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This article may be too technical for most readers to understand. Please improve this article to make it accessible to non-experts, without removing the technical details. (November 2009) |
Communication signals often have the form:
![A(t)\cdot \sin[2\pi ft + \phi(t)],](http://wpcontent.answers.com/math/e/6/7/e67aecb272872d50c78b14d09f34990d.png)
which is called envelope-and-phase form.
An equivalent representation, called quadrature-carrier form, is:
where 
represents a carrier frequency, and:
![I(t)\ \stackrel{\mathrm{def}}{=}\ A(t)\cdot \cos[\phi(t)] \,](http://wpcontent.answers.com/math/e/b/1/eb1747f245ada8e2c7456472bd8d64d3.png)
![Q(t)\ \stackrel{\mathrm{def}}{=}\ A(t)\cdot \sin[\phi(t)].\,](http://wpcontent.answers.com/math/3/b/d/3bd8a83881b27a027bdce00543e381f9.png)
and 
represent possible modulation of a pure carrier wave: 
The modulation alters the original 
component of the carrier, and creates a (new) 
component, as shown above. The component that is in phase with the original carrier is referred to as the in-phase component. The other component, which is always 90° (
radians) out of phase, is referred to as the quadrature component.
References
See also
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