Review of Spectral Characteristics of Periodic and Aperiodic signals-generation and

Transcription

1 CHETTINAD COLLEGE OF ENGINEERING & TECHNOLOGY DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING Faculty Name: Ms.P.Nalini Class/Sem : II Year / IV Sem Subject Name: Communication Theory Subject code: EC 2252 EC2252 COMMUNICATION THEORY UNIT I AMPLITUDE MODULATION SYSTEMS Review of Spectral Characteristics of Periodic and Aperiodic signals-generation and Demodulation of AM-DSBSC,SSB and VSB Signals, Comparison of Amplitude Modulation Systems, Frequency Translation, FDM, Non-linear distortion OBJECTIVE: To study about the amplitude modulation techniques. CONTENTS PAGE NO HISTORY OF COMMUNICATION SYTEMS....2 INTRODUCTION PERIODIC AND APERIODIC SIGNALS AMPLITUDE MODULATION Modulation Need for Modulation Introduction To AM History Of AM Principles of Amplitude Modulation AM GENERATION Frequency Spectrum & Bandwidth Modulation Index and Percent Modulation DSBSC (Double SideBand Suppressed Carrier) Full AM or DSBLC (Double SideBand Large Carrier) Demodulation of DSBLC signals SSBSC (Single Sideband Suppressed Carrier) To demodulate SSBSC signal Types Of SSB Generation 30 1

2 1.6 RECOVERING THE MODULATED INFORMATION FROM AN AM SIGNAL AM VESTIGIAL SIDEBAND (VSB) FDM-FREQUENCY DIVISION MULTIPLEXING NONLINEAR DISTORTION..41 Summary..44 Keyterms..45 Multiple choice questions 46 Review Questions with answers PART-A..47 PART-B..50 Self check Questions.56 HISTORY OF COMMUNICATION SYTEMS: 1831 Samuel Morse invents the first repeater and the telegraph is born 1837 Charles Wheatstone patents "electric telegraph" 1849 England to France telegraph cable goes into service -- and fails after 8 days Morse patents "clicking" telegraph England-France commercial telegraph service begins. This one uses gutta-percha, and survives August 18 - First transatlantic telegraph messages sent by the Atlantic Telegraph Co. The cable deteriorated quickly, and failed after 3 weeks The first transcontinental telegraph line is completed 1865 The first trans-atlantic cable goes in service 1868 First commercially successful transatlantic telegraph cable completed between UK and Canada, with land extension to USA. The message rate is 2 words per minute The trans-atlantic message rate is increased to 20 words per minute Baudot invents a practical Time Division Multiplexing scheme for telegraph. Uses 5-bit codes & 6 time slots bps max. rate. Both Western Union and Murray would use this as the basis of multiplex telegraph systems Typewriter invented. 2

3 1876 Alexander Graham Bell and Elisa Grey independently invent the telephone (although it may have been invented by Antonio Meucci as early as 1857) 1877 Bell attempts to use telephone over the Atlantic telegraph cable. The attempt fails Oliver Heaviside's analysis shows that a uniform addition of inductance into a cable would produce distortionless transmission Test calls placed over five miles of under-water cable San Francisco-Oakland gutta-percha cable begins telephone service Alexander Graham Bell incorporated AT&T 1885 James Clerk Maxwell predicts the existence of radio waves 1887 Heinrich Hertz verifies the existence of radio waves 1889 Almon Brown Strowger invents the first automated telephone switch 1895 Gugliemo Marconi invents the first radio transmitter/receiver 1901 Gugliemo Marconi transmits the first radio signal across the Atlantic 1901 Donald Murray links typewriter to high-speed multiplex system, later used by Western Union 1905 The first audio broadcast is made 1910 Chesapeake Bay cable is first to use loading coils underwater 1911 The first broadcast license is issued in the US 1912 Hundreds on the Titanic were saved due to wireless 1915 USA transcontinental telephone service begins (NY-San Francisco) The first video signal is broadcast 1927 First commercial transatlantic radiotelephone service begins 1929 The CRT display tube is invented 1935 Edwin Armstrong invents FM 1939 The blitzkrieg and WW II are made possible by wireless 1946 The first mobile radio system goes into service in St. Louis 1948 The transistor is invented 3

4 1950 Repeatered submarine cable used on Key West-Havana route The first trans-atlantic telephone cable, TAT-1, goes into operation. It uses 1608 vacuum tubes The first artificial satellite, Sputnik goes into orbit 1968 The Carter phone decision allows private devices to be attached to the telephone 1984 The MFJ (Modification of Final Judgement) takes effect and the Bell system is broken up 1986 The first transatlantic fiber optic cable goes into service INTRODUCTION: Introduction to Communication Systems Communication Basic process of exchanging information from one location (source) to destination (receiving end). Refers process of sending, receiving and processing of information/signal/input from one point to another point. FLOW OF INFORMATION SOURCE DESTINATION Fig -A simple communication system Electronic Communication System defined as the whole mechanism of sending and receiving as well as processing of information electronically from source to destination. Example Radiotelephony, broadcasting, point-to-point, mobile communications, computer communications, radar and satellite systems. Communication System to produce an accurate replica of the transmitted information that is to 4

5 transfer information between two or more points (destinations) through a communication channel, with minimum error. NEED FOR COMMUNICATION Interaction purposes enables people to interact in a timely fashion on a global level in social, political, economic and scientific areas, through telephones, electronic-mail and video conference. Transfer Information Tx in the form of audio, video, texts, computer data and picture through facsimile, telegraph or telex and internet. Broadcasting Broadcast information to masses, through radio, television or teletext. Key Terms Related To Communications Message physical manifestation produced by the information source and then converted to electrical signal before transmission by the transducer in the transmitter. Transducer Device that converts one form of energy into another form. Input Transducer placed at the transmitter which convert an input message into an electrical signal. Example Microphone which converts sound energy to electrical energy. Message Input Transducer Electric Signal Output Transducer placed at the receiver which converts the electrical signal into the original message. Example Loudspeaker which converts electrical energy into sound energy. Electric Signal Output Transducer Message Signal electrical voltage or current which varies with time and is used to carry message or information from one point to another. Elements of a Communication System The basic elements are : Source, Transmitter, Channel, Receiver and Destination 5

6 Function of each Element Information Source the communication system exists to send messages. Messages come from voice, data, video and other types of information. Transmitter Transmit the input message into electrical signals such as voltage or current into electromagnetic waves such as radio waves, microwaves that is suitable for transmission and compatible with the channel. Besides, the transmitter also do the modulation and encoding (for digital signal). Block Diagram of a Transmitter Channel/Medium is the link or path over which information flows from the source to destination. Many links combined will establish a communication networks. There are 5 criteria of a transmission system; Capacity, Performance, Distance, Security and Cost which includes the installation, operation and maintenance. 2 main categories of channel that commonly used are; line (guided media) and free space (unguided media) Receiver Receives the electrical signals or electromagnetic waves that are sent by the transmitter through the channel. It is also separate the information from the received signal and sent the information to the destination. Basically, a receiver consists of several stages of amplification, frequency conversion and filtering. 6

7 1.1 Periodic and Aperiodic Signals: In data communications, we commonly use periodic analog signals and nonperiodic digital signals. Periodic: completes a pattern within a measurable time frame (period), and repeat the pattern over subsequent identical period. Nonperiodic: changes without exhibiting a pattern. Comparison of analog and digital signals NOTE: In data communication, we commonly use periodic analog signals and aperiodic digital signals. Periodic Analog Signals Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves. A sine wave, each cycle consists of a single arc above time axis followed by a single arc below it. A sine wave, can be represented by three parameters: Peak Amplitude Frequency Phase Peak Amplitude Peak Amplitude of a signal is the absolute value of its highest intensity, proportional to the energy it carries. 7

8 Period and frequency Period refers to the amount of time, in seconds, a signals need to complete 1 cycle. Frequency refer to the number of periods in 1 s. Frequency and period are inverses of each other. F=1/T Units of periods and frequencies: Unit Equivalent Unit Equivalent Seconds (s) 1 s hertz (Hz) 1 Hz Milliseconds (ms) 10 3 s kilohertz (KHz) 103 Hz Microseconds (ms) 10 6 s megahertz (MHz) 106 Hz Nanoseconds (ns) 10 9 s gigahertz (GHz) 109 Hz Picoseconds (ps) s terahertz (THz) 1012 Hz 8

9 Example-1 Express a period of 100 ms in microseconds, and express the corresponding frequency in kilohertz. Soln: From Table we find the equivalent of 1 ms.we make the following substitutions: 100 ms = s = ms = 105 ms Now we use the inverse relationship to find the frequency, changing hertz to kilohertz100 ms = s = 10-1 s f = 1/10-1 Hz = KHz = 10-2 KHz Periodic & Nonperiodic signal:: Periodic analog signals Periodic analog signals can be classified as Simple sine wave (cannot be decomposed into simpler signals) Composite composed of multiples sine waves Sine wave Sine wave can be represented by 3 parameters peak amplitude, frequency and the phase Sine wave examples 9

10 Wavelength Wavelength describes how far the wave can travel in 1 period time. Depends on the frequency and the medium. Propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal λ: wavelength, f : frequency, c : speed of light 3 x 108 ms-1 c Bandwidth f The bandwidth is a property of a medium: It is the difference between the highest and the lowest frequencies that the medium can satisfactorily pass. 10

11 Example 3: If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is the bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V. Soln: B = fh - fl = = 800 Hz The spectrum has only five spikes, at 100, 300, 500, 700, and 900 : Example 4: A signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all integral frequencies of the same amplitude Soln: B = fh - fl 20 = 60 - fl fl = = 40 Hz 11

12 1.2 AMPLITUDE MODULATION Modulation: Introduction: Information signals are transported between transmitter and receiver through a transmission medium. Original information must be transformed into a suitable form for transmission. Modulation is a process of impressing low frequency information signals onto a high frequency carrier signal. The device used for modulation process is called Modulator Demodulation is the reverse process of the modulation where the received signals are transformed back to their original form. The device used for the demodulation process is called Demodulator. Modulation is the process of varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Often a high-frequency sinusoid waveform is used as carrier signal to convey a lower frequency signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device the performs the inverse operation of modulation is known as a demodulator or detector. A device that can do both operations is a modem (short for "Modulator-Demodulator") Analog modulation methods: In analog modulation, the modulation is applied continuously in response to the analog information signal 12

13 A low-frequency message signal (top) may be carried by an AM or FM radio wave. Common analog modulation techniques are: Amplitude modulation (AM) (here the amplitude of the modulated signal is varied) Double-sideband modulation (DSB) Double-sideband modulation with unsuppressed carrier (DSBWC)(used on the AM radio broadcasting band) Double-sideband suppressed-carrier transmission (DSB-SC) Double-sideband reduced carrier transmission (DSB-RC) Single-sideband modulation (SSB, or SSB-AM), SSB with carrier (SSB-WC) SSB suppressed carrier modulation (SSB-SC) Vestigial sideband modulation (VSB, or VSB-AM) Quadrature amplitude modulation (QAM) Need for Modulation: 1) To reduce the antenna height 2) To multiplex the more number of signals 3) To reduce the noise and distortions 4) To narrow banding the signals 5) To reduce equipment complexity Introduction To Am There are three ways to put information on an RF carrier: AM( Amplitude Modulation) FM( Frequency Modulation) PM( Phase Modulation) 13

14 1.2.4 History Of AM: Of the three, amplitude modulation (AM) was the first to be used for communications, probably because AM has the simplest transmitter and receiver circuits. Marconi's original transatlantic radio transmissions that took place in the first decade of the 20th century used a primitive form of AM. Morse code was sent by switching the carrier wave on and off. Incidentally, this was also the first digital modulation technique! To generate AM, it is necessary to multiply two signals together: 1. an RF sine wave, of frequency fc and amplitude Vc0, known as the carrier 2. the audio signal (information) that is to be transmitted. For simplicity's sake we will consider the audio signal to be a pure sine wave of frequency fm and amplitude Vm0. Our results will, however, apply to any modulating signal. This multiplication is accomplished be using a circuit known as a modulator. Please note that we are not summing the two signals, we are multiplying them. A modulator is a circuit with a non-linear response. The output is not merely the sum of all the inputs, multiplied by a gain factor, but also includes combinations of the input signals. A typical AM modulator output consists principally of the following signals: the carrier at frequency fc an upper sideband at frequency fc+ fm a lower sideband at frequency fc- fm There are some other signals present, such as harmonics of the carrier, but the modulator is designed to keep these other signals very small. The simplest and historically earliest form of radio was the transmission of Morse code by simply switching a carrier ON and OFF. The carrier was generated by applying a series of pulses to a tuned circuit by means of a spark gap. This is a form of AM but not suitable for audio transmission. Reginald Aubrey Fessenden, a prolific radio inventor and engineer made an attempt to develop AM radio using the vacuum tube. After many unsuccessful tries, Fessenden transmitted a few words by using a spark gap transmitter which produces 10,000 sparks per second with a carbon microphone connected in series with the antenna. A continuous transmission is possible by this. 14

15 1.2.5 Principles of Amplitude Modulation: Amplitude Modulation is a process of changing the amplitude of a relatively high frequency carrier signal in proportion with the instantaneous value of the modulating signal (information). AM is a linear modulation process since the output envelope produces a linear variation with respect to the input modulating signal. AM Modulators are non-linear devices with two inputs and one output. One input is a single high frequency carrier signal of constant amplitude. Another input is comprised of relatively low frequency information signals which may be a single frequency or a complex waveform of many frequencies. Radio Frequencies (RF) are high frequencies which are efficiently radiated by an antenna and propagated through free space. In Modulator, the information acts on RF carrier producing a modulated waveform. Information signal is a single frequency or a range of frequency. Eg:Typical voice grade communication system (300 Hz to 3000 Hz). AM The graphs below show the carrier, modulating signal and the resulting AM signal produced by the modulator. Note that the amplitude of the modulator output follows the amplitude of the modulating signal. 15

16 We can also look at AM signals in the frequency domain. Here is the frequency of the unmodulated carrier. I contains a single peak at the carrier frequency, 10 KHz. The modulating signal also consists of a single peak. However, this peak is located at 400 Hz, the modulating frequency: 16

17 The output of the modulator consists of 3 frequencies. The carrier is still present but there are also two sidebands, each separated from the carrier by the value of the modulating frequency. In this example, the modulating frequency is 400 Hz and the carrier frequency is 10 KHz. After modulation, there are 3 frequencies: a 10 KHz carrier, a 9.6 KHz lower sideband and a 10.4 KHz upper sideband The sidebands and the carrier all have constant amplitudes, yet their sum has a time varying amplitude. This happens because there is a continuously changing phase difference between the two sidebands and the carrier. When all three are in phase, the modulated signal reached its maximum value. When both sidebands are completely out of phase with the carrier, the modulated signal reaches its minimum value. 1.3 AM GENERATION: Amplitude Modulation Double Side Band Full Carrier (AM DSB FC) is the most commonly used AM, which is also called as conventional AM. For conventional AM, Carrier Signal is VC (t) Modulating Signal is Vm(t) Modulated Wave is Vam(t) A single frequency modulating signal acts on a high frequency carrier signal to produce an AM Waveform. The output waveform contains all frequencies that make up AM signal. AM Envelope is the modulated output waveform from the AM modulator. 17

18 The shape of the modulated wave is called AM envelope. With no modulating signal, the output waveform is the carrier signal. Figure: AM Generation When a modulating signal is applied, the amplitude of the output wave varies in accordance with the modulating signal. The reception rate of the envelope is equal to the frequency of the modulating signal. The shape of the envelope is identical to the shape of the modulating signal. 18

19 1.3.1 Frequency Spectrum & Bandwidth: AM Modulator is a non-linear device since a non-linear mixing occurs in the modulation process. The output envelope is a complex wave made up of a DC voltage, carrier frequency, sum (fc + fm) and the difference (fc - fm). The sum and difference are displaced from fc by an amount equal to fm. AM Signal Spectrum contains the frequency components spaced fm Hz on either side of the carrier. Modulated Wave does not contain a frequency component equal to the modulating signal frequency. fc. Modulation translates the modulating signal in frequency domain so that it is reflected symmetrically about AM Spectrum extends from fc - fm(max) to fc + fm(max) modulating signal frequency. Where fc is the carrier frequency,fm(max) is the highest Lower Side Band (LSB) extends from fc - fm(max) to fc. Upper Side Band (USB) extends from fc to fc+ fm(max). Lower Side Frequency (LSF) is any frequency in LSB. Upper Side Frequency (USF) is any frequency within USB. 19

20 Bandwidth of AM DSB FC is the difference between USFmax and LSFmax or it is given by Bandwidth, B = 2 fm(max) (1.1) For radio wave propagation, fc and all frequencies in LSB and USB must be high to propagate through earth s atmosphere Modulation Index and Percent Modulation: Modulation Index or coefficient of modulation is the amount of amplitude change in AM waveform. m = Em / EC (1.2) where m is the modulation coefficient (unitless). Em is the peak change in amplitude of output waveform voltage (volts). EC is the peak amplitude of unmodulated carrier voltage (volts). From the equation (1.2), Em = m EC or EC = Em / m (1.3) Percent Modulation is the coefficient of modulation stated as percentage.it gives the percentage change in amplitude of the output wave when carrier is acted on by a modulating signal. M = (Em / EC) x 100 or M = m x 100 (1.4) Under Modulation Maximum (Perfect Modulation) Over Modulation Figure : AM Envelope for Different Values of Modulation Indices 20

21 Over Modulation results in the intersection of the two side band signals,so it is not possible to reconstruct the same original modulating signal in the demodulator. If the modulating signal is a pure single frequency sine wave and the modulation process is symmetrical (positive and negative excursions of the envelopes amplitude are equal), then Figure :Modulation Coefficient, Em and Ec 21

22 Also Em = Eusf + Elsf (1.8) where Em is the peak change in amplitude of output wave. Eusf is the voltage of USF or peak amplitude of USF (volts). Elsf is the peak amplitude of LSF (volts). Eusf = Elsf = (Em / 2) = (Vmax Vmin) / 4 (1.9) For 100% modulation, m=1, Em = EC, Vmin = 0 V. (1.10) This is the maximum percentage modulation without distortion. For 50% modulation, Em = (EC / 2) (1.11) Percent Modulation is also expressed as peak change in voltage of modulated wave with respect to the peak amplitude of unmodulated carrier, given by, Figure : Relationship between Modulation Index, Em and Ec 22

23 1.4 DSBSC (Double SideBand Suppressed Carrier): Double-sideband suppressed-carrier transmission (DSB-SC):transmission in which (a) frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and (b) the carrier level is reduced to the lowest practical level, ideally completely suppressed. In the double-sideband suppressed-carrier transmission (DSB-SC) modulation, unlike AM, the wave carrier is not transmitted; thus, a great percentage of power that is dedicated to it is distributed between the sideband, which implies an increase of the cover in DSB-SC, compared to AM, for the same power used. DSB-SC transmission is a special case of Double-sideband reduced carrier transmission. This is used for RDS (Radio Data System) because it is difficult to decouple. To generate: Simply use a mixer to multiply the RF carrier by the base band (audio ) signal. The transmitted signal then looks like: (Note the phase reversal at the zero crossings of the audio signal.) 23

24 To demodulate: Multiply by the carrier again and low pass filter to remove the double frequency term in the mixer output. For DSBSC this carrier provided by a local oscillator in the receiver must be at exactly the same frequency and phase as the carrier on which the signal enters the receiver. This makes DSBSC impractical in many applications since it requires either the transmission of a pilot carrier or the use of a more complicated form of PLL (a Costas loop) to recover the carrier from the received signal. 24

25 The final audio output is the same as the audio input in the transmitter Full AM or DSBLC (Double SideBand Large Carrier) Generation of DSBLC can be done in a number of ways including the direct modulation of the amplitude of an oscillator output. It can also be generated by first creating DSBSC and then adding the carrier to the mixer output. 25

26 The modulation index, m, controls the depth of modulation and the amplitude of m times s(t) must never exceed Demodulation of DSBLC signals The d.c. level in the output of this envelope detector is removed by a.c. coupling and the audio signal can then be amplified. 26

27 Square Law Detection Detection (demodulation) can also be achieved by biasing a diode to act as a square law detector. Squaring the received signal gives: Low pass filtering then gives: The squaring process is effectively a mixing process in which every frequency component in the signal mixes with every other frequency component. It is to just mix every signal frequency component with the carrier, which is the same as the mixing process used in DSBSC demodulation. However, unless the carrier is very much larger than every signal frequency component, the results of mixing frequency components in the signal with other frequency components are significant. To get a good demodulation from this process we want the carrier component in the input signal to the demodulator to be very large. However having a large carrier component relative to the signal sidebands, which involves a very small modulation index, is a very inefficient use of power in the RF transmission - it involves transmitting largely unmodulated carrier that carries no information. The solution is to transmit a signal with a large modulation index (e.g. 0.9) but to include a phase locked loop carrier recovery circuit in the receiver to generate a strong additionalcarrier component to mix with the incoming signal. This process is obviously morecomplicated and expensive but can produce a better quality output than the simple envelope detector. 27

28 1.5 SSBSC (Single Sideband Suppressed Carrier) There are two ways to generate a SSBSC modulated signal. It is used to generate DSBSC by mixing and then to use a filter to select either the upper or the lower Sideband This is very demanding on the filter used after the mixer: it must have a very steep roll off to pass all of the required sideband while stopping all of the other sideband. If the carrier frequency used in this operation is a standard IF frequency, 28

29 then good single sideband filters are readily available.there is another method of generating SSBSC that gets around this problem by using a single sideband mixer which actually comprises two mixers and some phase shifting circuitry. An analysis of this technique requires an understanding of Hilbert transforms, and is beyond the scope of this module To demodulate SSBSC signals We simply multiply by the carrier again and low pass filter, to remove the double frequency term in the mixer output, exactly as for DSBSC modulation For SSBSC the frequency and phase requirements of the carrier provided by a local oscillator in the receiver are rather less stringent than for DSBSC which is the required audio signal plus a double carrier frequency component. Low pass filtering will then yield the required audio signal. The output of the mixer, plotted in the time domain looks something like: In the frequency domain this mixing and filtering demodulation process involves: 29

30 1.5.2 Types Of SSB Generation An AM signal contains a carrier and two sidebands, each of which contains all the information that was in the modulating signal. Also, as we shall see in a later section, most of the power in an AM signal is in the carrier, which contains no information. It would be a better use of bandwidth and power to send just one of the sidebands, without the carrier. Such an AM signal, consisting of only one sideband, is known as a single sideband suppressed carrier signal, which is thankfully often shortened to SSB. There are four ways to generate an SSB signal, two of which we will look at in more detail: the filter method and the phasing method. 30

31 The Filter Method The filter method uses a special modulator known as a balanced modulator to generate an AM signal that has both sidebands and no carrier. This type of AM is known as double sideband suppressed carrier, or DSB. Then the DSB signal is passed through a filter that removes the unnecessary sideband. Fig:SSB Modulator, Block Diagram The filter method is quite simple to explain, but when one tries to build a SSB modulator using the filter method, some problems appear. The first is that the balanced modulator circuit must be completely balanced electrically, or some carrier will appear in the output. To achieve good balance, some type of diode ring mixer is usually used, as shown below. 31

32 Doubly Balanced Mixer The second problem is design and construction of filters that will pass the desired sideband and not the other. In order to select the desired sideband, two band pass filters are required; one that passes only the lower sideband and another that passes only the upper sideband. We have to have the capability to keep either sideband because some communications services use only the upper sideband while others use only the lower. The audio signals transmitted via SSB typically have a bandwidth of Hz, which leaves only 600 Hz between the sidebands as shown below: 32

33 The carrier frequency is generally above 1 MHz, so the filter must have very sharp selectivity to pass only one of two signals whose frequencies differ by less than 0.1%. The filter requirement imposes a constraint on the design of an SSB transmitter: it is not practical to change the operating frequency of the transmitter by directly changing the carrier frequency at the balanced modulator. This would require a filter that had both high selectivity and a variable pass band, which is very difficult to do. Instead, the SSB signal is generated at a fixed carrier frequency that is usually much lower than the final operating frequency.. The output of the SSB modulator is then fed into a mixer and mixed with a variable frequency RF signal that up converts the SSB signal to the desired operating frequency. The operating frequency is changed by changing the frequency of the variable frequency oscillator. The Phasing Method The phasing method of SSB generation does not require any filters, but the circuitry is more complex, as shown in the diagram below: The degree of carrier suppression is dependent on the accuracy of the phase shift networks as shown in the table below: 33

34 In most applications, the carrier must be suppressed at least 40 db, which limits the phase error to approximately 1 degree. Designing a phase shift network for the carrier frequency is not difficult; the carrier signal is fixed in frequency and has a narrow bandwidth. On the other hand, providing a constant 90 degree phase shift over the entire audio pass band ( Hz) is more challenging. In spite of this difficulty, the phasing method was the first method to be implemented, during the 1950's. As filter technology improved, the phasing method was abandoned the circuitry associated with the filter method was simpler and less expensive. 1.6 RECOVERING THE MODULATED INFORMATION FROM AN AM SIGNAL Detection, or demodulation, is the process by which information is recovered from a modulated RF signal. For AM, demodulation works very much like modulation. Recall that AM was generated by mixing the carrier and information signals in a nonlinear device. The non-linear device had the following outputs: 1. The carrier frequency 2. The modulating frequencies 3. The sum of the carrier and modulating frequencies (upper sideband) 4. The difference of the carrier and the modulating frequencies (lower sideband). In general, the output of a mixer contains the sum and difference of the two input frequencies. If we put an AM signal into a non-linear device and mix the carrier and the two sidebands, we get the following outputs: 1. The carrier 34

35 2. The upper sideband 3. The lower sideband 4. The sum of the upper sideband and lower sideband 5. The sum of the carrier and upper sideband 6. The sum of the carrier and lower sideband 7. The difference of the upper and lower sidebands 8. The difference of the carrier and the lower sideband (the original modulating signal) 9. The difference of the upper sideband and the carrier (the original modulating signal) Of the 9 output signals, 2 are the original modulating signal (audio), while the others are RF signals of much higher frequency. By filtering the output of the non-linear device through a low pass filter, only the modulating (audio) signals will pass through to the next stage. The diagram below shows the input and output signals from a non-linear device. This relatively complex process can be realized by a very simple device: the diode. The diode, which only allows current to pass in one direction, is a very non-linear device. By rectifying the RF signal with a diode and filtering the output, audio can be recovered from an AM RF signal. This is exactly the type of detector used in the earliest receivers, the crystal sets. The metal cat's whisker that touched the surface ofthe galena crystal formed a Schottky diode (although it was not called that at the time - German physicist Walther Schottky did not publish his work on metal semiconductor diodes until 1938). The capacitor paralleled across the headphones provided the low pass filtering that recovered the audio. The diode detector is very simple and is still used widely today for AM reception. However, it does have some drawbacks: 1. A diode detector has no gain - in fact losses through a diode detector can be 6dB or more, requiring more amplifier stages ahead of the diode. 2. A diode detector is so non-linear that additional distortion is introduced into the recovered signal. 35

36 3. The requirement for a low pass filter at the output limits the diode detector's ability to handle wide bandwidth waveforms. To address these drawbacks, the synchronous detector was developed. The synchronous detector is a specially designed mixer whose inputs are the AM signal and a locally generated signal of the same frequency and phase as the carrier signal. The outputs of this mixer are: 1. The difference of the carrier signal and the local signal = 0 Hz 2. The difference of the upper sideband and the locally generated signal = the original audio signal 3. The difference of the locally generated signal and the lower sideband = the original audio signal 4. The sum of the locally generated carrier and the AM signal = an AM signal at twice the frequency of the input AM signal. The additional AM signal generated in the synchronous detector is at a much higher frequency than the original carrier and can be filtered out quite easily. The remaining outputs are the audio signals that were present in the sidebands of the original signals. The synchronous detector can provide gain, and can also demodulate wideband signals. Distortion levels of less than 1% can be realized through careful design. While this detector cures most of the drawbacks of the diode detector, it requires significantly more complex circuitry and is generally used only where wide bandwidth is required. For most other applications, the diode detector is used. The diagram below shows a basic synchronous detector and the input and output signals. 36

37 1.7 AM VESTIGIAL SIDEBAND (VSB) Carrier is transmitted with full power One complete sideband is also transmitted Only part of the second sideband is transmitted Lower modulating signal frequencies are transmitted double sideband and the higher modulating signals are transmitted single sideband Thus lower sideband experience 100% modulation while the upper sideband cannot achieve more than 50% modulation Television broadcasts (regardless of NTSC, PAL, or SECAM analog video format) use this method if the video is transmitted in AM, due to the enormous bandwidth used. Transmits USB or LSB and vestige of other sideband Reduces bandwidth by roughly a factor of 2 Generated using standard AM or DSBSC modulation, then filtering Standard AM or DSBSC demodulation VSB used for image transmission in TV signals 37

38 where W is the bandwidth of the message. 1.8.FDM-Frequency Division Multiplexing FDM is an analog multiplexing technique that combines signals. FDM is an analogue technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted. Signals generated by each sending device use separate bandwidth ranges of the link and these ranges are the channels through which the signals travel. Channels must be separated by strips of unused bandwidth (guard bands) to prevent signals from overlapping 38

39 FDM process FDM demultiplexing example EXAMPLE 5: Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands. Solution Shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure shown below 39

40 EXAMPLE 6: Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 x x 10 = 540 KHz, as shown in Figure shown below 40

41 1.9 NONLINEAR DISTORTION: Memory - Present output value ~ function of present input + previous input values - contain L & C No memory - Present output values ~ function only of its present input values. Circuits : linear + no memory resistive ciruits - linear + memory RLC ciruits (Transfer function) Assume no memory Present output as a function of present input in t domain If the amplifier is linear 41

42 42

43 43

44 Summary: Modulation is a process of impressing low frequency information signals onto a high frequency carrier signal. Demodulation is the reverse process of the modulation where the received signals are transformed back to their original form. Digital modulation is to transfer a digital bit stream over an analog pass band channel. Analog modulation is to transfer an analog base band (or low pass) signal. Amplitude Modulation is a process of changing the amplitude of a relatively high frequency carrier signal in proportion with the instantaneous value of the modulating signal. Double-sideband suppressed-carrier transmission (DSB-SC): transmission in which (a) frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and (b) the carrier level is reduced to the lowest practical level, ideally completely suppressed. In Vestigial sideband Carrier is transmitted with full power 44

45 Key terms: A Amplitude Modulation(12) Analog Modulation(12) B Bandwidth(10) C Communication (4) Channel(6) D DSBSC -Double SideBand Suppressed Carrier (23) Doubly Balanced Mixer(32) F Frequency(8) FDM-Frequency Division Multiplexing(38) I Information signal(15) Inter Modulation distortion(43) R Receiver(6) Radio Frequencies(15) S SSBSC -Single Sideband Suppressed Carrier(28) T Transmitter(6) U Under Modulation(20) USB-Upper Side Band(19) V VSB-Vestigial SideBand (37) W Wavelength(10) L LSB-Lower Side Band(19) M Modulation Modulation index(20) Medium(6) N Non Linear Distortion(41) O Over Modulation(20) P Periodic(7) Percent Modulation(20) 45

46 MULTIPLE CHOICE QUESTIONS: 1) is the process of impressing low frequency information signals on to a high frequency carrier signal a)modulation (b)demodulation (c)multiplexing (d)demultiplexing 2) is the process where the received signals are transformed back to their original form (a)modulation (b)demodulation (c)multiplexing (d)de-multiplexing 3) AM modulators are devices (a)linear (b)non-linear (c)complex (d)segregate 4)The modulated output waveform from an AM modulator is called as (a)fm wave (b)pm wave (c)am envelope (d)encapsule 5)The other name for AM envelope is called as (a)dsp-hc (b)dsp-fc (c)dsb-fc (d)dsb-hc 6)The band of frequencies between fc - fm(max) and fc is called (a)u.s.b (b)l.s.b (c)u.s.f (d)l.s.f 7)The band of frequencies between fc and fc + fm(max) is called (a)u.s.b (b)l.s.b (c)u.s.f (d)l.s.f 8) Amplitude modulation is a modulation. i) Linear ii) Non-linear iii) Active iv) Passive 9) When there is no modulating signal, the output waveform of the Amplitude modulator is signal. i) Modulated ii) Modulating iii) Carrier iv) Unmodulated 10) For 0% modulation, the total transmitted power is equal to. i) Signal power ii) Half of carrier power iii) Half of signal power iv) Carrier power ANSWERS: 1. Modulation 2. Demodulation 3.Non-linear 4. AM envelope 5. DSB-FC 6.L.S.B 7. U.S.B 8. Non-linear 9. Carrier 10. Carrier power 46

47 REVIEW QUESTIONS: PART-A 1. Define modulation? Modulation is a process by which some characteristics of high frequency carrier signal is varied in accordance with the instantaneous value of the modulating signal. 2.What are the types of analog modulation? Amplitude modulation. Angle Modulation 1. Frequency modulation 2. Phase modulation. 3.Define depth of modulation. It is defined as the ratio between message amplitude to that of carrier amplitude. m=em/ec 4. What are the degrees of modulation? Under modulation. m<1 Critical modulation m=1 Over modulation m>1 5.What is the need for modulation? Needs for modulation: _ Ease of transmission _ Multiplexing _ Reduced noise _ Narrow bandwidth _ Frequency assignment _ Reduce the equipments limitations. 6.What are the types of AM modulators? There are two types of AM modulators. They are _ Linear modulators _ Non-linear modulators Linear modulators are classified as follows _ Transistor modulator There are three types of transistor modulator. _ Collector modulator _ Emitter modulator _ Base modulator _ Switching modulators Non-linear modulators are classified as follows _ Square law modulator _ Product modulator _ Balanced modulator 7.Give the classification of modulation. There are two types of modulation. They are _ Analog modulation _ Digital modulation 47

48 Analog modulation is classified as follows _ Continuous wave modulation _ Pulse modulation Continuous wave modulation is classified as follows _ Amplitude modulation _ Double side band suppressed carrier _ Single side band suppressed carrier _ Vestigial side band suppressed carrier _ Angle modulation _ Frequency modulation _ Phase modulation Pulse modulation is classified as follows _ Pulse amplitude modulation _ Pulse position modulation _ Pulse duration modulation _ Pulse code modulation Digital modulation is classified as follows _ Amplitude shift keying _ Phase shift keying _ Frequency shift keying 8.What is single tone and multi tone modulation? If modulation is performed for a message signal with more than one frequency component then the modulation is called multi tone modulation. If modulation is performed for a message signal with one frequency component then themodulation is called single tone modulation. 9.The antenna current of an AM transmitter is 8A when only carrier is sent. It increases to 8.93A when the carrier is modulated by a single sine wave. Find the percentage modulation. Solution: Given: Ic =8A It=8.93A m=0.8 Formula: It=Ic (1+m2/2)½ 8.93=8(1+m2/2) ½ m=0.701 It=8 (1+0.82/2)½ It=9.1A 10.Compare AM with DSB-SC and SSB-SC. 48

49 11.What are the advantages of VSB-AM? 1.It has bandwidth greater than SSB but less than DSB system. 2.Power transmission greater than DSB but less than SSB system. 3.No low frequency component lost. Hence it avoids phase distortion. 12 Compare linear and non-linear modulators. Linear modulators Non-linear modulators 1.Heavy filtering is not required 1.Heavy filtering is required. 2.These modulators are used in high 2.These modulators are used in low level modulation level modulation.. 3.The carrier voltage is very much 3.The modulating signal voltage is greater than modulating signal voltage very much greater than the carrier signal voltage. 13.How will you generating DSBSC-AM? There are two ways of generating DSBSC-AM such as 1.balanced modulator 2.ring modulators 14. What are advantages of ring modulator? 1.Its output is stable. 2. It requires no external power source to activate the diodes. 3.Virtually no maintenance. 4. Long life. 15. Define demodulation. Demodulation or detection is the process by which modulating voltage is recovered from the modulated signal. It is the reverse process of modulation. 16. What are the types of AM detectors? 1. Nonlinear detectors 2. Linear detectors 17.What are the types of linear detectors? 1.Synchronous or coherent detector. 2.Envelope or non coherent detector. 49

50 50

51 51

52 52

53 53

54 54

55 55

56 SELF CHECK QUESTIONS: PART-A 1. Define Amplitude Modulation. 2. What is the basic operation of an AM modulator? 3. What is meant by the term RF and AM envelope? For 100% modulation, what are the maximum and minimum amplitudes of AM envelope? 4. Mention the number of inputs given to an AM modulator. 5. Explain the terms: Modulating signal, Carrier signal, Modulated wave and AM envelope. 6. What is meant by the repetition rate of the AM envelope? 7. Describe Upper Side Band (USB), Lower Side Band (LSB), Upper Side Frequencies (USF) and Lower Side Frequencies (LSF). 8. What is the relation between the modulating signal frequency and bandwidth in a conventional AM system? 9. Define modulation coefficient and percent modulation. 10. Without causing excessive distortion, what is the highest modulation coefficient in conventional AM? 11. For 100% modulation, what is the relationship between the voltage amplitudes of the side frequencies and carrier? 12. Describe each term of the following expression: v am (t) = 10 Sin (2 500kt) 5 Cos (2 515kt) + 5 Cos (2 485kt) 13. What is the effect of modulation on the amplitude of the carrier component of the modulated spectrum? 14. What does AB DSB FC stand for? 15. Write the relation between the carrier and side band powers in an AM DSB FC wave. 16. Mention the advantages and disadvantages of AM DSB FC wave. PART-B 1. Derive the necessary equations for calculating the maximum and minimum voltage for an AM envelope. Prove that the maximum amplitude of AM envelope is 2E c when it undergoes 100% modulation. 2. Starting from the representation of unmodulated carrier power, prove that the total power of AM DSB FC envelope is equal to the carrier power, for the highest modulation coefficient. 3. For an AM DSB FC modulator with a carrier frequency f c = 100 KHz and a maximum modulating signal frequency f m(max) = 5 KHz, determine a. Frequency limits for the upper and lower side bands, b. Bandwidth, c. Upper and lower side frequencies produced when the modulating signal is a single frequency 3 KHz tone. d. Also draw the output frequency spectrum. 4. What is the maximum modulating signal frequency that can be used with an AM DSB FC system with a 56

57 20 KHz bandwidth? 5. If a modulated wave with an average voltage of 20 V p changes in amplitude ± 5V, determine the minimum and maximum envelope amplitudes, modulation coefficient and percent modulation. Also Sketch the envelope. 6. For an unmodulated carrier amplitude of 16 V p, and a modulation coefficient of m = 0.4, determine the amplitudes of the modulated carrier and side frequencies. 7. One input to an AM DSB FC modulator is an 800 KHz carrier with an amplitude of 40 V p. The second input is a 25 KHz modulating signal whose amplitude is sufficient to produce a ± 10 V change in amplitude of the envelope. Determine a. Upper and lower side frequencies b. Modulation coefficient and percent modulation c. Maximum and minimum positive peak amplitudes of the envelope d. Draw the output frequency spectrum e. Draw the envelope 8. Write the expression for an AM voltage wave with the following waves: i. Unmodulated carrier amplitude = 20 V p ii. Modulation coefficient = 0.4 iii. Modulating signal frequency = 5 KHz iv. Carrier frequency = 200 KHz 9. For an AM DSB Fc wave with an unmodulated carrier voltage of 18 V p and a load resistance of 72, determine a) Unmodulated carrier power b) Modulated carrier power c) Total side band power d) Upper and lower side band powers e) Total transmitted power f) Also draw the power spectrum. 57