarerelated?Explain(May/June2013)

10. Explain the amstrongmethodofFMSignal(May/june2014)

11. Explain the functionofanyFMdetectorcircuit(May/June2014)

12. Explain how FM is achieved usingvaractordiodes(May/June2014)

13. MaketheatleastyfivecomparisonsofAMandFM system(May/June2014)

14. Derivethesingletonefrequencymodulationand draw itis frequencyresponse(may/june2011)

15. An anglemodulatedwaveis described bytheequationV(t)=10Cos(2*10t+10Cos200t)

Find1. Powerofthemodulated signal2.Maximum frequencydeviation3.Band width

16. (i)Explain with diagrams the generation of FM using direct method.

(ii) With the phasorrepresentation explain the foster seelay discriminator.(May/June2015)

17. An angle modulated signal is described byXc(t)=10 cos [2π(106)t]+0.1sin(103)πt]

(i) Considering Xc(t) as aPM signal with kp=10, find m(t)

(ii) Considering Xc(t) as a FM signal with kp=10π, find m(t)(May/June2015)

UNIT IIIRANDOMPROCESS

PART A

1. Definearandom variable. Specifythesamplespace and the random variable fora coin tossingexperiment

(may/June2012)

2. Givethedefinitionofnoiseequivalent temperature(May/June2012)

3. Definearandom variable. Specifythesamplespace and the random variable fora coin tossingexperiment

(Nov/Dec2012)

4. Calculate thermal noisevoltageacrossthesimpleRCcircuit,(Nov/Dec2012)

5. Definewhitenoise(May/June2013)

6. Definenoisefigure(May/June2013)

7. statetheShannon’s theorem(May/June2014)

8. State theneedforpre-emphasis and de-emphasis circuitin the field of communication(May/june2014)

9. What is thebasicdifferencebetweenanAMsignal and anarrowbandFM signal?

10. What arethetwomethods ofproducinganFMwave?

11. CompareWBFMandNBFM.

12.ListthepropertiesoftheBesselfunction.

13. Givetheaveragepowerof an FM signal.

14. Definephasedeviation.

15. DefinefrequencyDeviation.

16. State theCarson’srule.

17. Definethedeviation ratio D fornon-sinusoidal modulation.

18. What is theuseofcrystalcontrolled oscillator?

19. .What arethedisadvantagesofFMsystem?

20. How willyougeneratemessage from frequency-modulated signals?

21. What arethetypesofFM detectors?

22. What arethetypesofphasediscriminator?

23. What arethedisadvantagesofbalancedslopedetector?

24. Defineprobability.

25. Defineprobabilitydensityfunction.

26. Define Q factor of a receiver.(May/June2015)

27. Write the equation for the mean square value of thermal noise voltage in a resistor.(May/June2015)

28.List the necessary and sufficient conditions for the process to be WSS. (APR/MAY 2017)

29.State Wiener Khintchine theorem. (APR/MAY 2017)

30.State Central Limit Theorem. .(MAY /JUNE 2016)

31.Define Auto correlation function. .(MAY /JUNE 2016)

32.State Central Limit theorem. .(NOV /DEC 2016)

33.Write Einstein – Wiener – Khintchine relation. .(NOV /DEC 2016)

34.Defie random variable. (NOV/DEC 2015)

35..StateBaye’s rule. (NOV/DEC 2015)

36.Define the Q factor of a receiver. .(APR/MAY 2015)

37.Write the equation for the mean square of thermal noise voltage in a resistor. .(APR/MAY 2015)

38.Give the mathematical definition for random process. (Apr/May 2018)

39.What is narrowband Noise.(Apr/May 2018)

PART B

1.Listthedifferenttypesof random process andgivethe definition(May/june2012)

2. Write short notes on shot noise(May /june2012)

3. Writethe definition, powerspectraldensityandautocorrelationfunctionforwhitenoiseand narrow band noise(May/June2012)

4. What causes thermal noiseinamaterial?Writethe expression forRMSvalueofthenoise

5. Derivetheexpressionforshot noise voltage(Nov/Dec2012)

6.Givethepropertiesofautocorrelation function(Nov/Dec2012)

7. A mixerstagehasanoisefigof20 dband this isprecededbyanamplifier that has anoisefigof9db and an amplifiergain of15db.calculatethe overallnoisefigurereferred to theinput(Nov/Dec2012)

8.Areceiverhasanoisefig of12dbanditisfadebyallownoiseamplifierthathasagainof50dbandatemperature of90k.Calculatethenoisetemp.ofthereceiverandtheoverallnoisetempnofthereceiving systemtakeroomtemp is290K(Nov/Dec2012)

9. writeshort noise and thermal noise(may/june2013)

10. Derivetherelationship between noise figandequivalentnoisetemp(may/june2013)

11. Explain the followingterms mean correlationcovariance,ergodicity(may/june2013)

12. How doyou represent narrowband noise(may/june2013)

13. Define and explain the following:(may/june2014)

i. Gaussian noise and Gaussian distribution ii. Thermal noiseiii. Shot noise

14. What typeofGaussian noise follow

15. (i)Give arandomprocess, X(t)=Acos(!t+µ), whereAand !are constants and µ is auniform random variable. Show that X(t)is ergodicinbothmeanand autocorrelation

(ii)Writeashortnoteonshot noise and also explain aboutpowerspectraldensityofshot noise.(April/may2010)

16. (i)Define Noise. Explain the various types of internal noise.

(ii) Explain with derivation the effect of noise in cascaded amplifier circuit.(May/June2015)

17. Derive the SNR performance of DSB system and the AM system. Also prove that the output SNR in AM is at least 3 dB worse than that of DSB system.(May/June2015)

UNIT IVNOISE CHARACTERIZATION

PART A

1.Whatarethe characteristics ofsuperheterodynereceivers?

2. What arethemethodsto improve FM thresholdreduction?

3. comparethenoiseperformanceofDSBSCreceiverusingcoherent detection with AM receiverusingenvelope detection?(april/may 2011)

4.whatarethemethods to improve FM threshold reduction?(april/may 2011)

5.definethresholdeffectin AM receiver?(Nov/Dec2011)

6.what is FM threshold effect?(Nov/Dec2011)

7.whatarethe characteristics ofsuperheterodynereceivers?

8.whatarethemethods to improve FM threshold reduction?

9. definethresholdeffectinFM receiver?

10.definepre-emphasisand de-emphasis.(april/may 2010)

11.draw thecircuitdiagramofpre-emphasis filter?(april/may 2010)

12.what is meant bycapture effect?(Nov/Dec2010)

13.what is thesignificanceofpre-emphasis andde-emphasiscircuit?(Nov/Dec2010)

14.what is coherent system?(may/june2013)

15.what is carson’srule?(may/june2013)

16.WhatarethedisadvantagesofFMsystem?

17.Howwillyougeneratemessage from frequency-modulated signals?

18.WhatarethetypesofFM detectors?

19.Whatarethetypesofphasediscriminator?

20.Whatarethedisadvantagesofbalanced slope detector?

21.Defineprobabilitydensityfunction.

22.Definenoise.

23.Givetheclassificationof noise.

24.WhatarethetypesofExternal noise?

25.Whataretypesofinternal noise?

26.Whatarethetypesofextraterrestrial noise andwritetheir origin?

27. What is pre-emphasis? Why it is needed?(May/June2015)

28. Define threshold effect in AM systems.(May/June2015)

29.Specify the cause of threshold effect in AM systems. (APR/MAY 2017)

ment the of pre-emphasis and de-emphasis circuit in SNR improvement. (APR/MAY 2017)

31.Give the definition of noise equivalent temperature. .(MAY /JUNE 2016)

32.Define Capture effect. .(MAY /JUNE 2016)

33.Two resistors of 20 k, 50 k are at room temperature (290k).For a bandwidth of 100 kHz. Calculate the thermal noise voltage generated by two resistors in series. .(NOV /DEC 2016)

34.Define noise figure and noise equivalent temperature. .(NOV /DEC 2016)

35.Define noise figure. (NOV/DEC 2015)

36.Wha is threshold effect? (NOV/DEC 2015)

37.What is preemphasis?Why it is needed? .(APR/MAY 2015)

38.Define threshold effect in Am systems. .(APR/MAY 2015)

39.Define the reason why SNR of receiver should be high? (Apr/May 2018)

40.How Pre emphasis and de emphasis provide overall SNR improvement inFM Systems.(Apr/May 2018)

PARTB

1. DeterminetherangeoftuningofalocaloscillatorofasuperHeterodynereceiverwhenflo>fc.thebroadcast frequencyrangeis 542Hzto 1600hz

Assumeif=455Khz(May/June2012)

2. What is captureeffectinFM(May/June2012)

3. ComparethenoiseperformanceofDSBSCreceiverusingcoherentdetection with AM receiver usingenvelopedetection(Nov/Dec2012)

4.Definepre-emphasisand de emphasis(Nov/Dec2012)

5.Drawthesuperheterodynereceiverand explain theoperationofeach block(may/june2013)

6.Derivethefigureofmeritfor non coherentsystem with suitable assumption(may/june2013)

7. Derivethefigureofmerit of aFMsystem(may/june2013)

8. Explain FM threshold(may/june2013)

9. Explain the advantages oftheusageofsuperheterodynereceivermay(may/june2014)

10. Explain the envelopedetection withasuitablediag(may/june2014)

11. Express method of coherent detection(may/june2014)

12. Compare at leastthreeimportantcharacteristics ofvariousfmsystems(may/june2014)

13. DeriveanexpressionforSNRat input (SNRc)andoutputof (SNRo)ofa

coherentdetector

14. Explain pre-emphasisandDe-emphasisin detail.(April/May2010)

15. ComparetheperformancesofAMandFMsystems(April/May2010)

16. (i) Let X and Y be real random variables with finite second moments. Prove the Cauchy-Schwarz inequality. (E[XY]) 21)

5. What is the need for modulation?

Needs for modulation:

a) Ease of transmission , b) Multiplexing ,c) Reduced noise, d) Narrow bandwidth ,e) Frequency assignment, f) Reduce the equipments limitations.

6. Give the Classification of Modulation.

There are two types of modulation. They are a)Analog modulation b)Digital modulation

Analog modulation is classified as follows (i)Continuous wave modulation (ii)Pulse modulation

Continuous wave modulation is classified as follows

(i)Amplitude modulation (ii)Double side band suppressed carrier (iii)Single side band suppressed carrier (iv)Vestigial side band suppressed carrier

Angle modulation

(i)Frequency modulation (ii)Phase modulation

Pulse modulation is classified as follows

(i)Pulse amplitude modulation (ii)Pulse position modulation (iii)Pulse duration modulation (iv)Pulse code modulation

Digital modulation is classified as follows (i) Amplitude shift keying (ii)Phase shift keying (iii) Frequency shift keying.

7. What is the difference between high level and low level modulation?

In high level modulation, the modulator amplifier operates at high power levels and delivers power directly to the antenna. In low level modulation, the modulator amplifier performs modulation at relatively low power levels. The modulated signal is then amplified to high power level by class B power amplifier. The amplifier feeds power to antenna.

8. Define Detection.

Detection is the process of extracting modulating signal from the modulated carrier. Different types of detectors are used for different types of modulations.

9. Define Amplitude Modulation.

In amplitude modulation, the amplitude of a carrier signal is varied according to variations in amplitude of modulating signal.

The AM signal can be represented mathematically as, eAM = (Ec + Em sinωmt ) sinωct. and the modulation index is given as,m = Em EC

10. What is Super Heterodyne Receiver?

The super heterodyne receiver converts all incoming RF frequencies to a fixed lower frequency, called intermediate frequency (IF). This IF is then amplitude and detected to get the original signal.

11. What is single tone and multi tone modulation?

If modulation is performed for a message signal with more than one frequency component then the modulation is called multi tone modulation. If modulation is performed for a message signal with one frequency component then the modulation is called single tone modulation.

12. Compare AM with DSB-SC and SSB-SC.

13. What are the advantages of VSB-AM?

1. It has bandwidth greater than SSB but less than DSB system.

2. Power transmission greater than DSB but less than SSB system.

3. No low frequency component lost. Hence it avoids phase distortion.

14. How will you generating DSBSC-AM?

There are two ways of generating DSBSC-AM such as a).Balanced modulator b).Ring modulators

15. What are advantages of ring modulator?

a).Its output is stable.

b). It requires no external power source to activate the diodes. c).Virtually no maintenance.

d). Long life.

16. Define Demodulation.

Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation. The devices used for demodulation or detection are called demodulators or detectors. For amplitude modulation, detectors or demodulators are categorized as, a) Square-law detectors b) Envelope detectors

17. Define Multiplexing.

Multiplexing is defined as the process of transmitting several message signals Simultaneously, over a single channel.

18. Define SSB-SC.

(i) SSB-SC stands for Single Side Band Suppressed Carrier

(ii) When only one sideband is transmitted, the modulation is referred to as Single side band modulation. It is also called as SSB or SSB-SC.

19. Define DSB-SC.

After modulation, the process of transmitting the sidebands (USB, LSB) alone and suppressing the carrier is called as Double Side Band-Suppressed Carrier.

20. What are the disadvantages of DSB-FC?

(i) Power wastage takes place in DSB-FC

(ii) DSB-FC is bandwidth inefficient system.

21. Define Coherent Detection.

During Demodulation carrier is exactly coherent or synchronized in both the frequency and phase, with the original carrier wave used to generate the DSB-SC wave. This method of detection is called as coherent detection or synchronous detection.

22. What is Vestigial Side Band Modulation?

Vestigial Sideband Modulation is defined as a modulation in which one of the sideband is partially suppressed and the vestige of the other sideband is transmitted to compensate for that suppression.

23. What are the advantages of signal sideband transmission?

a) Power consumption b) Bandwidth conservation c) Noise reduction

24. What are the disadvantages of single side band transmission?

a) Complex receivers: Single side band systems require more complex and expensive receivers than conventional AM transmission.

b) Tuning difficulties: Single side band receivers require more complex and precise tuning than conventional AM receivers.

25. Compare linear and non-linear modulators?

|S.No |Linear Modulators |Non Linear Modulators |

|1. |Heavy filtering is not required. |Heavy filtering is required. |

| | | |

|2. |These modulators are used in high level |These modulators are used in low level modulation. |

| |modulation. | |

| | |The modulating signal voltage is very much greater than the |

|3. |The carrier voltage is very much greater than |carrier signal voltage. |

| |modulating signal voltage. | |

|3. | | |

26. What is BW for AM wave?

The difference between these two extreme frequencies is equal to the bandwidth of the AM wave. Therefore, Bandwidth, B = (ωc + ωm) - (ωc - ωm) B = 2ωm

27. What is the BW of DSB-SC signal?

Bandwidth, B = (ωc + ωm) - (ωc - ωm) B = 2ω It is obvious that the bandwidth of DSB SC modulation is same as that of general AM waves.

28. What are the demodulation methods for DSB-SC signals?

The DSB-SC signal may be demodulated by following two methods: (i) Synchronous detection method.

(ii) Using envelope detector after carrier reinsertion.

29. Write the applications of Hilbert transform?

(i) For generation of SSB signals, (ii) For designing of minimum phase type filters, (iii) For representation of band pass signals.

30. What are the methods for generating SSB-SC signal?

SSB-SC signals may be generated by two methods as under:

(i)Frequency discrimination method or filter method. (ii)Phase discrimination method or phase-shift method.

UNIT-II ANGLE MODULATION

1. What do you understand by narrowband FM?

When the modulation index is less than 1, the angle modulated systems are called low index. The bandwidth requirement of low index systems is approximately twice of the modulating.

2. Define frequency modulation.

Frequency modulation is defined as the process by which the frequency of the carrier wave is varied in accordance with the instantaneous amplitude of the modulating or message signal.

3. Define modulation index of frequency modulation.

It is defined as the ratio of maximum frequency deviation to the modulating frequency

4. What do you meant by multitone modulation?

Modulation done for the message signal with more than one frequency component is called multitone modulation.

5. Define phase modulation.

Phase modulation is defined as the process of changing the phase of the carrier signal in accordance with the instantaneous amplitude of the message signal.

6. What are the types of Frequency Modulation?

Based on the modulation index FM can be divided into types. They are Narrow band FM and Wide band FM. If the modulation index is greater than one then it is wide band FM and if the modulation index is less than one then it is Narrow band FM

7. What is the basic difference between an AM signal and a narrowband FM signal?

In the case of sinusoidal modulation, the basic difference between an AM signal and a narrowband FM signal is that the algebraic sign of the lower side frequency in the narrow band FM is reversed.

8. What are the two methods of producing an FM wave?

Basically there are two methods of producing an FM wave. They are, i) Direct method: In this method the transmitter originates a wave whose frequency varies as function of the modulating source. It is used for the generation of NBFM

ii) Indirect method: In this method the transmitter originates a wave whose phase is a function of the modulation. Normally it is used for the generation of WBFM where WBFM is generated from NBFM

9. Compare WBFM and NBFM.

|S.NO |WBFM |NBFM |

| |Modulation index is greater |Modulation index less than 1 |

|1 |than 1 | |

|2 |Frequency deviation 75 KHz |Frequency deviation 5 KHz |

|3 |Bandwidth 15 times NBFM |Bandwidth 2fm |

|4 |Noise is more suppressed |Less suppressing of noise |

10. Give the average power of an FM signal.

The amplitude of the frequency modulated signal is constant .The power of the FM signal is same as that of the carrier power. P =1 2 E 2c

11. Define phase deviation.

The maximum phase deviation of the total angle from the carrier angle is called phase deviation.

12. Define frequency Deviation.

The maximum departure of the instantaneous frequency from the carrier frequency is called frequency deviation.

13. Define the deviation ratio D for non-sinusoidal modulation.

The deviation ratio D is defined as the ratio of the frequency deviation f, which Corresponds to the maximum possible amplitude of the modulation signal m (t), to the highest modulation frequency. D = ∆f/ f m

14. What is the use of crystal controlled oscillator?

The crystal-controlled oscillator always produces a constant carrier frequency there by enhancing frequency stability.

15. What are the disadvantages of FM system?

1. A much wider channel is required by FM.

2.FM transmitting and receiving equipments tend to be more complex and hence it is expensive.

16. How will you generate message from frequency-modulated signals?

First the frequency-modulated signals are converted into corresponding amplitude- modulated signal using frequency dependent circuits. Then the original signal is recovered from this AM signal.

17. What are the types of FM detectors?

The types of FM detectors are

(i) Slope detector and (ii) Phase discriminator.

18. What are the types of phase discriminator?

The types of phase discriminator are (i) Foster seeley discriminator and (ii) Ratio detector.

19. What are the disadvantages of balanced slope detector?

1. Amplitude limiting cannot be provided

2. Linearity is not sufficient

3. It is difficult to align because of three different frequency to which various tuned circuits to be tuned.

4. The tuned circuit is not purely band limited.

20. Write the advantages and disadvantages of foster-seely discrimination method?

Advantages:

a) It is much easier to design

b) Only two tuned circuits are necessary and they are tuned to same frequency c) Linearity is better

Disadvantages:

a) It requires Amplitude limiting circuit.

21. What are the applications of phase locked loop?

Phase locked loops are used for various purposes in AM and FM communication. (i)Automatic frequency correction in FM transmitter uses PLL to keep carrier frequency constant.

(ii)PLL is used direct FM Tramitter uses PLL to keep carrier frequency constant. (iii)PLL is also used in FM demodulators.

22. A 80 MHz carrier is frequency modulated by a sinusoidal signal of 1V amplitude and the frequency sensitivity is 100 Hz/V. Find the approximate bandwidth of the FM waveform if the modulating signal has a frequency of 10 kHz.

Ans: Frequency Sensitivity = 100 Hz/ volt. Amplitude of modulating signal = 1V

Hence maximum frequency deviation, δ =100 Hz / volt ×1V= 100 kHz

Frequency of modulating signal, fm = 10kHz

∴ BW = 2 [δ + f m (max)]

= 2 [100 +10×103]

BW = 20.2 kHz

UNIT-III RANDOM PROCESS

1. Define noise.

Noise is defined as any unwanted form of energy, which tends to interfere with proper reception and reproduction of wanted signal.

2. Give the classification of noise.

Noise is broadly classified into two types. They are (i)External noise (ii)Internal noise.

3. What are the types of External noise?

External noise can be classified into

1. Atmospheric noise 2. Extraterrestrial noises 3. Man –made noises or industrial noises

4. What are types of internal noise?

Internal noise can be classified into

1. Thermal noise 2. Shot noise 3. Transit time noise 4. Miscellaneous internal noise

5. What are the types of extraterrestrial noise and write their origin?

The two type of extraterrestrial noise are solar noise and cosmic noise Solar noise is the electrical noise emanating from the sun.Cosmic noise is the noise received from the center part of our galaxy, other distant galaxies and other virtual point sources.

6. Define transit time of a transistor.

Transit time is defined as the time taken by the electron to travel from emitter to the collector.

7. Define flicker noise.

Flicker noise is the one appearing in transistors operating at low audio frequencies. Flicker noise is proportional to the emitter current and junction temperature and inversely proportional to the frequency.

8. State the reasons for higher noise in mixers.

1. Conversion transconductance of mixers is much lower than the transconductance of amplifiers.

2. If image frequency rejection is inadequate, the noise associated with the image frequency also gets accepted.

9. Define signal to noise ratio.

Signal to noise ratio is the ratio of signal power to the noise power at the same point in a system.

10. Define noise figure.

Noise figure (NF) and noise factor (F) are measures of degradation of the signal-to-noise ratio (SNR), caused by components in a radio frequency (RF) signal chain. It is a number by which the performance of an amplifier or a radio receiver can be specified, with lower values indicating better performance.

11. Define thermal noise. Give the expression for the thermal noise voltage across a resistor.

The electrons in a conductor possess varying amounts of energy. A small fluctuation in this energy produces small noise voltages in the conductor. These random fluctuations produced by thermal agitation of the electrons is called thermal noise.

12. Define noise temperature. (In terms of hypothetical temperature)

The available noise power is directly proportional to temperature and it is independent of value of resistance. This power specified in terms of temperature is called as noise temperature. It is denoted by Te . It is given as,Te = (F −1 )T

13. What is shot noise?

When current flows in electronic device, the fluctuations number of electrons or holes generates the noise. It is called shot noise. Shot noise also depends upon operating conditions of the device.

14. Give the expression for noise voltage in a resistor.

The Mean –Square value of thermal noise voltage is given by, Vn 2 = 4 k TBR K – Boltz man constant, R – Resistance T – Obsolute temperature, B Bandwidth

15. What is White Noise?

Many types of noise sources are Gaussian and have flat spectral density over a wide frequency range. Such spectrum has all frequency components in equal portion, and is therefore called white noise. The power spectral density of white noise is independent of the operating frequency.

16. What is narrowband noise?

The receiver of a communication system usually includes some provision for preprocessing the received signal. The preprocessing may take the form of a narrowband filter whose bandwidth is large enough to pass modulated component of the received signal essentially undistorted but not so large as to admit excessive noise through the receiver. The noise process appearing at the output of such filter is called narrow band noise.

17. Give the Friss formula in terms of noise temperature.

Friis' formula is used to calculate the overall noise figure of a receiver, which is in turn composed of a number of stages, each with its own noise figure and gain. It is given by

[pic]

18. Define noise equivalent bandwidth.

The noise equivalent bandwidth of the filter is defined as the bandwidth of an ideal filter at which the noise power passed by real filter and ideal filter is same.

19. Define noise factor.

Noise factor (F) is defined as the ratio of signal to noise power ratio at the input to signal to noise power ratio at the output

20. Give the characteristics of shot noise.

(i) Shot noise is generated due to fluctuations in the number of electrons or holes. (ii) Shot noise has uniform spectral density.

(iii) Mean square noise current depends upon direct component of current. (iv) Shot noise depends upon operating conditions of the device

UNIT-IV NOISE CHARACTERIZATION

1. What is FM threshold effect?

As the carrier to noise ratio is reduced, clicks are heard in the receiver output. As the carrier to noise ratio reduces further, crackling, or sputtering sound appears at the receiver output. Near the breaking point, the theoretically calculated output signal to noise ratio becomes large, but its actual value is very small. This phenomenon is called threshold effect.

2. What is capture effect in FM?

When the noise interference is stronger than FM signal, then FM receiver locks to interference. This suppresses FM signal. When the noise interference as well as FM signal are of equal strength, then the FM receiver locking fluctuates between them. This

phenomenon is called capture effect.

3. What is meant by figure of merit of a receiver?

The ratio of output signals to noise ratio to channel signal to noise ratio is called figure of merit.,

4. What is the Purpose of re-emphasis and de-emphasis in FM?

The PSD of noise at the output of FM receiver sally increases rapidly at high frequencies but the PSD of message signal falls off at higher frequencies. This means the message signal doesn’t utilize the frequency band in efficient manner. Such more efficient use of frequency band and improved noise performance can be obtained with the help of re-emphasis and de-emphasis.

5. What are extended threshold demodulators?

Threshold extension is also called threshold reduction. It is achieved with the help of FMFB demodulator. In the local oscillator is replaced by voltage controlled oscillator (VCO).The VC frequency changes as per low frequency variations of demodulated signal. Thus the receiver responds only to narrow band of noise centered around instantaneous carrier frequency. This reduces the threshold of FMFB receiver.

6. What is threshold effect with respect to noise?

When the carrier to noise ratio reduces below certain value, the message information is lost. The performance of the envelope detector deteriorates rapidly and it has no proportion with carrier to noise ratio. This is called threshold effect.

7. Define pre-emphasis and de-emphasis.

Pre-emphasis: It artificially emphasizes the high frequency components before modulation. This equalizes the low frequency and high frequency portions of the PSD and complete band is occupied.

De-emphasis: This circuit attenuates the high frequency components. The attenuation characteristic is exactly opposite to that of pre-emphasis circuit. De-emphasis restores the power distribution of the original signal.

The signal to noise ratio is improved because of pre-emphasis and de-emphasis circuits.

8. Define superheterodyne principle.

It can be defined as the process of operation of modulated waves to obtain similarly modulated waves of different frequency. This process uses a locally generated carrier wave, which determines the change of frequency.

9. Define signal to noise ratio.

Signal to noise ratio is the ratio of signal power to the noise power at the same point in a system.

10. What is threshold effect in an envelope detector? Explain.

When a noise is large compared to the signal at the input of the envelope detector, the detected output has a message signal completely mingled with noise. It means that if the input SNR is below a certain level, called threshold level, the noise dominates over the message signal, threshold is defined as value of the input signal to noise ratio (So/No) below which the output signal to noise ratio (Si/Ni) deteriorates much more rapidly than the input signal to noise ratio. The threshold effect in an envelope detector whenever the carrier power-to-noise power ratio approaches unity or less.

UNIT V SAMPLING AND QUANTIZATION

1. Define Nyquist rate.

Let the signal be bandlimited to „W‟ Hz. Then Nyquist rate is given as, Nyquist rate = 2W samples/sec Aliasing will not take place if sampling rate is greater than Nyquist rate

2. What is meant by aliasing effect?

Aliasing .effect .takes .place .when .sampling .frequency .is .less .than .Nyquist .rate. Under .such .condition, .the .spectrum .of .the .sampled .signal .overlaps .with .itself. Hence higher frequencies take the form of lower frequencies. This interference of the frequency components is called as aliasing effect.

3. Define PWM.

PWM is basically pulse width modulation. Width of the pulse changes according to amplitude of the modulating signal. It also referred as pulse duration modulation or PDM.

4. . State Sampling theorem.

A bandlimited signal of finite energy, which has no frequency components higher than W Hz, may be completely recovered from the knowledge of its samples taken at the rate of 2W samples per second.

5. How the message can be recovered from PAM?

The .message .can .be .recovered .from .PAM .by .passing .the .PAM .signal .through reconstruction filter integrates amplitude of PAM pulses. Amplitude reconstruction signal is done to remove amplitude discontinuities due to pulses.

6. . Write an expression for bandwidth of binary PCM with N messages each with a maximum frequency of fm Hz.

If „v‟ number of bits are used to code each input sample, then bandwidth of PCM is given as, BT ≥ N.v.fm Here v. fm is the bandwidth required by one message.

7. How is PDM wave converted into PPM message?

The .PDM .is .signal .is .clock .signal .to .monostable .multivibrator. The .multivibraor triggers on falling edge. Hence a PPM pulse of fixed width is produced after falling edge of PDM pulse. PDM represents the input signal amplitude in the form of width of the pulse. A PPM pulse is produced after the width of PDM pulse. In other words, the position of the PPM pulse depends upon input signal amplitude.

8. Mention the use of adaptive quantizer in adaptive digital waveform co d ing schemes

Adaptive quantizer changes its .step .size .according variance of .the .input signal. Hence quantization error is significantly reduced due to the adaptive quantization. ADPCM uses adaptive quantization. The bit rate of such schemes is reduced due to adaptive quantization.

9 What do u understand from adaptive coding?

In adaptive coding, the quantization step size and prediction filter coefficients are changed as per properties of input signal. This reduces the quantization error and number of bits to represent the sample value. Adaptive coding is used for speech coding at low bits rates.

9. What is meant by quantization?

While converting the signal value from analog to digital, quantization is performed. The .analog value is assigned to nearest digital value. This is called quantization. The . quantized .value .is .then .converted .into .equivalent .binary .value. .The quantization levels are fixed depending upon the number of bits. Quantization is performed in every Analog to Digital Conversion.

10. The signal to quantization noise ratio in a PCM system depends on what criteria?

The signal to quantisation noise ratio in PCM is given as, (S/N)db ≤(4.8+6v)dB Here v is the number of bits used to represent samples in PCM. Hence signal to quantization noise ratio in PCM depends upon the number of bits or quantization levels

-----------------------

Expander

Compressor

Uniform Quantizer

|S.No |AM signal |DSB-SC |SSB-SC |

1Bandwidth = 2fmBandw彺 彻 悮 悯 拾 挀 擪 擬 擮 擰 擴 擶 擺 擼 攀 攂 攆 攈 攊 攌 攎 攐 攒 攔 攖 攘 攚 攜 攞 攠 攢 攤 攦 攪 攬 攸 攺 敂 敄 敖 敘 敮 数 斔 斖 斸 於 旈 旊 旐 ���틖틖틖틖샄쒼변변변샄삼꺼껼껼틼ꎧꎧꎧ覗覗—ᔛᅨ﹃ᘀ⑨㬱㔀脈血

|2 |Contains USB,LSB,Carrier |Contains USB,LSB |USB,LSB |

|3 |More Power is required for |Power required is less than |Power required is less than |

| |transmission |that of AM. | |

| | | |AM &DSB-SC |

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