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Principles Of Communication Engineering

Principles Of Communication Engineering. Chapter 2. Amplitude Modulation. Topics Covered in Chapter 2. 2-1 : AM Concepts 2-2 : Modulation Index and Percentage of Modulation 2-3 : Sidebands and the Frequency Domain 2-4: Single-Sideband Modulation 2-5: AM Power. 2-1 : AM Concepts.

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Principles Of Communication Engineering

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  1. Principles Of Communication Engineering

  2. Chapter 2 Amplitude Modulation

  3. Topics Covered in Chapter 2 • 2-1: AM Concepts • 2-2: Modulation Index and Percentage of Modulation • 2-3: Sidebands and the Frequency Domain • 2-4: Single-Sideband Modulation • 2-5: AM Power

  4. 2-1: AM Concepts • In the modulation process, the voice, video, or digital signal modifies another signal called the carrier. • In amplitude modulation(AM) the information signal varies the amplitude of the carrier sine wave. • The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal. • An imaginary line called the envelope connects the positive and negative peaks of the carrier waveform.

  5. 2-1: AM Concepts Figure 1-1: Amplitude modulation. (a) The modulating or information signal.

  6. 2-1: AM Concepts Figure 1-2: Amplitude modulation. (b) The modulated carrier.

  7. 2-1: AM Concepts • In AM, it is particularly important that the peak value of the modulating signal be less than the peak value of the carrier. Vm < Vc • Distortion occurs when the amplitude of the modulating signal is greater than the amplitude of the carrier. • A modulator is a circuit used to produce AM. Amplitude modulators compute the product of the carrier and modulating signals.

  8. 1-1: AM Concepts Figure 1-3: Amplitude modulator showing input and output signals.

  9. 2-2: Modulation Index and Percentage of Modulation Figure 1-5: AM wave showing peaks (Vmax) and troughs (Vmin).

  10. 2-2: Modulation Index and Percentage of Modulation • The modulation index (m) is a value that describes the relationship between the amplitude of the modulating signal and the amplitude of the carrier signal. m =Vm / Vc • This index is also known as the modulating factor or coefficient, or the degree of modulation. • Multiplying the modulation index by 100 gives the percentage of modulation.

  11. Vmax−Vmin Vm = 2 2-2: Modulation Index and Percentage of Modulation Percentage of Modulation • The modulation index is commonly computed from measurements taken on the composite modulated waveform. • Using oscilloscope voltage values: • The amount, or depth, of AM is then expressed as the percentage of modulation (100 × m) rather than as a fraction.

  12. Determining modulation index from Vmax and Vmin EKT343 –Principle of Communication Engineering

  13. Cont’d… m = modulation index Em = peak change in the amplitude output waveform (sum of voltages from upper and lower side frequencies) Ec = peak amplitude of the unmodulated carrier • Mathematically, the modulation index is • And the percentage of modulation index is EKT343 –Principle of Communication Engineering

  14. Cont’d… • If the modulating signal is a pure, single-freq sine wave and the process is symmetrical then the modulation index can be derived as follows: • Therefore, EKT343 –Principle of Communication Engineering

  15. Cont’d… • Since the peak change of modulated output wave Em is the sum of the usf and lsf voltages hence, • Then Eusf = peak amplitude of the upper side frequency (volts) Elsf = peak amplitude of the lower side frequency (volts) EKT343 –Principle of Communication Engineering

  16. 2-2: Modulation Index and Percentage of Modulation Overmodulation and Distortion • The modulation index should be a number between 0 and 1. • If the amplitude of the modulating voltage is higher than the carrier voltage, m will be greater than 1, causing distortion. • If the distortion is great enough, the intelligence signal becomes unintelligible.

  17. 2-2: Modulation Index and Percentage of Modulation Overmodulation and Distortion • Distortion of voice transmissions produces garbled, harsh, or unnatural sounds in the speaker. • Distortion of video signals produces a scrambled and inaccurate picture on a TV screen.

  18. 2-2: Modulation Index and Percentage of Modulation Figure 1-4: Distortion of the envelope caused by overmodulation where the modulating signal amplitude Vmis greater than the carrier signal Vc.

  19. 2-3: Sidebands and the Frequency Domain • Side frequencies, or sidebands are generated as part of the modulation process and occur in the frequency spectrum directly above and below the carrier frequency.

  20. 2-3: Sidebands and the Frequency Domain Sideband Calculations • Single-frequency sine-wave modulation generates two sidebands. • Complex wave (e.g. voice or video) modulation generates a range of sidebands. • The upper sideband (fUSB) and the lower sideband (fLSB) are calculated: fUSB = fc + fmand fLSB = fc− fm

  21. 2-3: Sidebands and the Frequency Domain Figure 1-6: The AM wave is the algebraic sum of the carrier and upper and lower sideband sine waves. (a) Intelligence or modulating signal. (b) Lower sideband. (c ) Carrier. (d ) Upper sideband. (e ) Composite AM wave.

  22. 2-3: Sidebands and the Frequency Domain Frequency-Domain Representation of AM • Observing an AM signal on an oscilloscope, you see only amplitude variations of the carrier with respect to time. • A plot of signal amplitude versus frequency is referred to as frequency-domain display. • A spectrum analyzer is used to display the frequency domain as a signal. • Bandwidth is the difference between the upper and lower sideband frequencies. • BW = fUSB−fLSB = [fc + fm(max)] – [fc –fm(max) ] = 2fm(max)

  23. Figure 1-8: The relationship between the time and frequency domains. 2-3: Sidebands and the Frequency Domain

  24. 2-3: Sidebands and the Frequency Domain Frequency-Domain Representation of AM • Example 1: • For a conventional AM modulator with a carrier freq of fc= 100 kHz and the maximum modulating signal frequency of fm(max)= 5 kHz, determine: • Freq limits for the upper and lower sidebands. • Bandwidth. • Upper and lower side frequencies produced when the modulating signal is a single-freq 3-kHz tone. • Draw the output freq spectrum.

  25. Example 2 • Suppose that Vmax value read from the graticule on an oscilloscope screen is 4.6 divisions and Vmin is 0.7 divisions. Calculate the modulation index and percentage of modulation. EKT343 –Principle of Communication Engineering

  26. Example 3 • For the AM waveform shown in Figure below, determine • Peak amplitude of the upper and lower side frequencies. • Peak amplitude of the unmodulated carrier. • Peak change in the amplitude of the envelope. • Modulation index. • Percent modulation. EKT343 –Principle of Communication Engineering

  27. AM Envelope for Example 3 EKT343 –Principle of Communication Engineering

  28. The Mathematical Representation and Analysis of AM • Representing both the modulating signal Vm(t) and the carrier signal Vc(t) in trigonometric functions. • The AM DSBFC modulator must be able to produce mathematical multiplication of these two analog signals EKT343 –Principle of Communication Engineering

  29. Cont’d… • Substituting Vm = mVc gives: Constant + mod. signal Unmodulated carrier EKT343 –Principle of Communication Engineering

  30. Cont’d… • The constant in the first term produces the carrier freq while the sinusoidal component in the first term produces side bands frequencies Carrier frequency signal (volts) Upper side frequency signal (volts) Lower side frequency signal (volts) EKT343 –Principle of Communication Engineering

  31. Cont’d… • From the equation it is obvious that the amplitude of the carrier is unaffected by the modulation process. • The amplitude of the side frequencies depend on the both the carrier amplitude and modulation index. • At 100% modulation the amplitudes of side frequencies are each equal to one-half the amplitude of the carrier. EKT343 –Principle of Communication Engineering

  32. Generation of AM DSBFC envelope showing the time-domain of the modulated wave, carrier & sideband signals EKT343 –Principle of Communication Engineering

  33. Voltage spectrum for an AM DSBFC wave EKT343 –Principle of Communication Engineering

  34. Example 4 • One input to a conventional AM modulator is a 500-kHz carrier with an amplitude of 20 Vp. The second input is a 10-kHz modulating signal that is of sufficient amplitude to cause a change in the output wave of ±7.5 Vp. Determine • Upper and lower side frequencies. • Modulation index and percentage modulation. • Peak amplitude of the modulated carrier and the upper and lower side frequency voltages. • Maximum and minimum amplitudes of the envelope. • Expression for the modulated wave. EKT343 –Principle of Communication Engineering

  35. 2-3: Sidebands and the Frequency Domain Pulse Modulation • When complex signals such as pulses or rectangular waves modulate a carrier, a broad spectrum of sidebands is produced. • A modulating square wave will produce sidebands based on the fundamental sine wave as well as the third, fifth, seventh, etc. harmonics. • Amplitude modulation by square waves or rectangular pulses is referred to as amplitude shift keying (ASK). • ASK is used in some types of data communications.

  36. 2-3: Sidebands and the Frequency Domain Figure 1-11: Frequency spectrum of an AM signal modulated by a square wave.

  37. 2-3: Sidebands and the Frequency Domain Figure 1-12: Amplitude modulation of a sine wave carrier by a pulse or rectangular wave is called amplitude-shift keying. (a) Fifty percent modulation. (b) One hundred percent modulation.

  38. 2-3: Sidebands and the Frequency Domain Pulse Modulation • Continuous-wave (CW) transmission can be achieved by turning the carrier off and on, as in Morse code transmission. • Continuous wave (CW) transmission is sometimes referred to as On-Off keying (OOK). • Splatter is a term used to describe harmonic sideband interference.

  39. 2-4: Single-Sideband Modulation • In amplitude modulation, two-thirds of the transmitted power is in the carrier, which conveys no information. • Signal information is contained within the sidebands. • Single-sideband (SSB) is a form of AM where the carrier is suppressed and one sideband is eliminated.

  40. 2-4: Single-Sideband Modulation DSB Signals • The first step in generating an SSB signal is to suppress the carrier, leaving the upper and lower sidebands. • This type of signal is called a double-sideband suppressed carrier (DSSC) signal. No power is wasted on the carrier. • A balanced modulator is a circuit used to produce the sum and difference frequencies of a DSSC signal but to cancel or balance out the carrier. • DSB (or DSSC) is not widely used because the signal is difficult to demodulate (recover) at the receiver.

  41. 2-4: Single-Sideband Modulation Figure 1-16: A frequency-domain display of DSB signal.

  42. 2-4: Single-Sideband Modulation SSB Signals • One sideband is all that is necessary to convey information in a signal. • A single-sideband suppressed carrier (SSSC) signal is generated by suppressing the carrier and one sideband.

  43. 2-4: Single-Sideband Modulation SSB Signals • SSB signals offer four major benefits: • Spectrum space is conserved and allows more signals to be transmitted in the same frequency range. • All power is channeled into a single sideband. This produces a stronger signal that will carry farther and will be more reliably received at greater distances. • Occupied bandwidth space is narrower and noise in the signal is reduced. • There is less selective fading over long distances.

  44. 2-4: Single-Sideband Modulation Disadvantages of DSSC and SSBSC • Single and double-sideband SC’s are not widely used because the signals are difficult to recover (i.e. demodulate) at the receiver. • A low power, pilot carrier is sometimes transmitted along with sidebands in order to more easily recover the signal at the receiver.

  45. 2-4: Single-Sideband Modulation Signal Power Considerations • In SSB, the transmitter output is expressed in terms of peak envelope power (PEP), themaximum power produced on voice amplitude peaks. Applications of DSB and SSB • A vestigial sideband signal (VSB) is produced by partially suppressing the lower sideband. This kind of signal is used in TV transmission.

  46. 2-5: AM Power • In radio transmission, the AM signal is amplified by a power amplifier. • A radio antenna has a characteristic impedance that is ideally almost pure resistance. • The AM signal is a composite of the carrier and sideband signal voltages. • Each signal produces power in the antenna. • Total transmitted power (PT) is the sum of carrier power (Pc ) and power of the two sidebands (PUSB and PLSB).

  47. 2-5: AM Power • When the percentage of modulation is less than the optimum 100, there is much less power in the sidebands. • Output power can be calculated by using the formula PT = (IT)2R where IT is measured RF current and R is antenna impedance

  48. 2-5: AM Power • The greater the percentage of modulation, the higher the sideband power and the higher the total power transmitted. • Power in each sideband is calculated PSB = PLSB = PUSB = Pcm2 / 4 • Maximum power appears in the sidebands when the carrier is 100 percent modulated.

  49. fc-fm fc+fm fc AM Power Review: conventional AM(DSB-FC) Frequency spectrum: Bandwidth=2 x fmmax Total Power=Pcarrier +Pusb+Plsb EKT343 –Principle of Communication Engineering

  50. DSB FULL CARRIER

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