Definition

phase modulation (PM)

Part of the Electronics glossary:

Also see modulation and phase-shift keying.

Phase modulation (PM) is a method of impressing data onto an alternating-current (AC) waveform by varying the instantaneous phase of the wave. Thisscheme can be used with analog or digital data.

In analog PM, the phase of the AC signalwave, also called the carrier, varies in a continuous manner. Thus, thereare infinitely many possible carrier phase states. When the instantaneous data inputwaveform has positive polarity, the carrier phase shifts in one direction; when theinstantaneous data input waveform has negative polarity, the carrier phase shifts in theopposite direction. At every instant in time, the extent of carrier-phase shift(the phase angle) is directly proportional to the extent to which the signalamplitude is positive or negative.

In digital PM, the carrier phase shifts abruptly, rather than continuouslyback and forth. The number of possible carrier phase states is usually a power of2. If there are only two possible phase states, the mode is called biphasemodulation. In more complex modes, there can be four, eight, or more differentphase states. Each phase angle (that is, each shift from one phase state to another)represents a specific digital input data state.

Phase modulation is similar in practice to frequency modulation (FM). When theinstantaneous phase of a carrier is varied, the instantaneous frequency changes as well. The converse also holds: When the instantaneous frequency is varied, theinstantaneous phase changes. But PM and FM are not exactly equivalent, especially inanalog applications. When an FM receiver is used to demodulate a PM signal, or whenan FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between phase and frequency variations is not linear;that is, phase and frequency do not vary in direct proportion.

This was last updated in September 2005
Posted by: Margaret Rouse

Related Terms

Definitions

  • geo-fencing (geofencing)

    - Geo-fencing is a feature in a software program that uses the global positioning system (GPS) or radio frequency identification (RFID) to define geographical boundaries. A geofence is a virtual barr... (WhatIs.com)

  • bandwidth

    - Definition: Learn what bandwidth is and how the term is used as a synonym for signal frequency range, data transfer rate and capacity. (SearchEnterpriseWAN.com)

  • quad-core processor

    - A quad-core processor is a chip with four independent units called cores that read and execute central processing unit (CPU) instructions such as add, move data, and branch. (WhatIs.com)

Glossaries

  • Electronics

    - Terms related to electronics, including definitions about electrical components and words and phrases about computers, laptops parts, digital cameras, televisions and home appliances.

  • Internet applications

    - This WhatIs.com glossary contains terms related to Internet applications, including definitions about Software as a Service (SaaS) delivery models and words and phrases about web sites, e-commerce ...

Ask a Question. Find an Answer.Powered by ITKnowledgeExchange.com

Ask An IT Question

Get answers from your peers on your most technical challenges

Ask Question

Tech TalkComment

Share
Comments

    Results

    Contribute to the conversation

    All fields are required. Comments will appear at the bottom of the article.