Bài giảng Mạch điện tử thông tin - Chapter 6: Analog Modulators - Đỗ Hồng Tuấn
Chapter 6:
Analog Modulators
Dept. of Telecomm. Eng.
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Analog Modulation
❑ Amplitude Modulation:
▪ Standard amplitude modulation (AM) with carrier.
▪ Suppressed-carrier double-sideband modulation (DSB).
▪ Single-sideband modulation (SSB).
▪ Vestige-sideband modulation (VSB).
❑ Frequency Modulation
❑ Phase Modulation
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Standard AM (1)
❑ AM signal and spectra:
The AM signal is:
where is positive constant called AM modulation index, x(t) is message,
Accosct is carrier signal.
Dept. of Telecomm. Eng.
Faculty of EEE
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Standard AM (2)
Message spectrum with bandwidth W:
AM spectrum: AM bandwidth is 2W
Dept. of Telecomm. Eng.
Faculty of EEE
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Standard AM (3)
AM transmitted power:
where Pc represents unmodulated carrier power:
and Psb represents power per sideband:
where Sx is average message power.
If fc >>W and 1, then the message can be extracted from xc(t) by a
simple envelope detector.
Dept. of Telecomm. Eng.
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DHT, HCMUT
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Standard AM (4)
❑ Product modulator for AM:
▪ Using Multiplier:
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Standard AM (5)
▪ Using square-law element:
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Standard AM (6)
We assumed that the nonlinear element approximates the square
law transfer curve:
Thus, with
▪ High-level modulation using class C amplifier:
Dept. of Telecomm. Eng.
Faculty of EEE
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DHT, HCMUT
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Standard AM (7)
❑ AM demodulation:
▪ Synchronous detection (coherent detection): The oscillator of
demodulator is exactly synchronized (both phase and frequency) with
the carrier.
Using PLL for carrier synchronization.
Dept. of Telecomm. Eng.
Faculty of EEE
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DHT, HCMUT
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Standard AM (8)
▪ Envelop detection:
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(Suppressed-Carrier) DSB (1)
❑ DSB signal and spectra: The wasted carrier power in AM can be
eliminated by setting = 1 and suppressing the carrier-frequency
component. The resulting DSB signal is:
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Faculty of EEE
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(Suppressed-Carrier) DSB (2)
DSB transmitted power:
❑ Product DSB modulator:
▪ Balanced modulator:
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(Suppressed-Carrier) DSB (3)
▪ Ring modulator:
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(Suppressed-Carrier) DSB (4)
❑ DSB demodulator:
▪ Synchronous detection (coherent detection): As AM demodulator.
Required carrier synchronization.
▪ Envelope reconstruction (for suppressed carrier modulation):
Required carrier synchronization.
Dept. of Telecomm. Eng.
Faculty of EEE
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(Suppressed-Carrier) DSB (5)
▪ Using pilot carrier:
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(Suppressed-Carrier) SSB (1)
❑ SSB signals and spectra:
SSB bandwidth and power:
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(Suppressed-Carrier) SSB (2)
❑ SSB modulator:
▪ Based on DSB and filter:
Two-step SSB modulator:
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(Suppressed-Carrier) SSB (3)
▪ Phase-shift method:
HQ(f): 90o phase shifter
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(Suppressed-Carrier) SSB (4)
▪ Weaver method:
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(Suppressed-Carrier) SSB (5)
❑ SSB demodulator: Similar to DSB demodulator.
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