Presentation on theme: "TVS, By Ya Bao 1 AMPLITUDE MODULATION 1.DEFINING AM A carrier frequency whose amplitude is varied in proportion to the instantaneous amplitude of a modulating."— Presentation transcript:
TVS, By Ya Bao 1 AMPLITUDE MODULATION 1.DEFINING AM A carrier frequency whose amplitude is varied in proportion to the instantaneous amplitude of a modulating voltage 2.GENERATING THE AM nonlinear device: diode or transistor biased in its nonlinear region
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 RatioPowerVoltage(dB) 1 0 0 101020 1002040 1,0003060 10,0004080
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 16 6.4 Filtering the SSB LSB or USB Dual Conversion: up-converting the mod ulating 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:"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. Type of the filters LC filtersceramicmechanicalcrystal Order of Qs 200200010,00050,000
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.