Electronic Communication system/George Kennedy Similar Free eBooks Advanced Electronic Communications Systems Wayne Tomasi Sixth Edition. Kennedy. George, date. Electronic Communication system/George Kennedy, Bernard Davis,. 4th ed p. cm. Includes bibliographical references and index. Contents PREFACE xv 1 INTRODUCTION TO COMMUNICATIONS SYSTEMS 1 COMMUNICATIONS 1 COMMUNICATIONS SYSTEMS 2
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I Electrnagnetic Radhuion I Atlienmis wilb Parabolic Reflectors J Rectangular Waveguides I Methods of Exciting Waveguides This chapter serves to introduce t he reader to the subject of communication systems, and also this book as a whole.
Modulation mt: The final section briefly discusses about basics or signal representation and analy: Explain the term channel noise and its effects. T hus, in a broad sense, the term communication refers to the transmission of infom1ation from one place to the other. The infommtion transmission between humans si tting very close example, across a table may take place via one or more of the following means: Among these, the most effective one is via speech mode.
However, the speech mode of communication is also limited by how loud a person can produce the speech signal and is effective only over few te ns of meters. For long-distance communication, initially humans employed non-electrical means like drum beats, smoke signals, running messengers, horses a nd pigeons.
The electrical means of communication started with wire telegraphy in the eighteen forties, dcveioping with tdephony some decades later in the eighteen seventies and radio at the begilming of the twentieth cenntry.
Later, the use of satellites and fibre optics made communication even more w idespread with an increasing emphasis on wireless. Presently, in the early period of twenty-first century, we live in a modem society where several electrical modes of communication are at our di sposal. Some of these include, landline telephone, telev ision set, fax machine, mobile phone, computer w ith internet and personal digital assistant. At the same time, they al: Due to miniaturization, most of these communication aids have become gadgets in the hands of the current generation.
After enjoying these facilities in our daily routines, we are in such a stage that it is difficult to imagine c1 modern society without all these modes of communication. By observing all these developments.
Several new modes of electrical communication emerge from time to time due to the continuous techno- logical progress. For instance, this progress only brought us from the era of wired telegraphy to the present era ofwi,-eless mobile communication. Even though this change occurs, the basic objective of electrical co 1trnnication remain:: The different steps involved in the transn-1ission of information may be outlined as follows: Origin of information in the mind of the person who w,mts to communicate Generation of message signal carrying the infonnation Cunvc,ting the message sif,rnal into electrical fom1 using a suitable trans?
Once this is done. This book aims at giving qualitative exposure to ctifferent concepts in the commu. After this. Any logical order may be used, but the one adopted here is basic systems, communication processes and circuits, and then more complex systems.
I shows the generic block diagram of a communication system. Any communication system will have five blocks, including the information source and destination blocks. However, f-rom the practical design point of view, we are intereste? This i: Also, the communication in electrical fonn takes place mainly in these three blocks. Introduction to Co11w11mic: The infoTTTiation comes from the infom1ation source, which originates it. Information is a very generic word signifying at the abstract level anything intended for communication, whieh may include some thought, news, feeling, visual scene, and so on.
The infomiation source converts this information into a physi- cal quantity. This physical manifestation of the infonnation is tenned as mes:. Even though we use the words infonnation and message interchangeably, it is better to understand the basic difference between lhc two.
In the study of electrical communication systems, we are mainly interested in transmitting the information manifested as the message signal to the receiving point, as efficiently as possible.
However, the message signal also usually will be in the non-electrical fom1. For electrical communication purpose, first we need to convert the message signal to the electrical form, which is achieved using a suitable transducer. Trru1sducer is a device wh. It is raining today at my place is the information and the speech corresponding to it is the message signal.
The speech signal is nothing but the acoustic pressure variations plotted as a function of time. These acoustic pressure variations are converted into electrical fom1 using microphone as the transducer. The electrical version of the message signal is the actual input to the nansmitter block of the communication system.
Chawl is a physical medium which connects the transmitter block with the receiver block. This is because, in the first case the channel is a simple copper wire i.: The block diagram of typical radio transmitter is shown in Fig. This transmitter bl. The high-frequency signal is essential fQr carrying ot,t an important opt: This high-frequency signal is more commonly tenned carrier and i:.
The carrier signal is characterized by Lhc three parameters amplitude, frequency and phase. The modulation process involves varying one of these three parameters in accordance with the variation of the message signal.
Even though, modulation is also a generic word indicating the operation of modifying one of the parameter, of a given signal 1 we will still stick to the above context, unless specified otherwise.
The modulated signal from the modulator is transmitted or radiated into the atmosphere using an antenna as the transducer. Modulator Modulation voltage Modulation Modulal1on in processing ,. The physical medium includes copper wire, coaxial cable, fibre optic cable, wave guide and free space or atmosphere. The choice of a particuJar channel depends on the feasibility and also the purpose of communication system.
For instance if the objective is to provide connectivity for speech commtmication among a group of people working in one physically localized place, then copper wire may be the best choice. Alternatively, if the information needs to be sent to millions of people scattered in a geographical area like radio and television broadcasting, then free space or atmosphere is the best choice. The nature of modification of message signal in the transmitter block is based on the choice of the communication channel.
This is becau: The message signal in the modified form travels through the channel to reach the entry point of the receiver. The fo llowing illustration may help us understand the functionality of channel: Suppose we have two water reservoirs connected through a mechanism canal for transferring water from one to the other, when needed.
The objective of the canal is just to cany the water frorn one reservoir to the other and nothing more. Of course, the amount of water which finally reaches the other reservoir depends on the condition of the canal. On similar lines, the amount message signal which finally reaches the receiver depends on the characteristics of the channel.
Finally, it should b. There are a great variety of receivers in communication systems, depending on the processing required to recreate the original message signal and also final presentation of the message to the destination.
Most of the receivers do conform broadly to the super heterodyne type, as does the simple broadcast receiver whose block diagram is shown in Fig. The super heterodyne receiver includes proctissing steps like reception. Among the different processing steps employed, demodulation is the most important one which converts the message signal available in the modified form ro the origina l electrical vcr sion of the message.
Thus demodulation is essentially an inverse operation of modulation. The purpose of receiver and form of output display influence its construction as much as the type of modu- lation system used.
Accordingly the receiver can be a very simple crystal receiver, with headphones, to a far more complex radar receiver, with its involved antenna ammgements and visual display system. Note that the transmitter and receiver must be in agreen1ent with modulation methods used. Intermediate Audio voltage frequency Demodulator and amplifier power amplifiers. Usually, humans will be the destination block.
The incoming message signal via speech mode is processed by the speech perception system to comprehend the infonnation. Similarly, the message signal vfa video or visual scene and written sc-ript is processed by the visual perception system to comprehend the infonnation.
This may also be due the fact that human brain is the least understood part of human body in tenns of its functional ability. The process of regulating is modulation. Thus, for regulation we need one physical quantity which is to be regulated and another physical quantity which dictates regulation. In electrical communication, the signal to be regulated is termed as carrier.
The signal which dictates regulation is termed as modulating signal. Message acts as modulating signal. The modulation process is the most important operation in the modem communication systems. Hence before studying the modulation and its types, it is essential to know the need for modulation. The following example may help to better understand the need for modulation. Assume that there is a spe- cial and rare cultural event from a reputed artist organized at a far distant place destination city from your geographical locatiot1 source city.
However, you have decided to attend the event and enjoy the live perfonnance. Then what will you do? The obvious choice is you will take the help of transportation vehicle to carry you from the source city to the destination city.
Thus there arc two important aspects to be observed in this example. The second one is the transpmtation vehicle which is the carrier. Once you reach the destination city, the purpose of the carrier is served. Exactly similar situation is present in au electrical communication. The message signal which is to be transmitted to the receiver is like you and cannot travel for long distance by itself. Hence it should take the help of a carrier which has the capacity to take the message to the receiver.
This is the basic reason why we need to do modulation; so that message can sit on U1e carrier and reach the receiver. In a more fonnal way, the need for modulation can be explained as follows. The distance that can be travelled by a signal in an open atmosphere is directly inversely proportional to its frequency wavelength. Most of the message signals like speech and music are in the audio frquency range 20 H. AltemaLively, for a signal at 15 kHz. A ve11ical antenna of this size is impracticable.
There is an even more important argument against transmitting signal frequencies directly; all message is concentrated within the same range 20 Hz kHz for speech and music, few MHz for video , so that all signals from the different sources would be hopelessly and inseparably mixed up.
In any city, only one broad- casting station can operate at a given ti. In order to separate Lhe various signals, it is n. Each must be given its own carrier frequency location. This also overcome:: Once signals have been translated, a tuned circuit is employed in the front end of the receiver to make sure that the desired section of the spectrum is admitted and all unwanted ones are rejected.
The tuning of such a circuit is nonnally made variable and connected to the tuning control, so thaL the receiver can select any desired transmission within a predetermined range. The use of modulation process helps in shifting the given message signal frequencies to a very high frequency range where it can occupy only negligible percentage of the spectrum. For instance, at I kHz. But at I GHz.
This means that more number of message signals can be accommodated at higher frequencies. Although this separation of signals has removed a number of the difficulties encountered in the absence of modulation, the fact still remains that unmodulated carriers of various frequencies cannot, by themselves. An unmodulated carrier has a constant amplitude, ,r constant frequency and a constant phase relationship with respect to some reference.
Speech, for instance, is made up of rapid and unpredictable variations in amplitude volume and frequency pitch and resonances. Since it is impossible to represent these two variables by a set of three constant pa- rameters, an unmodulated canier cannot be used to convey infonnation. Therefore, at any i. In this fashion, enough informatiqn about the instantaneous amplitude and frequency is transmitted to enable the receiver to recreate the original message.
That is, the signal infom1ation is manifested as changing electric and magnetic field intensities at specified number of times per second. The ocsillations are sinusoidal in nature and measured as cycles per second or hertz Hz. The oscillations can be as low as I Hz and can extend up to a very large value. The entire range of frequencie!
Jntrod11 c: Table l. For the classification purpose, the EM spectrum is divided into small segments and each segment is given a nomenclature.
Each range is identified by end frequencies or wavelengths that differ by a factor of Even though these are not crisp boundaries, communication fa temity have accepted them as convenient classjfication for all further discussions.
Ln each range a typical application is only given as an example and is HOT exhaustive. Also, the choice of application is the one which is more common among the public. Apart from this detailed classification, the EM spectrnm is also broadly classified into two broad categories, namely, audio frequency AF for the frequency range 20 Hz - 20 kHz and the radio frequency RF range for freq uencies more than 20 kHz. Table 1. Ji llowing l'ulues: Frequency f Waveh!
It is typically measured in seconds sec. For instance, the duration ofa conversation with your friend using a mobile phone is charged in sec based on the time duration for which you used the service of the communication system. Frequency is defined as the number of oscillations per second and is measured ill hertz Hz. For instance, the message in a communication system is usually measured in tenns of the range of freq uencies and the carrier is one frequency f alue. Wavelength Wavelength il is yet another fimdamental quantity used as an alternative to frequency for distinguishing communication signals.
Wavelength is defined as the distance travelled by an EM wave during the time of one cycle. Spectntnz The frequency domain representation of the given signal. Bandwidth Bandwidth Bw is that portion of the EM spectrum occupied by a signal. More specifically it is the range of frequencies over which the infonnation is present io the original signal and hence it may also be termed as signal bandwidth. Cham,cl Bandwidth The range of frequencies required for the transmission of modulated signal.
Modulation In terms of signal and channel bandwidths, modulation is a process of traosfonning signal from signal bandwidth to channel bru1dwidth. Demodttlatiou On the similar lines, demodulation is the reverse process of moduJation, that is, transform-. Baseband Sig11a. Baseband Tra11smission Transmission of message signal in its original frequency range. Broadband Signal Message signal tn its modulated frequency range.
Broadband Transmission Transmission ofm. A high-fidelity audio signal requires a range of 50 to Hz, but a bandwidth of to Hz is adequate for a telephone conversation and is termed as nmowband speech.
For wideband speech the frequency range is from O to Hz.
When a carrier has been similarly modulated with each, a greater bandwidth will be required for the high-fidelity hi-fi trnnsrnissio. At this point, it is worth noting that the transmitted bandwidth need not be exactly the same as the bandwidth of the original signal, for reasons connected with the properties of the modulating systems. This will be made clear in Chapters 3 and 4. Before trying to estimate the bandwidth of a modulated transmission, it is essential know the bandwidth occupied by the modulating signal itself.
If this consists of sinusoidal signals, then there is no problem, and the occupied bandwidth will simply be the frequency range between the lowest and the highest sine wave signal. However, if the modulating signals are nonsinusoidal, a much more complex situation results. Since such nonsinusoidal waves occur very frequently as modulating signals in communications, their frequency requirements will be discussed in Section l.
Described mathematically in the time domain and in the frequency domain, this signal may be represented as follows: I where v 1 "" voltage as a function of time E, peak voltage 11 "". The symbolfin Equation 1. Next we will review the Fourier series, which is used to express periodic time functions ln the frequency domain, and the Fourier transform, which is used to express nonpcriodic time domain functions in the frequency domain.
A periodic waveform has amplitude and repeats itself during a specific time period T. Some examples of wav ;fonns are sine, square, rectangular, lnangular, and sawtooth. Figure 1. This simpl. The form for the Fourier series is 11s follows:. The makeup ofa square or rectangular wave is the sum of harmonics the sine wave components at various amplitudes. The Fourier coefficients for the rectangular waveform in Fig. Example 1.
The single pulse has a maximum voltage of 4 Vand a duration of2 s see fig. The bandwidth required will therefore be considerably greater than might have been expected if only the repetition rate of such a wave had been taken into account.
It may be shown that any nonsinusoidal, single-valued repetitive waveform consists of sine waves and! There are an i11finire number ofsuch harmonics. Some non-sine wave recurring at a rate of times per second will consist of a Hz fundamental sine wave, and hannonic-s at , and Hz, and so on.
As a general rule, it may be added that the higher the harmonic, the lower its energy level, so that in bandwidth calculations the highest hamllmics arc often ignored. The preceding statement may be verified in any one of three different ways. It may be proved mathemati- cally by Fourier analysis. Graphical synthesis may be used.
In this case adding the appropriate sine-wave components, taken from a formula derived by Fourier analysis, demonstrates the truth of the statement. An added advantage of this method. Finally, the presence of the component sine waves in the correct proportions may be demonstrated with a wave analyzer, which is basically a high-gain tunable amplifier with a narrow bandpass, enabling it to tune to each component sine wave and measure its amplitude.
Some fommlas for frequently encountered nonsinusoidal waves arc now given, and more may befound in handbooks.
Square wave: Triangular wave: Sawtooth wave: In each case several of the hannonics will be reqai. This, of colU'se, will greatly increase the required bandwidth. Circle the letter preceding the ber of harmonics line that correctly completes each sentence.
In a communication system, noise is m? St likely amplitude decreases with the harmonic number to affect the signal cl. Indicate the false statement.
Modulation is used to c. Fourier analysis c. The process of sending 4. From the transmitter and receiving started as early as the signal deterioration because of noise is usu- a. Which of the following steps is not included in d. Indicate the true statement. The acoustic channel is used for which of the d. The need for modula- a. UHF communications tion can best be exemplified by the following. An antenna in the standard broadcast AM band I0.
Amplitude modulation is the process of is 16, ft a. All sound is concentrated from 20 Hz to frequency 20kHz b. A message is composed ofunpredictable varia- frequency tions in both amplitude and frequency c. Review Questions 1.
Mention the elements of a communication system. Describe their functionality. Explain the need for modulation. Write the typical frequency ranges for the following classification of EM spectrum: The carrier performs certain functions in radio communications. What are they? Define noise.
Where is it most likely to affect the signal? What does modulation actually do to a me;sage and carrier'? List the basic functions of a radio transmitter and the corresponding functions of the receiver. Noise is probab. It is ever present and limits the perfonnancc of most systems. Measuring it is very contentious: After studying this chapter, you should oe familiar with the types and somces of noise.
The methods of calculating the noise produced by various:: The very important noise quantities,:. Calculate noise levels for a variety of conditions using the equations in the text. Noise may be defined, in electrical tenns, as any unwanted introduction of energy tending to interfere with the proper reception and reproduction of transmitted signals.
Many disturbances of an electrical nature produce noise in receivers; modifying the sii;,,nal in an unwanted manner. In radio receivers, noise may produce hiss in the loudspeaker output. In television receivers "snow" or "confetti" colored snow becomes superimposed on the picture.
Noise can limit the range of systems, for a given transmitted power. It may sometimes even force a reduction in the bandwidth of a system. There are numerous ways of classifying noise. It may be subdivided according to type, source, effect, or relation to the receiver, depending on circumstances. It is most convenient here to divide noise into two broad groups: External noise is difficult to treat quantitatively, and there is often little that can be done about it, short of moving the system to another location.
Note how radiotelescopes are always located away from industry, whose processes create so much electrical noise. International satellite earth station Internal noise is both more quantifiable and capable of being reduced by appropriate receiver design. Because noise has such a limiting effect, and also because it is often possible to reduce its effects through intelligent circuit use and design, it is most important for all those connected with commw,ications to be well informed about noise and its effects.
An astonishing variety of strange sounds will be heard, all tend- ing to interfere with the program. Most of these sounds arc the result of spurious radio waves which induce voltages in the antertna. The majority of these radfo waves come from natural sources of disturbance. They represent atmospheric noise, generally called static. Static is caused by lightning discharges in thunderstonns and other natural electric d. It originates in the fonn of amplitude-modulated impulses, and because such processes are random in nature, it is spread over most of the RF spectrum normally used for broadcasting.
Atmospheric noise consists of SpLtrious radio signals with components distributed over a wide range of freq uencies. The static is likely to be n16re severe but less frequent if the storm is local. Field strength is inversely proport ional to frequency, so that th. Such noJse consists of impulses, and these nonsinuso. Static from distant sources will vary in intensity actord. The usual increase in its level talccs place at night, at both broadcast and shortwave frequencies.
Atmospheric noise becomes Jess severe at frequencies above about 30 MHz because of two separate factors. First, the higher frequ,encies are limited to line-of-sight propagation i. Second, the nature of the mechanism generating thisI noise is such that very little of it is created in the VHF range and above. For convenience, a division into two subgroups will suffice.
Even though the additional noise produced comes from A limited portion of the sun's surface, it may still be orders of magnitude greater than that received during periods of quiet sun. The noise received is called thennal or black-body noise and is distributed fairly uniformly over the entire sky. We also receive noise from the center of our own galaxy the Milky Way , fro1n other galaxies, and from other vi. Summary Space noise is observable at frequencies in the range from about 8 MHz to somewhat above 1.
Not very much of it below 20 MHz penetrates down through the ionosphere, while its eventual disappearance at frequencies in excess of 1. Under this heading, sources such as automobile and aircraft ignition, electric motors and switching equipment; leakage from high-voltage lines and a multitude of other heavy electric machines are all included. Fluorescent lights are another powerful source of such noise and therefore should not be used where sensitive receiver reception or testing is being conducted.
The noise is produced by the arc discharge present in all these operations, and under these circumstances it is not surprising that this noise should be most intense in industrial and densely populated areas. The nature of industrial noise is so variable that it is difficult to analyze it on any basis other than the sta- tistical. Such noise is generally random, impossible to treat on an individual voltage basis i.
Random noise power is proportional to the bandwidth ove, which ii is measured. It is due to the rapid and random motion of the molecules atoms and electrons inside the component itself. In thenriodynamics, kinetic theory shows that the temperature of a particle is a way of expressing its internal kinetic energy.
Thus the "temperature" of a body is the statistical root mean square nns value of the veloc- ity of motion of the particles in the body. Therefore P II ex: Example 2. Tf an ordinary resistor at the standard temperature of 17C K is not connected to any voltage source, it might at first be thought that there is no voltage to be measured across it. That is correct if the measuring instrument is a direct current de voltmeter, but it is incorrect if a very sensitive electronic voltmeter is used. The resistor is a noise generatur, and there may even be quite a large voltage across it.
Since it is random and therefore has a finite nns value but no de component, only the alternating current ac meter will register a reading. This noise voltage is caused by the random movement of electrons within the resistor, which consti- tutes a current.
It is tnte that as many electrons arrive at one end of the resistor as at the other over any long period oftirne. At any instant of time, there are bound to be more electrons arriving at one particuJar end than at the other because their movement is random. The rate of arrival of electrons at either end of the resistor therefore varies randomly, and so does the potential difference between the two ends. A random voltage across the resistor definitely exists and may be both measured and calculated.
It must be realized that all fonn ulns referring to random noise arc applicable only to the m1s value of such noise, not to its instantaneous value, which is quite unpredictable. Using Equation 2. Assmne that RL is noiseless and is receiving the maximum noise power generated by R; under these conditions of maximum power transfer, R,.
Then P. Note especially that the generated noise voltage is quite independent of the frequency at which it is measured. This stems from the fact that it is random and therefore evenly distributed over the frequency spectmm. What is therms noise voltage at the input to this amplifier if tlte ambient tenzpernture is 27C? A low voltage fed to this amplifier would be masked by the noise and lost.
The most important of all the other sources is the shot effect, which leads to shot noise in all amplifying devices and virtually all active devices. It is caused by rando,n variations in the arrival ofelectrons or holes at the output electrode of an amplifying device and appears as a randomly varying noise current superimposed on the output. When amplified, it is sup- posed to sound as though a shower of lead shot were falling on a metal sheet.
Hence the name shot noise. Although the average output current of a device is governed by tlle various bias voltages, at any instant of time there may be more or fewer electrons arriving at the output electrode. In bipolar transistors, this is mainly a result of the random drift of the discrete current carriers across the junctions.
Shot noise behaves in a similar manner to thennal agitation noise, apart from the fact that it has a different source. Many variables are involved in the generation of this noise in the various amplifying devices, and so it is customary to use approximate equations for it. The most convenient method of dealing with shot noise is to find the value or fortnula for an equivalent input-noise resistor. This precedes the device, which is now assumed to be noiseless, and has a value such that the same amount of noise is present at the output of the equivalent system as in the practical amplifier.
Tbc noise current has been replaced by a resistance so that it is now easier to add shot noise to thermal noi. The value of the equivalent shot-noise resistance R. Approximate formulas for equivalent shot- noise resistances are also available. They all show that such noise is inversely proportional to transconductance and also directly proportional to output current. So far as the use of R is concerned, the important thing to realize is that it is a completely ficti.
For noise only, this resistance is treated as though it were an ordinary noise-creating resistor, at the same temperature as all the other resistors, and located in series with the input electrode of the device. The minute currents induced in the input of the device by random fluctuations in the output current become of great importance at such frequencies and create random noise frequency distortion.
Once this high-frequency noise makes its presence felt, it goes on increasing with frequency at a rate that soon approaches 6 decibels 6 dB per octave, and this random noise then quickly predominates over the other forms. The result of all this is that it is preferable to measure noise at such high frequencies, instead of trying to calculate an input equivalent noise resistance for it. RF transistors are remarkably low-noise. A noise figure see Sect-ion 2.
The sum of two such nns voltages in series is given by the square root of the sum of their squares, so that we have. IL is seen from the previous equations that in order to find the tota. The bandwidth of the amplifier is 6 MHz, and the teniperatwe is 17C. To calculate the noise voltage due to several resistors in parallel; find the total resistance by standard methods, and then substitute this resistance into Equation 2. This means that the total noise voltage is less than that due to any of the individual resistors; but, as shown in Equation 2.
Tt may appear logical to combine all the noise resistances at the input, calculate their noise voltage, multiply it by the gain of the first stage and add this voltage to the one generated at the input of the second stage. The process might then be continued, and the noise voltage at the output, due to all the intervening noise sources, would be found. Admittedly, there is nothing wrong with such a procedure. The result JJ useless because the argument assumed that it is important to find the total output noise voltage, whereas the important thing is to find the equivalent input noise voltage.
It is even better to go one step further and find an equivalent resistance for such an input voltage, i. This is. Consider the Lwo-stage amplifier of Fig. The gain of the first stage is A1 and that of the second is A2 The first stage has a total input-noise resi: The nns noise voltage at the output due to R3 is. The same noise voltage would be present at the output if there were no R3 there.
Tnstead R; was present at the input of stage 2, such that. R; where is the resistance which if placed at the input of tbe second stage would produce the same noise volt- age at the output as does R3 Therefore. Equation 2. Now the noise resistance actually present at the input of the second stage is R2, so that the equivalent noise resistance at the input of the second stage, due to the second stage and the output resistance, is.
It is possible to extend Equation 2. As Example 2. For the second stage, these values are 25, 81 kfl, 10 k! Calculate the equivalent input-noise 1'esistance of this two-stage amplifier.
To either side ofresonance the presence of the tuned circuit affects noise in just the same way as any other voltage, so that the tuned circuit limits the bandwidth of the noise source by not passing noise outside its own bandpass.
The more interesting case is a tuned circuit which is not ideal, i. In the preceding sections dealing with noise calculations, an input noise resistance has been used. Consider Fig. The-series resistance of the coil, whlch is the noise source here, is shown as a resistor in series with a noise generator and with the coil.
I required to determine the noise voltage across the capacitor, i. This will allow us,to calculate the resistance which may be said to be generating the noise. The noise current in the circuit will be. In the second instance, and also when equivalent noise resistance is difficult to obtain, the signal-to-noise ratio SIN is very often used.
It is defined as the ratio of signal power to noise power at the same point. 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. 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 not necessary to include the entire name of the publisher.
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