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CS601 - Data Communication - Lecture Handout 19

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Conversions

Analog To Analog Conversion

  • Representation of Analog information by an Analog signal
  • For Example: Radio

Analog To Analog Conversion

Analog To Analog Conversion Methods

Analog To Analog Conversion Methods

Amplitude Modulation (AM)

  • Amplitude of carrier signal is changed according to the amplitude of modulating signal
  • Frequency and phase of the carrier remain the same

Amplitude Modulation (AM)

AM Bandwidth

Bandwidth of AM signal (modulated signal) = 2 * bandwidth of modulating signal
Significant spectrum of AM audio = 5 KHz
⇒ 10 KHz bandwidth for an AM station

AM Bandwidth

AM Band Allocation

AM Band Allocation

Example 5.18

We have an audio signal with a BW of 4 KHz. What is the BW needed, if we modulate the signal using AM?

Solution:

  • AM signal requires twice the BW of original signal
  • BW = 2 * 4 KHz = 8 KHz

Frequency Modulation (FM)

  • Frequency of carrier signal is changed according to the amplitude of modulating signal
  • Amplitude and Phase of the carrier signal remain constant

Frequency Modulation (FM)

FM Bandwidth

Bandwidth of FM signal (modulated signal) = 10 * bandwidth of modulating signal
Significant spectrum of FM audio = 15 KHz
⇒ Minimum 150 KHz bandwidth

FM Band

FM Band

FM Band Allocation

FM Band Allocation

Example 5.19

We have an Audio signal with a BW of 4 MHz. What is the BW needed if we modulate the signal using FM?

Solution:

  • BW = 10 * 4 MHz = 40 MHz
  • Phase modulation (PM)
    • Simpler hardware requirements
    • Phase is modulated with the amplitude
    • Amplitude & Frequency of the carrier signal remain constant

Summary

  • Analog-to Analog Conversion
  • Frequency Shift Keying (FSK)
  • Phase Shift Keying (PSK)
  • Quadrature Amplitude Modulation (QAM)

Reading Sections

  • Section 5.4, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan