Analog Communication: Amplitude Modulation Systems

Amplitude Modulation (AM)

• Amplitude Modulation is the process of changing the amplitude of a high-frequency carrier signal in accordance with the amplitude of the modulating signal (information).
• The carrier amplitude is varied linearly by the modulating signal, which usually consists of a range of audio frequencies.
• The frequency of the carrier is not affected.

Applications of AM

• Radio broadcasting
• TV pictures (video)
• Facsimile transmission

Frequency Range and Bandwidth of AM

• Frequency range: 535 kHz – 1600 kHz
• Bandwidth: 10 kHz

Forms of Amplitude Modulation

• Conventional Amplitude Modulation (Full AM or Double Sideband Large carrier modulation (DSBLC) / Double Sideband Full Carrier (DSBFC))
• Double Sideband Suppressed carrier (DSBSC) modulation
• Single Sideband (SSB) modulation
• Vestigial Sideband (VSB) modulation

Time and Frequency Domain Description

• Amplitude modulated signal: s(t) = Ac [1 + kam(t)] cos(ωct)
• Fourier Transform: S(ω) = π Ac/2 (δ(ω − ωc) + δ(ω + ωc)) + kaAc/ 2 (M(ω − ωc) + M(ω + ωc))

Single Tone Modulation

• Modulating wave: m(t) = Acm cos(ωmt)
• Amplitude modulated signal: s(t) = Ac [1 + kam cos(ωmt)] cos(ωct)

Power Relations in AM Waves

• Ratio of total sideband power to total power in the modulated wave is called the efficiency of the AM system

Generation of AM Waves

• Two basic amplitude modulation principles:
  • Square law modulation
  • Switching modulator

Square Law Modulator

• Output of a non-linear device is not directly proportional to the input throughout the operation
• Input-Output relation: y(t) = a1x(t) + a2x^2(t)
• Square law device output: V0 = Vdc + Vm cos(ωmt)

Switching Modulator

• Total input for the diode: v1 = Ac cos(ωct) + m(t)
• When c(t) is positive, v2=v1, and when c(t) is negative, v2=0
• Switching response of the diode: v2 = v1 * s(t)

Detection of AM Waves

• Demodulation is the process of recovering the original message signal from the amplitude-modulated wave.