Amplitude Modulation Transmission and Reception

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Amplitude Modulation Transmission and Reception
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outline

• Theory of AM
• AM envelope
• AM transmitter
• Double-side band
• Demodulation
• AM receiver

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Principle of AM

• Amplitude modulation
• a process of changing the amplitude of
  relatively high frequency carrier signal in
  proportion with instantaneous value of the
  modulating signal
• inexpensive, low quality of modulation
• used for commercial broadcasting (audio
  video), 2 way mobile radio communication

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AM Envelope

• AM double-sideband full carrier (DSBFC)
• most commonly used
• conventional AM
• the carrier -
• the modulating signal -
• the modulated wave -

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AM Envelope

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AM Frequency Spectrum and Bandwidth

• AM nonlinear device
• non linear mixing occurs
• the output envelope is complex wave, made of
• dc voltage, carrier frequency, sum and
  difference frequencies

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AM Frequency Spectrum and Bandwidth

• AM signal contains frequency components spaced
  fm Hz on either side of the carrier
• modulated wave does not contain a frequency
  component that is equal to the modulating signal
• the effect of modulation to translate the
  modulating signal in the frequency domain so that
  it is reflected symmetrically about the carrier
  frequency

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AM Frequency Spectrum and Bandwidth

• AM spectrum
• fc fm(max) to fc fm(max)
• fc fm(max) lower sideband (LSB) lower side
  frequency
• fc fm(max) upper sideband (USB) upper side
  frequency
• Bandwidth (B) the difference between the
  highest upper side freq. and the lowest side
  freq. B 2 fm(max)

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AM Frequency Spectrum and Bandwidth

• AM DSBFC modulator
• carrier frequency, fc 100 kHz
• maximum modulating signal frequency, fm(max) 5
  kHz
• determine
• Frequency limits for the upper and lower
  sidebands
• Bandwidth
• Upper and lower side frequencies produced when
  the modulating signal is 3 kHz tone

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AM Frequency Spectrum and Bandwidth
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AM Transmitter

• Low level transmitter
• Source of modulating signal
• Microphone, magnetic tape, CD, phonograph record
• Preamplifier sensitive, high impedance
• To raise the amplitude of the source to a usable
  level
• Producing minimum nonlinear distortion
• Adding as little thermal noise as possible

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AM Transmitter

• Modulating signal driver
• Linear amplifier
• Amplifies the information signal to an adequate
  level to sufficiently drive the modulator
• RF oscillator
• Any oscillator configurations
• Crystal-controlled oscillator

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AM Transmitter

• Buffer amplifier
• Low-gain, high input impedance linear amplifier
• Function
• to isolate the oscillator from high power
  amplifiers
• Modulator
• To combine the modulating signal with the carrier
  frequency

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AM Transmitter

• Low-level transmitter
• Used for low power, low-power, low capacity
  systems
• Wireless intercoms, remote control units, pager,
  short range walkie-talkie

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AM Transmitter

• High-level Transmitter
• Modulating signal process as same as low level
• Power amplifier
• Because the carrier is at full power at the
  modulation occurrence.
• Additional carrier power amplifier

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AM Transmitter

• High level modulator
• Provides the necessary circuits for modulation to
  occur
• Final power amplifier
• Frequency up-converter
• Translate the low frequency signal to radio
  frequency signals
• To be efficiently radiated from an antenna

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AM Demodulation

• Reverse process of Am modulation
• The receiver simply converts a received AM wave
  back to the original source information
• The receiver must capable of band-limiting the
  total radio freq. spectrum tuning the receiver

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AM Demodulation

• Reverse process of Am modulation
• The receiver simply converts a received AM wave
  back to the original source information
• The receiver must capable of band-limiting the
  total radio freq. spectrum tuning the receiver

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AM Demodulation

• RF section detecting, band-limiting and
  amplifying the received signal
• The mixer/converter concerts the the received
  RF freq. to intermediate freq (IF)
• IF section amplify the intermediate freq.
• AM detector demodulates the AM wave
• Audio section amplify the recovered information