questions in analog digital comm

7/29/2019 Questions in Analog Digital Comm

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Q. 1. What are Analog modulation techniques?

Ans. The Analog modulation techniques are:1. Amplitude modulation2. Angle modulation(a) Frequency modulation

(b) Phase modulation.

Q. 2. Define amplitude modulation.

Ans. This is defined as the modulation, in which amplitude of carrier ischanged in accordance to the amplitude of the modulating signal.

Q. 3. What are the limitations of amplitude modulation?


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standard AM technique used for broadcasting etc.A3A Telephony : Single side band with reduced carrier (SSBRC) , also

called pilot carrier system.A3H Telephony : Single side band with full carrier (SSBFC).A3J Telephony : Single side band with supressed carrier (SSBSC).A3B Telephony Independent side band (ISB).A5C Television Vestigial (trace of) side band with full carrier (VSBFC).A41AX : Fascimile (FAX) also called slow scan TV system.

A9B Telegraphy : Telephony or telegraphj with independent sidebands. Note : When a number digit preceeds a system! emissiondesignation, it indicates allowable BW in Kiloherz e.g.OA is a slow speed radio telegraph ON OFF emission BW is nearly zero.1A2 BW = 1KHzA3J BW = 3KHz

A5C BW=5KHz

A9B BW = 9 KHz and so on.

Q. 5. What do you know by angle modulation?


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Ans. It is possible to convey or transmit an information by varying itsfrequency as well as angle of phase. These are known as frequency

and phase modulations respectively and both cbllectively are known asAngle Modulation.

The frequency and phase modulation systems have similarcharacteristics with minor differences.Briefing we can say angle modulations of two types:(1) Frequency modulation

(ii) Phase modulationThe angle modulation has many advantages over amplitudemodulation such as noise reduction, improved system fidelity andmore efficient use of power.Angle modulation has some disadvantages also such as increased B.W.and need of more complex circuits.

Angle modulation has the following applications:

(i) Radio broadcasting(ii) Two way mobile radio(iii) Microwave communication(iv) TV sound transmission


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(v) Cellular radio(vi) Satellite communication.

Q. 6. What do you mean by single tone frequency modulation?

Ans. In this modulation, the modulating signal contains a singlefrequency and modulates a carrier i.e. frequency of the carrier is

varied by the signal containing a single frequency component. This isthe simplest case.

Q. 7. Write a note on Power of a FM wave.

Ans. Power of FM Wave : Though the frequency of FM wave varieswith time, the carrier amplitude remains constant. Therefore, it can beshown that average power of a FM wave remains always equal to thecarrier power. When modulation is applied, the total power of thecarrier is redistributed among all the components of the spectrum. At


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certain value of nz1, when the carrier component becomes zero, allthe power in carried by the side frequencies.

Q. 8. Write a note on BW of an FM wave.

Ans. Transmission BW of FM Wave : Theoretically a FM wave containsan infinite

number of side frequencies, the BW required to transmit such a signalshall be also infinite. But in practice, an FM have a finite number ofside frequencies so we can specify BW required for transmission of anFM wave.The number of significant sidebands produced in a FM wave may beobtained from the function J. m1. if n > ni1 the values of J,, m1 arenegligible. At 1, the


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number of significant sidebands Note that rn is frequency modulation factor,fTfl is. the modulatingfrequency, and 6 is frequency devition. Thus we can say that BW of aFM is twice the frequency deviation, when 1.

Q. 9. Explain phase modulation obtained from frequency

modulation.

Ans. Generation of PM Signal from frequency Modulation : (Figure 1)It can be mathematically verified that, if the modulating signal isdifferentiated before it is applied to a frequency modulator, the output


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is a PM signal. In a frequency modulator, the instantaneous frequency of themodulated signal varies according to the signal applied at its input. Inthis case, the instantaneous frequency will vary as per differential ofthe modulating signal that is, the integral of instantaneous frequencywill vary according to the modulating signal. In other words, in

instantaneous phase is modulated in accordance to the modulatingsignal, the output therefore, is a PM signal.

Q. 10. Explain frequency modulation obtained from phase

modulation.

Ans. Generation of FM signal from phase modulation: It can bemathematically verified that if the modulating signal is integrated,


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before it is fed to a phase modulator the output is a FM signal.

In Figure 2 the input to the phase modulator is the integral of themodulating signal. The instantaneous phase of the modulated signalvaries in accordance with the modulating signal. In other words,derivative of the instantaneous phase varies as per the modulatingsignal. Since the time rate of change of phase is frequency, we can saythat the instantaneous frequency varies as per the modulating signal,which implies that output is an FM signal.

Q. 11. Describe designations of various frequency or phasemodulation system emissions.

Ans. The various FM/PM systems or emissions are:Fl : Telegraphy : Frequency shift keyedF2 : Telegraphy, on off, frequency modulated tone.


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F3 : Telephony or phase modulatedF4 : FAX

F5 : TelevisionF6 . : Telegraphy four frequency diplex.

Q. 12. Draw the block diagram for generation of PM signalusing FM modulator and FM signal using PM modulator.

Ans.

Q. 13. What is the maximum value of sideband power in AM?

Ans. 33% in both side bands.


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Q. 14. Why are frequencies allotted for AM broadcasting

smaller than that for FM broadcasting?

Ans. This is due to difference in Bandwidths.

Q. 15. What are different methods of generating FM?

Ans. The methods of FM generation are:(1) Direct methods(ii) Indirect methods.

Q. 16. Why is higher percentage of modulation desirable in AM?


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Ans. In AM, 100% modulation is ideal. Below 100% causes

undermodulation and above 100% causes overmodulatiorL bothleading to distortion.

Q. 17. Define frequency modulation. Derive the expression for

instantaneous amplitude of FM wave. Define modulation index.

Ans. When frequency of a carrier is varied as per modulatingfrequency (amplitude and phase remaining constant) the process iscalled frequency modulation The modulation index for FM is the ratioof maximum frequency deviation to the modulating frequencyFor derivation of FM wave Refer Chapter 2, Q

Q. 18. Define Modulation Index of AM.


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Ans. Modulation Index of AM.: Is defined as measure of extent ofamplitude variation about an unmodulated maximum carrier,

Q. 19. Define Deviation ratio of FM.

Ans. Deviation of FM : The maximum change in the instantaneousfrequency from the average carrier frequency is called frequency

deviation

Q. 20. Define Narrow banding.

Ans. Narrow banding : When the value of modulation index is small,the BW of the modulated wave is less or, narrow. This type of


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modulation system is called narrow band system. We have narrowband FM and narrow band PM system.

Q. 21. What are the different methods of frequency modulationExplain in detail Armstrong method.

Ans. The methods of frequency modulation are :(i) Narrow band FM(ii) Wide band FM.For Armstrong modulation. Refer Chapter 5,

Q. 22. For what the Narrow band FM signal is used for

Ans. The NBFM is used for FM mobile communication such as policewireless, ambulance etc. (for speech transmission).


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Q. 23. (a) Define % Modulation Index of a AM.

Ans. Percent modulation index of AM. The ratio of maximum value ofthe modulating signal to the maximum value of the carrier multiplied

by 100 is called percent modulation index

(b) What are limitations of AM?Ans. Limitations of AM:(i) Power of carrier and of one side band is useless.(ii) The AM reception is noisy.(iii) The BW is much less.(iv) Only two S.Bs are available.

Q. 24. (a) What is the frequency deviation and carrier swing.

What value of there is necessary to provide 75% modulation in

the FM broadcast band?


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Ans.(a)Change or shift either above or below the centre frequency(fe) is called Frequency deviation.The total variation of frequency from the lowest to highest point iscalled carrier swing.

For FM broadcast band, for 75% FM, the frequency deviationnecessary is 75 kHz.

The swing required is 2 x 75 150 kHz.

(b) Repeat above Q. for a FM signal serving as the audio portion of aTV broadcast.Ans. For sound portion of TV broadcast, the frequency deviationnecessary is 25 kHz and the swing required is 2 x 25 = 50 kHz. (c). What is the cause of diagonal clipping in a diode detector? Ans.The diagonal clipping results in diode detectors because ac load

impedance becomes reactive at higher audio frequencies.


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SHORT & LONG ANSWER TYPE QUESTIONS

Part 1 : Amplitude Modulation

Q. 25. Explain Amplitude modulation (AM) with the help of aneat sketch.

Ans. In amplitude modulation, amplitude of the carrier wave ischanged according to the amplitude of the signal. The technique isvery much used in the transmission of radio signals.4 shows the process of amplitude modulation.


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Note that

Only amplitude of the carrier is varied while its frequency and phaseremain unchanged.Whenthere is no signal, the amplitude of the carrier is equal to theunmodulated amplitude. When signal is present, the amplitude of thecarrier changes in accordance with the instantaneous value of the


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signal.During positive cycle of the signal, the amplitude of the carrier

increases to the sum of the amplitudes of the carrier and the signal (E+ Em)(iv) During negative cycle of the signal, the amplitude of the carrierdecreases and becomes equal to the difference of the amplitudes ofthe carrier and the signal.

Q. 26. Explain theory of A.M. OR provide analysis of A.M.

(E E).


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Q. 27. Define modulator factor/Index/depth of modulation.Derive expression of AM wave is displaced on a CR0. Explain its

significance.


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Ans.(a) Modulation Factor/Index (m) : The modulation factor/index,(m) can be defined in the following ways.

1. It is the ratio of maximum value of the signal to the maximum valueof the carrier, i.e.,2.m Em/Eci or = mEIt is the ratio of the change in the amplitude of the carrier to itsoriginal amplitude, i.e.,

4. It is the ratio of minimum amplitude to the maximum amplitude tothe modulated (ratio) wave as shown below.(b) If modulation curve is displayed on a cathode ray oscilloscope(CR0), we get the curve as shown in figure 6. The modulation index mcan be calculated as follow:m AEC/EC3. It is the percentage change in the amplitude of the carrier, i.e.,m = 1LEC/EC)< 100Hence m can be expressed as the ratio of minimum amplitude to themaximum amplitude of the radio wave.Note:


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1. The vkof m lies between 0 and 1.2. T){value of m depends upon the amplitudes of the signal as well as

of the carrier.Significance of m (Table 1)The modulation factor m plays a very important role in the modulationprocess. This will be made clear by calculating the values of m fordifferent amplitudes of the signal and the carrier.1. Let the amplitude of signal be zero (i.e. signal is not present) and

amplitude of carrier is E. In this case, the amplitude of modulatedwave 0 +E = E.Change in the carrier amplitude E - E = 0. Modulation index 0/Er = 0(No modulation)2. Let the amplitude of carrier = E and amplitude of signal = E/2.


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Hence m depends on the amplitude of both the signal and the carrier.

The value of m decides the strength of the modulated wave and hencethat of the signal. When rn = 1(100%) the signal will be strongest,perfect and clear. In the case of overmodulation (m = 150%) themodulated wave will be clipped..off arid huge distortion will occur inthe reception. Hence, the ideal value of modulation is I or 100%.

Q. 28. Draw and explain frequency spectrum of AM wave.

Ans. The equation of A.M. wave is given by.

Note that the equation has three parts:First part is an unmodulated carrier wave, which remains unchanged inthe process.The maximum amplitude is E.Second part has a maximum amplitude of mE/2 and its frequency is


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equal to the difference of carrier and the signal frequencies. This iscalled lower side band (L.S.B.). Recall that angular velocity of the

carrier o 2rf, where f is the frequency of the carrier. Similarly tOrn =fmThird part has also max. amplitude of mE/2 and frequency equal to thesum of carrier and signal frequencies. This is called upper side band(U.S.B.) (See Figure 6b).


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Figure 6 shows frequency spectrum of an A.M. wave which is

equivalent to three sine waves as shown. Bandwidth (B.W.) of an A.M.

wave Hence, in amplitude modulation, the bandwidth is twice the signal


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frequency.

Q. 29. Derive AM power.

Ans. We know that the power contained in a voltage wave isproportional to the square of its amplitude (V2). Note that an A.M.

wave is a voltage wave.

The total power contained in an A.M. wave will be the sum of thepowers contained in the three parts of the wave.

Considering root mean square (R.M.S) values

1.Power contained in the carrier


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2.

3. Note that power in both the sidebands is equal and at m = 1, the

sidebands contain 1/3rd (33%) power and the carrier contains66% of the total power. Hence bands carry half the carrier powerof the wave. -

4. (d)Total power contained in two side bands = 1/3 = 33%5. Total power contained in each side band = 1/6 = 16.5%. V 6. As the signal is contained only in the sidebands, useful power is

contained in sidebands. This is the reason, that we are interestedonly in the sidebands. The power in the sidebands go on increasingwith the increase in the modulating index (m).

7.

8.

9. Q. 30. Explain AM current calculations OR obtain current

relation between amplitude modulated and unmodulated

wave.10.11. Ans. Let l = Unmodulated current (carrier current)12.1T = Modulated current of an A.M. wave (both in R.M.S. values)

and R = Resistance through which current flows. Assume it to be


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the same in both the cases.

Q 31 Explain amp modulation by several sine wavessimultaneously

OrExplain A.M. modulation with a complex wave.


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Ans. (a) Modulation of a single sinusoidal wave is rarely done,instead several sinusoidal waves are simultaneously used to

modulate the carrier. To calculate the modulated power, we havefirst to calculate modulation index which can be evaluated by any ofthe methods given below:

13. We know that

Where P is the power of the sideband.If several sine waves simultaneously modulate the carrier, thecarrier power will

remain the same but the total side band power will become equalto the sum of individual


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side band powers,


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