Modulation Techniques (Part One) - AM, FM, PM

Modulation Techniques (Part One)

Although, this article refers to telephone modems and analogue audio modulation, the same principles apply to satellite modems. This material has been used for the tutorial as it explains the principles well.

By Eugene Blanchard, Edited by Joshua Drake, Bill Randolph, Phuong Ma
Copyright © 2001 by Commandprompt, Inc.
Copyright © 2001 by Eugene Blanchard.
Copyright © 2000 by Eugene Blanchard.

This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.' to the license reference or copy.

• 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.

By Eugene Blanchard, Edited by Joshua Drake, Bill Randolph, Phuong Ma
Copyright © 2001 by Commandprompt, Inc.
Copyright © 2001 by Eugene Blanchard.
Copyright © 2000 by Eugene Blanchard.

This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.' to the license reference or copy.

Next we will move on to the more complex modulation techniques.