Chapter 22. Modulation Techniques
Modulation techniques are methods that are used to encode digital information in an analog world. The 3 basic modulation techniques are as follows:
AM (amplitude modulation)
FM (frequency modulation)
PM (phase modulation)
All 3 modulation techniques employ a carrier signal. A carrier signal is a single frequency that is used to carry the intelligence (data). For digital, the intelligence is either a 1 or 0. When we modulate the carrier, we are changing its characteristics to correspond to either a 1 or 0.
AM - Amplitude Modulation
Amplitude Modulation modifies the amplitude of the carrier to represent 1s or 0s.In the above example, a 1 is represented by the presence of the carrier for a predefined period of 3 cycles of carrier. Absence--or no carrier--indicates a 0.
Advantages:
Simple to design.
Disadvantages:
Noise spikes on transmission medium interfere with the carrier signal.
Loss of connection is read as 0s.
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.
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 that is required to generate and detect phase changes.
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