Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.[1]

Contents

Formula

The phase of an oscillation or wave refers to a sinusoidal function such as the following:

x(t) = A\cdot \cos( 2 \pi f t %2B \theta )\,
y(t) = A\cdot \sin( 2 \pi f t %2B \theta ) = A\cdot \cos( 2 \pi f t %2B \theta -\pi/2),\,

where A, f, and \scriptstyle \theta are constant parameters. These functions are periodic with period \scriptstyle T = 1/f, and they are identical except for a displacement of \scriptstyle T/4 along the \scriptstyle t axis. The term phase can refer to several different things:

Phase shift

Phase shift is any change that occurs in the phase of one quantity, or in the phase difference between two or more quantities.[1]

\scriptstyle \theta  is sometimes referred to as a phase-shift, because it represents a "shift" from zero phase. But a change in \scriptstyle \theta is also referred to as a phase shift.

For infinitely long sinusoids, a change in \scriptstyle \theta is the same as a shift in time, such as a time-delay.  If \scriptstyle x(t) is delayed (time-shifted) by  \scriptstyle \begin{matrix} \frac{1}{4} \end{matrix}  of its cycle, it becomes:

x(t - \begin{matrix} \frac{1}{4} \end{matrix}T) \, = A\cdot \cos(2 \pi f (t - \begin{matrix} \frac{1}{4} \end{matrix}T) %2B \theta) \,
= A\cdot \cos(2 \pi f t - \begin{matrix}\frac{\pi }{2} \end{matrix} %2B \theta ),\,

whose "phase" is now \scriptstyle \theta - \begin{matrix}\frac{\pi }{2} \end{matrix}.   It has been shifted by \begin{matrix}\frac{\pi }{2} \end{matrix} radians.

Phase difference

Phase difference is the difference, expressed in electrical degrees or time, between two waves having the same frequency and referenced to the same point in time.[1] Two oscillators that have the same frequency and different phases have a phase difference, and the oscillators are said to be out of phase with each other. The amount by which such oscillators are out of step with each other can be expressed in degrees from 0° to 360°, or in radians from 0 to 2π. If the phase difference is 180 degrees (π radians), then the two oscillators are said to be in antiphase. If two interacting waves meet at a point where they are in antiphase, then destructive interference will occur. It is common for waves of electromagnetic (light, RF), acoustic (sound) or other energy to become superposed in their transmission medium. When that happens, the phase difference determines whether they reinforce or weaken each other. Complete cancellation is possible for waves with equal amplitudes.

Time is sometimes used (instead of angle) to express position within the cycle of an oscillation.

In-phase and quadrature (I&Q) components

The term in-phase is also found in the context of communication signals:


A(t)\cdot \sin[2\pi ft %2B \phi(t)]
= I(t)\cdot \sin(2\pi ft) %2B Q(t)\cdot \underbrace{\cos(2\pi ft)}_{\sin\left(2\pi ft %2B \frac{\pi}{2} \right)}

and:


A(t)\cdot \cos[2\pi ft %2B \phi(t)]
= I(t)\cdot \cos(2\pi ft) \underbrace{{}- Q(t)\cdot \sin(2\pi ft)}_{{} %2B Q(t)\cdot \cos\left(2\pi ft %2B \frac{\pi}{2}\right)},

where \ f\, represents a carrier frequency, and


I(t)\ \stackrel{\text{def}}{=}\ A(t)\cdot \cos\left(\phi(t)\right), \,

Q(t)\ \stackrel{\text{def}}{=}\ A(t)\cdot \sin\left(\phi(t)\right).\,

A(t)\, and \phi(t)\, represent possible modulation of a pure carrier wave, e.g.:  \sin(2\pi ft)\, (or \cos(2\pi ft).\,) The modulation alters the original \sin\, (or \cos\,) component of the carrier, and creates a (new) \cos\, (or \sin\,) 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° (\begin{matrix} \frac{\pi}{2} \end{matrix} radians) "out of phase", is referred to as the quadrature component.

Phase coherence

Coherence is the quality of a wave to display well defined phase relationship in different regions of its domain of definition.

In physics, quantum mechanics ascribes waves to physical objects. The wave function is complex and since its square modulus is associated with the probability of observing the object, the complex character of the wave function is associated to the phase. Since the complex algebra is responsible for the striking interference effect of quantum mechanics, phase of particles is therefore ultimately related to their quantum behavior.

Phase compensation

Phase compensation is the correction of phase error (i.e., the difference between the actually needed phase and the obtained phase). A phase compensation is required to obtain stability in an opamp. A capacitor/RC network is usually used in the phase compensation to keep a phase margin. A phase compensator subtracts out an amount of phase shift from a signal which is equal to the amount of phase shift added by switching one or more additional amplifier stages into the amplification signal path.

See also

References

  1. ^ a b c Ballou, Glen (2005). Handbook for sound engineers (3 ed.). Focal Press, Gulf Professional Publishing. p. 1499. ISBN 0240807588. http://books.google.com/books?id=y0d9VA0lkogC&pg=PA1499. 

External links