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Chapter 2

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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Chapter 2

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  1. Chapter 2 Amplitude Modulation

  2. 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

  3. 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.

  4. 2-1: AM Concepts Figure 1-1: Amplitude modulation. (a) The modulating or information signal.

  5. 2-1: AM Concepts Figure 1-2: Amplitude modulation. (b) The modulated carrier.

  6. 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.

  7. 2-1: AM Concepts Figure 1-3: Amplitude modulator showing input and output signals.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 2-2: Modulation Index and Percentage of Modulation Figure 1-5: AM wave showing peaks (Vmax) and troughs (Vmin).

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

  15. 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.

  16. 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

  17. 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.

  18. 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)

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

  20. 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.

  21. 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

  22. 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

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

  24. 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

  25. 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.

  26. 2-3: Sidebands and the Frequency Domain Figure 1-11: Frequency spectrum of an AM signal modulated by a square wave.

  27. 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.

  28. 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.

  29. 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.

  30. 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 is not widely used because the signal is difficult to demodulate (recover) at the receiver.

  31. 2-4: Single-Sideband Modulation Figure 1-16: A frequency-domain display of DSB signal.

  32. 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.

  33. 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.

  34. 2-4: Single-Sideband Modulation Disadvantages of DSB and SSB • Single and double-sideband 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.

  35. 2-4: Single-Sideband Modulation Signal Power Considerations • In SSB, the transmitter output is expressed in terms of peak envelope power (PEP), the maximum 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.

  36. VESTIGIAL SIDEBAND (VSB) • VSB is similar to SSB but it retains a small portion (a vestige) of the undesired sideband to reduce DC distortion. • VSB signals are generated using standard AM or DSBSC modulation, then passing modulated signal through a sideband shaping filter. • Demodulation uses either standard AM or DSBSC demodulation. EKT343 –Principle of Communication Engineering

  37. Cont’d • Also called asymmetric sideband system. • Compromise between DSB & SSB. • Easy to generate. • Bandwidth is only ~ 25% greater than SSB signals. • Derived by filtering DSB, one pass band is passed almost completely while just a trace or vestige of the other sideband is included. EKT343 –Principle of Communication Engineering

  38. VSB Carrier LSB MSB fc fc Cont’d • AM wave is applied to a vestigial sideband filter, producing a modulation scheme – VSB + C • Mainly used for television video transmission. • VSB Frequency Spectrum EKT343 –Principle of Communication Engineering

  39. AM Power Distribution

  40. 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).

  41. 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

  42. 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.

  43. 2-5: AM Power • In any electrical circuit, the power dissipated is equal to the voltage squared (rms) divided by the resistance. • Mathematically power in unmodulated carrier is Pc = carrier power (watts) Vc = peak carrier voltage (volts) R = load resistance i.e antenna (ohms) EKT343 –Principle of Communication Engineering

  44. Cont’d • The upper and lower sideband powers will be • Rearranging in terms of Pc, EKT343 –Principle of Communication Engineering

  45. Cont’d… • The total power in an AM wave is • Substituting the sidebands powers in terms of PC yields • Since carrier power in modulated wave is the same as unmodulated wave, obviously power of the carrier is unaffected by modulation process. EKT343 –Principle of Communication Engineering

  46. Power spectrum for AM DSBFC wave with a single-frequency modulating signal EKT343 –Principle of Communication Engineering

  47. Cont’d… • With 100% modulation the maximum power in both sidebands equals to one-half the carrier power. • One of the most significant disadvantage of AM DSBFC is with m = 1, the efficiency of transmission is only 33.3% of the total transmitted signal. The less wasted in the carrier which brings no information signal. • The advantage of DSBFC is the use of relatively simple, inexpensive demodulator circuits in the receiver. EKT343 –Principle of Communication Engineering

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

  49. Two major Drawbacks of DSBFC • Large power consumption, where carrier power constitutes >2/3 transmitted power.{remember : carrier does not contain any information} • Utilize twice as much bandwidth – both the upper and lower sideband actually contains same information (redundant). Thus, DSBFC is both power and bandwidth inefficient EKT343 –Principle of Communication Engineering

  50. fc-fm fc+fm fc Double side band suppressed carrier(DSB-SC) • Frequency spectrum: • Bandwidth:2 x fmmax • Total Power= Pusb + Plsb EKT343 –Principle of Communication Engineering

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