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Jump to Page. Search inside document. Deepti Joshi. Justin Gan Jit Fook. The message signal which is to be transmitted to the receiver is like you and cannot travel for long distance by itself. This is the basic reason why we need to do modulation. In this fashion. Ln each range a typical application is only given as an example and is HOT exhaustive. Ji llowing l'ulues: Cellular telephony 3. Jntrod11 c: Even though these are not crisp boundaries.
It is typically measured in seconds sec. Table 1. For the classification purpose. Apart from this detailed classification. Frequency is defined as the number of oscillations per second and is measured ill hertz Hz.
TV Remote Frequency f Waveh! Each range is identified by end frequencies or wavelengths that differ by a factor of JOO kli z Bandwidth Bandwidth Bw is that portion of the EM spectrum occupied by a signal. Broadband Transmission Transmission ofm. Modulation In terms of signal and channel bandwidths. At this point. Broadband Signal Message signal tn its modulated frequency range.
Baseband Sig11a. EM waves travel at the speed of light in atmosphere or vacuum. Spectntnz The frequency domain representation of the given signal. Before trying to estimate the bandwidth of a modulated transmission. For wideband speech the frequency range is from O to Hz. 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.
Wavelength is defined as the distance travelled by an EM wave during the time of one cycle. A high-fidelity audio signal requires a range of 50 to Hz. Since such nonsinusoidal waves occur very frequently as modulating signals in communications.
Baseband Tra11smission Transmission of message signal in its original frequency range. When a carrier has been similarly modulated with each. Demodttlatiou On the similar lines. This will be made clear in Chapters 3 and 4.
Described mathematically in the time domain and in the frequency domain. If this consists of sinusoidal signals. I where v 1 "" voltage as a function of time E. A periodic waveform has amplitude and repeats itself during a specific time period T.
The form for the Fourier series is 11s follows: T27rl11 ] 1. Each term is a simple mathematical symbol and shall be explained as follows: This simpl. Next we will review the Fourier series. T represents time. Figure 1. Some examples of wav.. The symbolfin Equation 1. The makeup ofa square or rectangular wave is the sum of harmonics the sine wave components at various amplitudes. S Example 1. The Fourier coefficients for the rectangular waveform in Fig. Example 1. It may be shown that any nonsinusoidal.
Some non-sine wave recurring at a rate of times per second will consist of a Hz fundamental sine wave. 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.
For some wavefonns. F w nmx 8x! There are an i11finire number ofsuch harmonics. Indicate the false statement St likely amplitude decreases with the harmonic number to affect the signal cl. The preceding statement may be verified in any one of three different ways. Some fommlas for frequently encountered nonsinusoidal waves arc now given. In this case adding the appropriate sine-wave components.. Graphical synthesis may be used. It may be proved mathemati- cally by Fourier analysis.
An added advantage of this method. Fourier analysis c. In each case several of the hannonics will be reqai. Square wave: Modulation is used to c. Circle the letter preceding the ber of harmonics line that correctly completes each sentence. Indicate the false statement. In a communication system. As a general rule. The process of sending 4.
A message is composed ofunpredictable varia. Define noise. What are they? The acoustic channel is used for which of the d. The carrier performs certain functions in radio communications. Write the typical frequency ranges for the following classification of EM spectrum: Where is it most likely to affect the signal?
Which of the following steps is not included in d. What does modulation actually do to a me. List the basic functions of a radio transmitter and the corresponding functions of the receiver. The need for modula. An antenna in the standard broadcast AM band I0. All sound is concentrated from 20 Hz to frequency 20kHz b. Amplitude modulation is the process of is Mention the elements of a communication system.
From the transmitter and receiving started as early as the signal deterioration because of noise is usu. Explain the need for modulation. Indicate the true statement. UHF communications tion can best be exemplified by the following. Describe their functionality. It may be subdivided according to type. In radio receivers. Measuring it is very contentious: It is ever present and limits the perfonnancc of most systems. International satellite earth station It is most convenient here to divide noise into two broad groups: Noise can limit the range of systems.
External noise is difficult to treat quantitatively. After studying this chapter. Many disturbances of an electrical nature produce noise in receivers. Note how radiotelescopes are always located away from industry. It may sometimes even force a reduction in the bandwidth of a system. It affects the sensitivity of receivers.
The methods of calculating the noise produced by various:: Noise may be defined. Objectives Upon completing the material in Chapter 2. In television receivers "snow" or "confetti" colored snow becomes superimposed on the picture. Internal noise is both more quantifiable and capable of being reduced by appropriate receiver design. The very important noise quantities. There are numerous ways of classifying noise. Calculate noise levels for a variety of conditions using the equations in the text.
The usual increase in its level talccs place at night. The static is likely to be n16re severe but less frequent if the storm is local. It originates in the fonn of amplitude-modulated impulses.
The majority of these radfo waves come from natural sources of disturbance. Atmospheric noise consists of SpLtrious radio signals with components distributed over a wide range of freq uencies. Static is caused by lightning discharges in thunderstonns and other natural electric d.
An astonishing variety of strange sounds will be heard. Such noJse consists of impulses. Field strength is inversely proport ional to frequency. Most of these sounds arc the result of spurious radio waves which induce voltages in the antertna.
Atmospheric noise becomes Jess severe at frequencies above about 30 MHz because of two separate factors. For convenience. Static from distant sources will vary in intensity actord. Even though the additional noise produced comes from A limited portion of the sun's surface.
They represent atmospheric noise. As the theory states. 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.
We also receive noise from the center of our own galaxy the Milky Way. The nature of industrial noise is so variable that it is difficult to analyze it on any basis other than the sta- tistical. The noise received is called thennal or black-body noise and is distributed fairly uniformly over the entire sky. Under this heading. It is due to the rapid and random motion of the molecules atoms and electrons inside the component itself. In thenriodynamics. Not very much of it below 20 MHz penetrates down through the ionosphere.
The noise is produced by the arc discharge present in all these operations. Noise 17 can become significant. Summary Space noise is observable at frequencies in the range from about 8 MHz to somewhat above 1.
Fluorescent lights are another powerful source of such noise and therefore should not be used where sensitive receiver reception or testing is being conducted. Such noise is generally random. Random noise power is proportional to the bandwidth ove.. It must be realized that all fonn ulns referring to random noise arc applicable only to the m1s value of such noise.
A random voltage across the resistor definitely exists and may be both measured and calculated. Assmne that RL is noiseless and is receiving the maximum noise power generated by R. That is correct if the measuring instrument is a direct current de voltmeter.
Since it is random and therefore has a finite nns value but no de component. This noise voltage is caused by the random movement of electrons within the resistor. Then P. The resistor is a noise generatur. It is tnte that as many electrons arrive at one end of the resistor as at the other over any long period oftirne. Using Equation 2. At any instant of time. The rate of arrival of electrons at either end of the resistor therefore varies randomly.. In bipolar transistors. Although the average output current of a device is governed by tlle various bias voltages.
Hence the name shot noise. When amplified. This stems from the fact that it is random and therefore evenly distributed over the frequency spectmm. The most important of all the other sources is the shot effect. Example 2. A low voltage fed to this amplifier would be masked by the noise and lost. Noise 19 and V. V Fig. Note especially that the generated noise voltage is quite independent of the frequency at which it is measured.
The paths taken are random and therefore unequal. It is caused by rando. So far as the use of R is concerned. RF transistors are remarkably low-noise. The sum of two such nns voltages in series is given by the square root of the sum of their squares.
Once this high-frequency noise makes its presence felt. In addition. 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 Shot noise behaves in a similar manner to thennal agitation noise.
Many variables are involved in the generation of this noise in the various amplifying devices.
They all show that such noise is inversely proportional to transconductance and also directly proportional to output current. The result of all this is that it is preferable to measure noise at such high frequencies. A noise figure see Sect-ion 2. Approximate formulas for equivalent shot. For noise only. Tbc noise current has been replaced by a resistance so that it is now easier to add shot noise to thermal noi.
This precedes the device. The most convenient method of dealing with shot noise is to find the value or fortnula for an equivalent input-noise resistor. The value of the equivalent shot-noise resistance R. This greatly simplifies subsequent calculations. The process might then be continued.
This is. Solution V.
This means that the total noise voltage is less than that due to any of the individual resistors. Tt may appear logical to combine all the noise resistances at the input. It is even better to go one step further and find an equivalent resistance for such an input voltage. The bandwidth of the amplifier is 6 MHz.
The result JJ useless because the argument assumed that it is important to find the total output noise voltage. Now the noise resistance actually present at the input of the second stage is R2. Tnstead R. The same noise voltage would be present at the output if there were no R3 there.
The-series resistance of the coil. As Example 2. Consider Fig. The more interesting case is a tuned circuit which is not ideal.. To either side ofresonance the presence of the tuned circuit affects noise in just the same way as any other voltage. Calculate the equivalent input-noise 1'esistance of this two-stage amplifier. In the preceding sections dealing with noise calculations. Noise 23 ll.
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For the second stage. I required to determine the noise voltage across the capacitor. This will allow us. The second 1s companson of n01se and signal at the same point to ensure that the n01se 1s. The noise current in the circuit will be. Thus i. Equation 2. It is defined as the ratio of signal power to noise power at the same point.
Instead of equivalent noise resistance. In addi- tion. In the second instance. Therefore 2. As a matter offact. For example.
Electronic Communication Systems
At frequencies higher than that. 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.
It is fed from a source antenna of internal impedance R. Each is treated as a four-tenninal network having an input impedance R1. 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.
Noise 25 not excessive. An effort is naturally made to keep the signal. Calculate the generalized form of noise figure from steps 3 and 6 2. Each is now shown. It is seen from Fig. Determine the signal output power P.
Calculate Pno from Rcq if possible 2. Rt gain"' A Fig. The calculation procedure may be broken down into a number of general steps. Write P for the noise output power to be determined later 2. Note that Equation 2. It is convenient to define R: Putting it another way. For the ti.
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An actual fonnula for F may now be obtained by substitution for the output noise power.. 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. Noise 27 2. Under matched conditions R.. This is a situation exploited very often in prac6ce.
Note that this constitutes n lnrgc enough mismatch. Not the least reason for its use is convenience. Controversy exists regarding which is the better all-around measurement.
This may be seen from reexamining Equation 2. In defining the equivalent noise temperature of a receiver or amplifier. If this is to lead to the conect value of noise output - power. Tcq' the equivalent noise temperature.
Finally we have. It will be recalled that the equivalent noise resistance introduced in Section 2. 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. T0F -. Noise 29 P. It is then possible to use Equation' Noise figure c.
Solar noise line that correctly completes each sentence. Which two broad classifications of noise are the width. The value of a resistor creating thermal noise is 8. It is the d. Atmospheric noise great importance at high frequencies. Equivalent noise resistance vaJues. Circle the letter preceding the a. One of the following types of noise becomes of c. When dealing with random noise calculations it comparing the noise performance of receivers: HF mixers are generally noisier than HF ampli.
The square of the b. Noise temperature b. One of the following is not a useful quantity for 9. The noise power generated is therefore spectrum. Input noise voltage a. Space noise generally covers a wide frequency doubled. Cosmic noise L. Thermal noise is independent of the frequency a. Galactic noise a. Boltzmann's constant fiers. Industrial noise is usually o the impulse c.
Indicate the noise whose source is in a category consists ofan incomplete statement followed by four different from that oftbe other three. If this receiver is connected to an antenna with an impedance of 75 fl.
When might the latter. Which of lhe following statements is tme? Review Problems I. What does the meter read now? The front end of a television receiver. A meter connected to the output of the amplifier reads I mV rms.
Noise in mixers is caused by inadequate image c. Define signal-to-noise ratio and noise figure ofa receiver. Calculate the minimum signal voltage that the receiver of Problem 2. How can some of them be avoided or minimized? What is the strongest source of extraterrestrial noise? Flicker is sometimes called demodulation a. The noise output of a resistor is amplified by a noiseless amplifier having a gain of 60 and a bandwidth of20 kHz.
A parallel-tuned circuit. The RF amplifier of a receiver has an input resistance of l n. Review Questions I. Describe briefly the forms of noise to which a transistor is prone. Noise 31 JO. An amplifier operating over the frequency range of to kHz bas a kfl input resistor. Discuss the types. The amplifier has a input resistor and a shot-noise equivalent resistance of fl. Random noise power is inversely proportional surement for comparing amplifier noise charac. Which of the following is the most reliable mea.
A random voltage across a resistance cannot Given that the bandwidth is 1. Write the expression for therms noise voltage. What is noise temperature'?
How is it related to noise figure? A receiver has an overall gain A. One of the terms of this formula will be the noise output power. Describe briefly how this can be measured using the diode generator. Derive the relation between noise figure and temperature. Write the relation for maximum noise power output of a resistor. What is ideal and practical values of noise figure? Why they arc so explain. What is transit-time effect? How it is generated? Based on this observation. Upon studying this chapter.
Understand the differences between AM and its variants. After studying the theory of amplitude modulation techniques. This is based on how many components of the basic amplitude modulated signal are chosen for transmission. The amplitude of the carrier signal is varied in accordance with the message to obtain modulated signal in case of amplitude modulation. Objectives Upon completing the material in Chapter Tbis chapter deals with the amplitude modulation techniques employed in analog communication.
This clas- sification is mainly based on the nature of message or modulating signal. The next chapter deals with angle modu.
Explain different approaches for the generation of AM. The angle modulation employs variation of angle of the carrier signal in proportion to the message. Like electronics and communication by George Kennedy etc. Please send me a link. Systems George Kennedy on Amazon. FREE shipping on qualifying offers. George Kennedy and Bernard Davis, Electronics. George Kennedy, Bernard Davis, , Sept. Electronics and communication System by George kennedy. Aircraft Materials and Processes by George F.
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