ELECTRONIC COMMUNICATION SYSTEMS BY GEORGE KENNEDY EBOOK
ELECTRONIC COMMUNICATION SYSTEM BY GEORGE cittadelmonte.info - Ebook download as PDF File .pdf), Text File .txt) or read book online. Electronic communication systems. Front Cover. George Kennedy. McGraw-Hill, - Technology & Engineering - pages. 0 Reviews. This is a very good book for us in electronics and communication industry! its very nice indeed. User Review QR code for Electronic Communication Systems.
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Kennedy. George, date. Electronic Communication system/George Kennedy, Bernard Davis,. 4th ed p. cm. Includes bibliographical references and index. Electronic Communication Systems. Fifth Edition. George Kennedy. Supervising Engineer. Overseas Telecommun/catlons Commission. Austral/a. Bernard Davis. electronic communication systems by george kennedy pdf free download. Hello, I would like to download all Electronics Engg ebook. Like electronics and.
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Therefore 2. An effort is naturally made to keep the signal. For example, it is hard to determine at a glance whether a receiver with an input impedance of SO! As a matter offact, the second receiver is the better one, as will be seen. Instead of equivalent noise resistance, a quantity known as noise figure, sometimes called noise fad or, is defined and used. The noise figure Fis defined as the ratio of the signal-to-noise power supplied to the input tenninals of a receiver or amplifier to the signal-to-noise power supplied to the output or load resistor.
Consequently, in a practical receiver, the output SIN will be lower than the input value, and so the noise figure will exceed 1.
However, the noise figure will be I for an ideal receiver, which introduces no noise of its own. Hence, we have the altemative definition of noise figure, which states that F is equal to the SIN of an ideal system divided by the SIN at the output of the receiver or amplifier under test, both working at the same temperature over the same bandwidth and fed from the same source.
In addi- tion, both must bt:: The noise figure of practical receivers can be kept to below a couple of decibels up to frequencies in the lower gigahertz range by a suitable choice of the first transistor, combined with proper circuit design and low-noise resistors. At frequencies higher than that, equally low-noise figures may be achieved lower, in fact by devices which use the transit- time effect or are relatively independent of it.
Each is treated as a four-tenninal network having an input impedance R1, an output impedance Ru and an overall voltage gain A. It is fed from a source antenna of internal impedance R, which may or may not be equal to R, as the circumstances warrant.
Voltage v, Rt gain"' A. The calculation procedure may be broken down into a number of general steps. Each is now shown, fol- lQwed by the number of the corresponding equation s to follow: Determine the signal output power P,0 2. Write P for the noise output power to be determined later 2. Calculate the generalized form of noise figure from steps 3 and 6 2. Calculate Pno from Rcq if possible 2. It is seen from Fig. R, 4kT 41 Ra V. The noise output power may be difficult to calculate.
For the ti. An actual fonnula for F may now be obtained by substitution for the output noise power, or from a knowledge of the equivalent noise resistance, or from measurement. Putting it another way, we see that all these resistances are added to R,, giving a lumped resistance which is then said to concentrate all the "noise ma.
All this applies here 1 with the minor exception that these noise resistances must now be added to the parallel combination of R0 and R,. It is convenient to define R: When Equation 2. This is a situation exploited very often in prac6ce, and it may now be applied to Equation 2.
Note that this constitutes n lnrgc enough mismatch. Controversy exists regarding which is the better all-around measurement, but noise temperature, derived from early work in radio astronomy, is employed extensively for antennas and low-noise microwave amplifiers. Not the least reason for its use is convenience, in that it is an additive like noise power. This may be seen from reexamining Equation 2.
Noise It will be recalled that the equivalent noise resistance introduced in Section 2. Similarly, Tcq' the equivalent noise temperature, may also be utilized if it proves convenient. It is then possible to use Equation' 2. Also, Te. It must be repeated that the equiva- lent noise temperature is just a convenient fiction. Jf all the noise of the receiver were generated by R0 , its temperature would have to be Tr,q.
Finally we have, from Equation 2. Multiple-Choice Questions Each of the fo llowing multiple-choice questions 5. Indicate the noise whose source is in a category consists ofan incomplete statement followed by four different from that oftbe other three. Circle the letter preceding the a. Solar noise line that correctly completes each sentence. Cosmic noise L. One of the following types of noise becomes of c.
Atmospheric noise great importance at high frequencies. It is the d. Galactic noise a. The square of the b. HF mixers are generally noisier than HF ampli- c. Boltzmann's constant fiers. Which two broad classifications of noise are the width. Thermal noise is independent of the frequency a.
Industrial noise is usually o the impulse c. The value of a resistor creating thermal noise is 8. Space noise generally covers a wide frequency doubled. The noise power generated is therefore spectrum, but the strongest interference occurs a. One of the following is not a useful quantity for 9. When dealing with random noise calculations it comparing the noise performance of receivers: Input noise voltage a.
Equivalent noise resistance vaJues. Noise temperature b. Noise figure c. Which of the following is the most reliable mea- a. Random noise power is inversely proportional surement for comparing amplifier noise charac- to bandwidth. Flicker is sometimes called demodulation a. Noise in mixers is caused by inadequate image c. A random voltage across a resistance cannot Which of lhe following statements is tme? Review Problems I.
An amplifier operating over the frequency range of to kHz bas a kfl input resistor. What is the rrns noise voltage at the input to this amplifier if the ambient temperature is I 7C?
The noise output of a resistor is amplified by a noiseless amplifier having a gain of 60 and a bandwidth of20 kHz. A meter connected to the output of the amplifier reads I mV rms. What does the meter read now? If this circuit is maintained at t 7C, what noise voltage will a wideband voltmeter measure when placed across it? The front end of a television receiver, having a bandwidth of 7 MHz and operating at a temperature of 27C, consists of an amplifier having a gain of 15 followed by a mixer whose gain is The amplifier has a input resistor and a shot-noise equivalent resistance of fl; for the converter, these values are 2.
O, respectively, and the mixer load resistance is kfl. Calculate Ri for this television receiver.
Electronic Communication Systems
Calculate the minimum signal voltage that the receiver of Problem 2. The RF amplifier of a receiver has an input resistance of l n, and equivalent shot-noise resistance of fl, a gain of 25, and a load resistance of kO. Given that the bandwidth is 1. If this receiver is connected to an antenna with an impedance of 75 fl, calculate the noise figure. Review Questions I. List, separately, the various sources ofrandom noise and impulse noise external to a receiver.
How can some of them be avoided or minimized? What is the strongest source of extraterrestrial noise? Discuss the types, causes and effects of the various fonns ofnoise which may be created within a receiver or an amplifier. Describe briefly the forms of noise to which a transistor is prone. Define signal-to-noise ratio and noise figure ofa receiver. When might the latter. One of the terms of this formula will be the noise output power. Describe briefly how this can be measured using the diode generator.
Write the relation for maximum noise power output of a resistor. Write the expression for therms noise voltage. What is transit-time effect? How it is generated? What is ideal and practical values of noise figure? Why they arc so explain. What is noise temperature'? How is it related to noise figure? Derive the relation between noise figure and temperature. The definition and meaning nf nmdulatinn in general, as well as the need for modulation, were introduced in Chapter 1.
This chapter deals with amplitude modulation techniques in detail. The communication process can be broadly divided into two types, namely. This clas- sification is mainly based on the nature of message or modulating signal. If the message to be transmitted is continuous or analog in nature, then such a communication process is termed as analog communication. Alternatively, if the message is discrete or digital in nature, then such a communication process is termed as digital communication.
In analog communication, message is analog and the carrier is sine wave, which is also analog in nature. The modulation techniques in analog communicatiot1 can be classified into amplitude modulation AM and angle modulation techniques. The amplitude of the carrier signal is varied in accordance with the message to obtain modulated signal in case of amplitude modulation. The angle modulation employs variation of angle of the carrier signal in proportion to the message.
Tbis chapter deals with the amplitude modulation techniques employed in analog communication. The next chapter deals with angle modu.
After studying the theory of amplitude modulation techniques, the students will be able to apprec-iate that an AM wave is made of a number of frequency components havi1ig a Specific relation to one another. This is based on how many components of the basic amplitude modulated signal are chosen for transmission. To summarize, this chaptt:!
Upon studying this chapter, the sn1dents will be able to understand the AM and its variants. The students will also be able to calculate the frequencies present, plot the spectmm, the power or current associated with different frequency components and finally bandwidth requirements.
This block diagram is drawn by referring to the communication system block diagram given in Fig. Analog carrier source. The continuous message signal is subjected to analog modulation with the help of a sine wave carrier at the transmitter. This results in the modulated signal which is also analog in nature. The analog modulated signal is transmitted via the cornmuication channel towards the receiver, after adding the requisite power levels. At the receiver the incoming modulated signal is passed through an analog demodulation process which extracts out the analog message signal.
The analog message is passed onto the final destination. As described above, the nature of signal starting from the information source till the final destination is analog and hence the name analog commWlication system.
This chapter deals with various amplitude modulation techniques employed in analog modulation block shown in Fig. In amplitude modulation, the amplitude of a carrier signal is varied by the modulating voltage, whose fre- quency is invariably lower than that of the carrier. In practice, the carrier may be high frequency HF while the modulation is audio. Fonnally; AM is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage.
Let the carrier voltage and the modulating voltage, ve and vm, respectively, be represented by Ve ;: Its inclusion here would merely complicate the proceedings, without affecting the result.
Amplitude Modulation From the definition of AM, you can see that the maximum amplitude V of the umnodulated carrier will have to be made proportional to the instantaneous modulating voltage viii sin w,,,t when the carrier is amplitude modulated.
Freqttettcy Spectnmi of the AM Wave We shall show mathematically that the frequencies present in the AM wave are the carrier frequency and the first pair of sideband frequencies, where a sideband frequency is defined as.
When a carrier is amplitude modulated, the proportionality constant is rnade equal to unity, and the instantaneous modulating voltage variations are superimposed onto the carrier amplitude. Thus when there is temporarily no modulation, the amplitude of the carrier is equal to its unmodulated value.
When modula- tion is present, the amplitude of the carrier is varied by its instantaneous value. The situation is illustrated in Fig. Figure 3. From Fig. We have A "' V,. Equation 3. It has thus been shown that the equation of an amplitude modulated wave contains three terms.
The first tenn is identical to Equation 3. It is apparent that the process of amplihtde modulation has the effect of adding to the unmodulated wave, rather than changing it.
The two additional terms produced are the two sidebands outlined. The very important conclusion to be made at this stage is that the bandwidth required for amplitude modulation is twice the frequency of the modulating signal. That is. In, V ,. Jn, r 3. The frequency spectrum ofAM wave is shown in Fig.
As illustrated, AM consists of three discrete frequencies. Of these, the central frequency, i. Example 3. If the oscillator output is modulated by audio frequencies up to 10 kH. Mod1rlatio11 Jt is derived from Fig. The maximum negative amplitude. The modulated wave extends between these two limiting envelopes and has a repetition rate equal to the VfflllX Vmnx - Vm a Vmn.
II is the standard method of evaluating the modulation index when calculating from a wavefonn such as may be seen on an oscilloscope, i.
It may not be used in any other siniation. When only the root mean square nns values oftbe carrier and the modulated voltage or current are known, or when the unmodulated and modulated output powers are given, it is necessary to understand and use the power relations in the AM wave.
Power Relations in the AM Wave It has been shown that the carrier component of the modulated wave has the same amplitude as the unmodulated carrier. The modulated wave contains extra energy in the two sideband components. Therefore, the modulated wave contains more power than the carrier had before the modulation took plac. Since amplitude of the sidebands depends on the modulation index V IV,C it is anticipated that the total power Ill.
This relation may now be derived. The total power in the modulated wave will be. The first tenn of Equation 3. It is interesting to know from Equation 3. This is important, because it is the maximum power that relevant amplifiers must be capable of handling without distortion. Calculate the total power in the modulated wave. P AM'!!!! How much of this is carrier power?
Current Relations in the AM Wave The situation which very often arises in AM is that the modu- lated and unmodulated currents are easily measurable, and it is then necessary to calculate the modula- tion index from them.
This occurs when the antenna current of the transmitter is metered, and the prob- lem may be resolved as follows. Find the percentage modulation. Determine the antenna current when the percent of modulation clumges to 0. Accordingly, a way has to be found to calculate the resulting power conditions.
The procedure consists of calculating the total modulation index and then substituting it into Equation 3. There are two methods of calculating the total modulation index. Let V1, V1, V3, etc. Then the total modulating voltage V, will be equal to the square root of the sum of the squares of the individual voltages; that is,.
I sideband power will now be the sum of individual sideband powers. It is seen that there are two approaches, both yield the same result. Note also that th. Calculate the modulation index. If another sine wave is simultaneously transmitted wW1 modulation index 0. It increases to 12 A as a result of simultaneous modulation. What is the modulation index due to this second wave?
Solution From Equation. Further, ifwe consider the power relation given by. II c 2 Therefore, the power required for the carrier component is given by. PAM 3. A significant saving in power requirement can be achieved by supressing the carrier before transmission. I - co. The next question will therefore be why AM is still in use?
DSBSC technique accordingly adds complexity at the receiving point to recover the message. Suppose your application requirement is cost ofreceiver needs to be significantly low, then AM is preferred, as in the. It is derived from Fig. The modulated wave extends between these two limiting envelopes and has a repetition rate equal to the unmodulated carrier frequency.
For better distinction, the bottom envelope is shown as dotted li. The top envelope crosses below ilie zero reference amplitude value and similarly, the bottom envelope crosses above the zero reference amplitude value. However, in case ofAM wave shown in Fig. At the most; the top envelope can touch the zero reference; but cannot cross it. Samething is true with respect of bottom envelope also. Thus the information from AM can be recovered uniquely either from top or bottom envelope by a simple envelope detector circuit assume it as diode rectifier for time being.
This is tbe price we pay by suppressing the carrier. Of course, as will be explained later, there are ways to overcome this problem for recovenng message.
The modulated wave contains energy only due to the two sideband components. Since amplitude of the sidebands depends on the modulation index V,,,IV,, it is anticipated that the total power in the DSBSC modulated wave will also depend on the modulation index.
SR 4 2R Substituting Equation 3. This is correct also, because, in case of AM wave, two-third of total power is utilized by the carrier component alone and rest one-third by both the sidebands.
How much of carrier power ill kW is required if we want to transmit the same message by an AM transmitter? We require 5. Hence saving in bandwidth can be achieved by suppressing one of the sidebands. Since only one of the sidebands is selected for transmis- sion, SSB needs a bandwidth equal to that of message.
Then the next question is why not use only SSB? The SSB technique further complicates the receiver structure to recover message. As will be explained later, an equally important limitation ofSSB is the practical difficulty in suppressing the unwanted sideband, since it lies close to the wanted sideband. The mathematical treatment here follows this assumption. If the cut-off frequencies are if. Aitematively, if the cut-off frequencies are J. The bandwidth required for SSB is the frequency of the modulating signal.
That is,. As illustrated, SSB consists ofone discrete frequency either atf.. The modulated wave will have only one sine wave. The only wave to distinguish is to compare with carrier signal. Its frequency will be either lower or more than carrier frequency by au amount of modulating signal frequency. The envelope of SSB does not contain message and hence a simple envelope detector circuit is not useful for recovering the message.
This is the price we pay by suppressing the carrier and one of the sidebands. Of course, here also, there are ways to overcome this problem to recover message. Carrier Va. The modulated wave contains energy only due to one sideband compcment. The total power in the SSB modulated wave will be. SO in 3. Calculate the total power in case of SSB tech- nique. Compare the powers required for SSB in both the cases and comment on the reason for change in tile power levels.
Case 2 Given, Pc"" W and m ;;: This infers that the total power in SSB also depends on the depth of modulation. How much of carrier power in kW is required if we want to transmit the same message by an AM transmitter? Ill2 o.. In total 6. It was observed in practice that such a process results in eliminating even some por- tion of the wanted sideband.
This is because, in many cases the message has information starting from zero frequency and spreads upto a maximum off,,, Hz.
Communication Systems by George Kennedy PDF | Telecommunication | Telecommunications
Therefore an attempt to attenuate unwanted component will in tum leads to attenuation of wanted component. One way to compensate for this loss is to allow a vestige or trace or fraction-of unwanted sideband along with the wanted sideband. Thif book also follows the same convention. We have. Depending on the cutoff frequencies, either LSB or USS comes out of the bandpasss filter, along with the vestige of the other. The bandwidth required for VSB is the frequency of the modulating signal plus vestiage, band.
That is, B. As illustrated, VSB consists of two discrete frequencies either at ifc - J , if. Amplitude Mod11latio11 The shape of the signal in the time cloinain depends on the value of vestige frequency. Alternatively, ifthef.. The modulated wave contains energy due to these two components. Since amplitude of the sidebands depends on the modulation index VJ Vr, the total power in the modulated wave will depend on the modulation index also.
BR 4 2R Substituling these equations in the total power equation, we have. This is correct also, because, in case ofVSB wave, one-sixth of total power is utilized by one sideband and a fraction of one-sixth for the transmission of the vestige.
Case 2 Given, P,. Solution G iven, Pvsll. Using Analog Multiplier The conceptual way to realize the generation of AM signal is with the help of an analog multiplier and a summer connected as shown in Fig.
The output of the analog multiplier is given by. Analog multiplier. Vme c. If the above equation refers to a resistor, then b is obviously its conductance. In-a nonlinear resistance, the current is still to a certain extent proportional to the applied voltage, but no longer directly as before. The previous linear relation seems to apply to ce1iain point, after which current increases more or less rapidly with vo ltage.
Whether the increase is more or less rapid depends on whether the device begins to saturate, or else some sort of avalanche current multiplication takes place. Current now becomes proportional not only to voltage but also to the square, cube and higher powers ofvoltage.
The reason that the initial portion of the graph is linear is simply that the coefficient c is much smaller than b. Therefore, c in practical nonlinear resistances is much greater than d, which is in t: Since Equation 3.
The devices like diodes, transistors and field effect transistors FET can be biased with suitable voltage to constrain them to exhibit the negative resistance property, Figure 3. The output of the diode is collected via a tuned ci. The diode is biased such that it exhibits the negative resistance property. Under this condition. The above equation has the standard AM signal components.
In this way we can generate the AM signal with the help of device that exibblts nonlinear resistance property. Using Analog Multiplier The conceptual way to reali'.
Analog ll "Vn1 Ve multiplier. Using a Bala11ced Modulator A baJanced modulator can be constructed using the non-linear devices like diodes and transistors. The balanced modulator using the diodes is given in Fig. The diodes use the nonlinear resistance property for generating modulated signals.
Both the diodes receive tbe carrier volt- age in phase; whereas the modulating voltage appears! The modulated output currents of the two diodes arc combined in the center-tapped prirnary of the output trilnsfonner.
They therefore subtract, as indicated by the direction of the arrows in the Fig. If this system is made cornpletely symmetrical, the carrier frequency will be completely canceled. No system can of course be perfectly symmetrical in practice, so that the carrier will be heavily supressed rather than completely removed. The output of the balanced modulator contait1s the two sidebands and some of the miscellaneous components which are taken care of by tuning the output tranfom1er's secondary winding.
The final output consists only of sidebands. As indicated. D1 and vf' - v at the input of diode Ht. D2 If perfect symmetry is assumed. Vo D2. The two diode output currents will be i. Let the constant of proportionality be a then. The tuning of the output transformer will remove the modulating frequencies from the output.
The output of the analog multiplier is given by ,. If the lower sideband is passed out then the output of the bandpass filter will be mVc v a cos wc - ro,. This results in the generation of SSB signal. Using the Filter Method The basis for the filter method is that after the balanced modulator the unwanted sideband is removed by a filter. The balanced modulator generates the DSBSC signal and the sideband suppression filter suprcsses-the unwanted sideband and al lows the wanted sideband.
Depending on the cut-off frequency values we can-represent the output of tbe fi Iler as v "" 2acV. In this way SSB is generated in case of filter method. Usittg the Phase Shift Method The phase shift method avoids filters and some of tbeir inherent disad- vantages, and instead makes use of two balanced modulators and two phase sh.
One of the balanced modulators, M1, receives the 90 phase shifted carrier and in phase message signal, whereas the other, M,, is fed with the 90 phase shifted message and in phase carrier signal. Both the modulators produce the two-sidebands.
One of the sidebands, namely, the upper sideband will be in phase in both the modulators, whereas, the lower sideband will be out of phase. Thus by suitable polarity for M1 output and addiJ. Let v. The 90 phase shifted versions of them are V,,, cos OJ.
Balanced modulator M,- v1. Balanced - 90 phase modulator shifter M2 V2. Usi,ig the Third Metltod The third method of generating SSB was developed by Weaver as a means of retaining the advantages of the phase shift method, such as its ability to generate SSB at any frequency and use of low audio frequencies, without the associated disadvantage of an audio frequency phase shift network required to operate over a large range of audio frequencies.
The block diagram oftbe third method is shown in Fig. We can see that the later part of this circuit is identical to that of the phase sbift method, but the way in which appropriate voltages are fed to the last two balanced modulators M3 and M4 has been changed. Thus the basic blocks remain same as in the case of SS B generation and the only difference is in the cut-off frequency values of the bandpass filter.
The output oftbe analog multiplier is given by. This signal is passed through a bandpass filter which, depending on the cut-off frequencies, will pass one sideband completely and a vestige of the other sideband. This results in the generation ofVSB signal. Using the Filte1 Method The basis for tbe filter method is, after the balanced modulator the unwanted ilideband is removed by a filter.
The balanced modulator generates the DSBSC signal and the sideband suppression filter supresses most oftbe unwanted sideband and allows a vestige ofit along with the other sideband. Depnding on the cut-off frequency values we can represent the output of the filter as1 ,, 2acVmVc cos a.
The study of all the amplitude modulation techniques gives 1 better understanding about their nature in time and frequency domains, and power and bandwidth requil. The basic technique. Amplit11tle Mod11lalio11 The SSB technique needs mini- mum power and bandwidth. This was followed by the study of different methods for the generation of AM and its variants.
The method using analog multiplier is concephJally simple to understand. Other methods are relatively different, but provide practical approaches for the generation.
Multiple-Choice Questions Each of the followi11g multiple-choice questions b. Circle the letter preceding the d. The AM wave will have 1. The bandwidth of AM wave is given by c. Amplitude modulation is defined as the system 8. The peak. The expression for total power in AM wave is Ill a. The instantaneous voltage of the AM wave is d. The maxi mum power of AM wave under distor b. VI" sincoCt tionless condition is c. THc modulation index of AM is given by C. The expression for total modulation index in case IfvSll is the instantaneous voltage ofone sideband, d.
The SSB wave wiU have a. The bandwidth of SSB wave is given by Iff,, is the vestige frequency, the bandwidth of a. The expression for total power in SSB wave is d.
The expression for total power in VSB wave is b. The maximum power ofSSB wave under distor- d. If F v;;, is the instantaneous voltage of vestiage d. DSBSC b. SSB c. VSB d. The outJ ut current of a nonlinear resistor caa. The instantaneous voltage of the VSB wave related to its input voltage by having f: JSB as wanted sideband is n. The balanced modulator can be used for the gen- DSBSC a.
SSB b. VSB c. The basic working principle of a balanced modu- The VSB wave will have lator is to a. The basic working principle of third method for A IkHz carrier is simultaneously modulated with H: Hz audio sine waves. What will be the frequencies present in the output? A broadcast AM transmitter radiates 50 kW of carrier power. What will be the radiated powerat 85 percent modulation?
When the modulation percentage is 75, an AM trai1smitter produces 10 kW. What would be the percentage power saving if the carrier and one of the sidebands were sup pressed before transmission took place?
ELECTRONIC COMMUNICATION SYSTEM BY GEORGE KENNEDY.pdf
A W carrier is simultaneously modulated by two audio waves with modulation percentages of55 and 65, respectively. What is the total sideband power radiated? A transistor class C amplifier has maximum permissible collector dissipation of 20 W and a collector efficiency of 75 percent. When a broadcast AM transmitter is 50 percent modulated, its antenna current is 12 A. What will the current be when the modulation depth is increased to 0.
The output current of a 60 percent modulated AM generator is 1. To what value will this current rise if the generator is modulated additionally by another audio wave, whose modulation index is 0.
What will be the percentage power saving if the carrier and one of the sidebands arc now suppressed? Amplitude Morlttlatio11 How do you distinguish between analog and digital communication? Define amplitude modulation? Write the expression for the peak amplitude of the AM wave? Write the expression for the instantaneous voltage of AM wave? Define modulation index of amplitude modulation? Mention the different components of AM wave?
How much is the bandwidth of AM wave? Derive the expression for the instantaneous voltage of AM wave? Derive the expression for the total powur in case of AM wave? Derive the expression for the total current in case of AM wave? Derive the expression for the total modulation index in case of modulation by several sine waves? Write the expression for the instantaneous vo ltage of SSB wave'?
Mention the different components of SSB wave? How much is the bandwidth ofSSB wave? Derive the expression for the instantaneous voltage of SSB wave? Derive the expression for the total power in case ofSSB wave? Write the expression for the instantaneous voltage ofVSB wave?
Mention the different components of VSB wave? How much is the bandwidth of YSB wave? Derive the expression for the instantaneous voltage of VSB wave? Derive the expression for the total power in case ofVSB wave? Describe the AM wave generation process using analog multiplier?
Describe the AM wave generation process using diode as nonlinear resistor? Describe the generation of SSB wave using analog multiplier? Describe the generation of SSB wave using frequency discrimination method? Describe the generation of SSB wave using phase shift method? Describe the generation of SSB wave using third method?
Describe the generation of VSB wave using analog multiplier and frequency discrimination methods?
Jn Chapter 3 we discussed in detail about the different ampljtude modulation techniques. The other important form of modulation used in analog communication is angle modulation.
This chapter gives a detailed treat- ment of angle modulation techniques. As mentioned in the previous chapter. There arc two variEmts iu angle modulation depending on which component of the angle is used, namely, frequency modulation FM and phase modula- tion PM. The freq uency and phase of the carrier are varied i11 accordance with the instantaneous variations of the message in case of FM and PM, repectivcly.
Following the pattern set in Chapter 3, this chapter covers the theory of angle modulation techniques and their general. Angle modulation is more difficult to detem1ine mathematically and has sideband behavior that is equally complex. After studying this chapter, the students will be able to undestand the similarity and importruH differences between FM and PM. They will also appreciate the fact that both PM and PM are similar in visual appearance, in fact, not possible to distinguish the two without reference message signal.
No doubt they equall y apply to PM also. In th. It will be seen that FM is the preferred forn, for most applications. UL1Uke amplitude modulation, FM is, or can be made, relati vely immune to tbe effect of noise. This point is discussed at length. It will be seen that tbe effect of noise in FM depends on tbe lloise sideband frequency, a point that is bwught out under the heading of noise triangle. FM and AM are then compared, on the basis that both are widely used practical systems.
The final topic studied in this chapter is the generation of FM. The second method is e in which basically phase modulation is generated, but circuitry is used to convert this to frequency modulation.
Both methods are used in practice. To summarize, thi:: Upon studying this chapter, the students will be able to understand the fM and PM, their differences; similarities, meriL-. The students will also be able to comment on the frequencies present, calculate frequency deviation, modulation index and finally bandwidth requfrements. Objectives Upon completing the material in Chapter 4, the student will be able lo:. Draw FM and PM waves ,.. Amplilude modulation, already discussed, is achieved when the amplitude V is varied.
Alteration of the phase angle q, will yield phase modulation. If the frequency of the carrier co. It is assumed that the modulating signal is sinusoidal. This signal has two important parameters which must be represented by the modulation process without distortion, specifically, its amplitu. It is understood that the phase relations of a complex modulation signal will be preserve. By the definition of frequency modulatior. The rate at which this frequency variation takes place is equal to the modulating frequency.
I, which shows the modulating voltage and the resulting frequency modulated wave. Figure 4. Similarly, all components of the modulating signal of the same frequency, wil.
The amplitude of the. This is the greatest single advantage of FM. The maximum deviation for this signal will occur when the sine tern, has its maximum value, I. J- 1; kf,,,, 4. This function represents an e angle and will b; called for convenience. The problem now is to determine the instantaneous value i.
As Fig. Ln this instance, the angular velocity is anything but constant. Thus Fig. Equauon 4. It is interesting to note that as the modulating frequency decreases and the modulating voltage amplitude remains constant, the modulation index.
This will be the basis for distinguishing frequency modulaw tion from phase modulation. Note that m1 , which is the ratio of two frequencies, is a dimensionless quantity in case of FM.
Angle Modulation Techniques Example 4. FM system, w71en the audio frcquen. If the AF voltage is now increased to7. Find the modulation index in each case. Altematively, the modulating frequency change did have to be taken into account in the modulation index calculation. What power will this FM wave dissipate in a 10 Q resistor?
The rate at which this phase variation changes is equal to the modulating frequency. The figure also shows the phase variation with time, which ca11 be seen to be the phase shifted version of the variation with time of the modulating voltage.
The result of using that modulating voltage to produce FM is also shown for comparison. Similarly, all components of the modulating signal of the same frequency, will deviate the carrier phase at the same rate per second, no matter what their individual amplitudes.
It can also be observed from the figure that, if only either FM or PM waves are given without reference message signal, then it is not possible to distinguish between the two. This is the close proximity between the two forms of angle modulatiofl. Hence in all further studies only FM will be dealt in detail. The observations can be easily mapped to PM. Under these conditions, the instantaneous phase will be ,1, "" ,1, -kV 4.
The problem now is to detcnnine the instantaneous value i. Note that the modulation index of PM is exprC! Substituting Equation 4. Hence the basis for distinguishing phase modulation from frequency modulation.
Note that mp is measured in radians. If the AF voltage is now increased to 7. Solution Case 1: CaSC 3: V l "" 1OV. This is a major difference between FM and PM.
However, this is not nearly as obvious as the difference between AM and FM, and it must be developed further. First, the similarity will be stressed.
In phase modulation, the phase deviation is proportional to the amplitude of the modulating signal and there- fore independent of its frequency. Tn frequency modulation, the frequency deviaiion is proportional to the amplitude of the modulating voltage. Also, if we take a refer- ence vector, rotating with a constant angular velocity which corresponds to the carrier frequency, then the FM vector will have a phase lead or lag with respect to the reference, since its frequency oscillates between 0- J;.
With this close similarity of the two forms of angle modulation established, it now remains to explan the difference. The larger the frequency deviation, the larger the pbasf deviation, so th'at the latter depends at least to a certain extent on the amplitude of the modufo. The difference is shown by comparing the definition of PM, which states in part that the modulation index is proportional to the modulating voltage only, with that of the FM, which states that the modulation index is also inversely proportional to the modulation frequency.
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