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AMPLITUDE MODULATION

AMPLITUDE MODULATION. DEFINING AM A carrier frequency whose amplitude is varied in proportion to the instantaneous amplitude of a modulating voltage GENERATING THE AM nonlinear device: diode or transistor biased in its nonlinear region. ANALYSIS OF THE AM WAVE.

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AMPLITUDE MODULATION

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  1. AMPLITUDE MODULATION • DEFINING AM • A carrier frequency whose amplitude is varied in proportion to the instantaneous amplitude of a modulating voltage • GENERATING THE AM nonlinear device: diode or transistor biased in its nonlinear region TVS, By Ya Bao

  2. TVS, By Ya Bao

  3. ANALYSIS OF THE AM WAVE TVS, By Ya Bao

  4. TVS, By Ya Bao

  5. 4. Different Carriers and AM Carriers are spaced at 20 kHz, beginning at 100kHz. Each carrier is modulated by a signal with 5kHz bandwidth. Is there interference from sideband overlap? TVS, By Ya Bao

  6. 5. MODULATION INDEX AND SIGNAL POWER TVS, By Ya Bao

  7. Moduiation Index and Power TVS, By Ya Bao

  8. Current Calculations Example A carrier of 1000 W is modulated with a resulting modulation index of 0. 8. What is the total power? What is the carrier power if the total power is 1000 W and the modulation index is 0.95? TVS, By Ya Bao

  9. 6. SUPPRESSED CARRIERS AND SINGLE SIDEBAND (SSB) 6.1 Signal Magnitudes and Ranges decibel (dB) TVS, By Ya Bao

  10. The dB gain is Ratio Power Voltage (dB) (dB) 1 0 0 10 10 20 100 20 40 1,000 30 60 10,000 40 80 TVS, By Ya Bao

  11. dB REFERENCE VALUES • dBm (reference is 1 mW), • dBV (reference is 1 V), • dBW (reference is 1 W). • Example: A signal of 200 mW could be expressed in dBm as An audio amplifier might have an output of 7 dBW. This corresponds to A signal of 8.2 V corresponds to TVS, By Ya Bao

  12. 6.2 Double Sideband Suppressed Carrier (DSBSC) When the carrier is reduced, this is called double-sideband suppressed-carrier AM, or DSB-SC. If the carrier could somehow be removed or reduced, the transmitted signal would consist of two information-bearing sidebands, and the total transmitted power would be information TVS, By Ya Bao

  13. For a 100-W total available power, compare the power in the sidebands when modulation is standard AM with a modulation index of 100%, versus a suppressed carrier design where 90% of the carrier power is suppressed. How many times greater is the sideband power in the suppressed carrier case? • A 500-W DSB-SC system with 100% modulation suppresses 50% of the carrier, and the suppressed carrier power goes to the sidebands. How much power is in the sidebands, and how much is in the carrier? By how many dB has the sideband power increased? TVS, By Ya Bao

  14. 6.3 Single-Sideband (SSB) • suppressing the carrier and one of the sidebands TVS, By Ya Bao

  15. TVS, By Ya Bao

  16. 6.4 Filtering the SSB LSB or USB • Dual Conversion: up-converting the mod­ulating frequency twice and selecting the upper or lower sideband for transmission. TVS, By Ya Bao

  17. 7. Filtering the SSB LSB or USB Their quality or "Q" factor: A filter is said to have a high Q if it selects or rejects a narrow range of frequencies compared with its centre frequency. Q is defined as centre frequency fo/3dB bandwidth. TVS, By Ya Bao

  18. Dual Conversion up-converting the mod­ulating frequency twice and selecting the upper or lower sideband for transmission. TVS, By Ya Bao

  19. 8. VESTIGIAL SIDEBAND • In VSB, a portion (or vestige) of the lower sideband is transmitted along with the full carrier power and upper-sideband content. TVS, By Ya Bao

  20. AM: Features and Drawbacks: • the AM signal is greatly affected by noise • impossible to determine absolutely the original signal level • conventional AM is not efficient in the use of transmitter power • AM is useful where a simple, low-cost receiver and detector is desired • The noise is either not a problem or can be tolerated in the ap­plication. TVS, By Ya Bao

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