27. Modulation Techniques (cont'd)

Introduction to Data Communications
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27b. FM - Frequency Modulation

Frequency Modulation modifies the frequency of the carrier to represent the 1s or 0s. In the above example, a 0 is represented by the original carrier frequency and a 1 by a much higher frequency ( the cycles are spaced closer together).

Advantages:

Immunity to noise on transmission medium.
Always a signal present. Loss of signal easily detected

Disadvantages:

Requires 2 frequencies
Detection circuit needs to recognize both frequencies when signal is lost.

27c. PM - Phase Modulation

Phase Modulation modifies the phase of the carrier to represent a 1 or 0.

The carrier phase is switched at every occurrence of a 1 bit but remains unaffected for a 0 bit. The phase of the signal is measured relative to the phase of the preceding bit. The bits are timed to coincide with a specific number of carrier cycles (3 in this example = 1 bit).

Advantage:

Only 1 frequency used
Easy to detect loss of carrier

Disadvantages:

Complex circuitry required to generate and detect phase changes.

Introduction to Data Communications
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