# pha ban đầu

In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other periodic function of some real variable (such as time) is an angle-like quantity representing the fraction of the cycle covered up vĩ đại . It is expressed in such a scale that it varies by one full turn as the variable goes through each period (and goes through each complete cycle). It may be measured in any angular unit such as degrees or radians, thus increasing by 360° or as the variable completes a full period.[1]

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This convention is especially appropriate for a sinusoidal function, since its value at any argument then can be expressed as , the sine of the phase, multiplied by some factor (the amplitude of the sinusoid). (The cosine may be used instead of sine, depending on where one considers each period vĩ đại start.)

Usually, whole turns are ignored when expressing the phase; so sánh that is also a periodic function, with the same period as , that repeatedly scans the same range of angles as goes through each period. Then, is said vĩ đại be "at the same phase" at two argument values and (that is, ) if the difference between them is a whole number of periods.

The numeric value of the phase depends on the arbitrary choice of the start of each period, and on the interval of angles that each period is vĩ đại be mapped vĩ đại.

The term "phase" is also used when comparing a periodic function with a shifted version of it. If the shift in is expressed as a fraction of the period, and then scaled vĩ đại an angle spanning a whole turn, one gets the phase shift, phase offset, or phase difference of relative vĩ đại . If is a "canonical" function for a class of signals, lượt thích is for all sinusoidal signals, then is called the initial phase of .

## Mathematical definition

Let be a periodic signal (that is, a function of one real variable), and be its period (that is, the smallest positive real number such that for all ). Then the phase of at any argument is

Here denotes the fractional part of a real number, discarding its integer part; that is, ; and is an arbitrary "origin" value of the argument, that one considers vĩ đại be the beginning of a cycle.

This concept can be visualized by imagining a clock with a hand that turns at constant tốc độ, making a full turn every seconds, and is pointing straight up at time . The phase is then the angle from the 12:00 position vĩ đại the current position of the hand, at time , measured clockwise.

The phase concept is most useful when the origin is chosen based on features of . For example, for a sinusoid, a convenient choice is any where the function's value changes from zero vĩ đại positive.

The formula above gives the phase as an angle in radians between 0 and . To get the phase as an angle between and , one uses instead

The phase expressed in degrees (from 0° vĩ đại 360°, or from −180° vĩ đại +180°) is defined the same way, except with "360°" in place of "2π".

### Consequences

With any of the above definitions, the phase of a periodic signal is periodic too, with the same period :

for all .

The phase is zero at the start of each period; that is

for any integer .

Moreover, for any given choice of the origin , the value of the signal for any argument depends only on its phase at . Namely, one can write , where is a function of an angle, defined only for a single full turn, that describes the variation of as ranges over a single period.

In fact, every periodic signal with a specific waveform can be expressed as

where is a "canonical" function of a phase angle in 0 vĩ đại 2π, that describes just one cycle of that waveform; and is a scaling factor for the amplitude. (This claim assumes that the starting time chosen vĩ đại compute the phase of corresponds vĩ đại argument 0 of .)

Since phases are angles, any whole full turns should usually be ignored when performing arithmetic operations on them. That is, the sum and difference of two phases (in degrees) should be computed by the formulas

and

respectively. Thus, for example, the sum of phase angles 190° + 200° is 30° (190 + 200 = 390, minus one full turn), and subtracting 50° from 30° gives a phase of 340° (30 - 50 = −20, plus one full turn).

## Phase shift 

The difference between the phases of two periodic signals and is called the phase difference or phase shift of relative vĩ đại .[1] At values of when the difference is zero, the two signals are said vĩ đại be in phase, otherwise they are out of phase with each other.

In the clock analogy, each signal is represented by a hand (or pointer) of the same clock, both turning at constant but possibly different speeds. The phase difference is then the angle between the two hands, measured clockwise.

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The phase difference is particularly important when two signals are added together by a physical process, such as two periodic sound waves emitted by two sources and recorded together by a microphone. This is usually the case in linear systems, when the superposition principle holds.

For arguments when the phase difference is zero, the two signals will have the same sign and will be reinforcing each other. One says that constructive interference is occurring. At arguments when the phases are different, the value of the sum depends on the waveform.

### For sinusoids

For sinusoidal signals, when the phase difference is 180° ( radians), one says that the phases are opposite, and that the signals are in antiphase. Then the signals have opposite signs, and destructive interference occurs. Conversely, a phase reversal or phase inversion implies a 180-degree phase shift.[2]

When the phase difference is a quarter of turn (a right angle, +90° = π/2 or −90° = 270° = −π/2 = 3π/2), sinusoidal signals are sometimes said vĩ đại be in quadrature (e.g., in-phase and quadrature components).

If the frequencies are different, the phase difference increases linearly with the argument . The periodic changes from reinforcement and opposition cause a phenomenon called beating.

### For shifted signals

The phase difference is especially important when comparing a periodic signal with a shifted and possibly scaled version of it. That is, suppose that for some constants and all . Suppose also that the origin for computing the phase of has been shifted too. In that case, the phase difference is a constant (independent of ), called the 'phase shift' or 'phase offset' of relative vĩ đại . In the clock analogy, this situation corresponds vĩ đại the two hands turning at the same tốc độ, so sánh that the angle between them is constant.

In this case, the phase shift is simply the argument shift , expressed as a fraction of the common period (in terms of the modulo operation) of the two signals and then scaled vĩ đại a full turn:

If is a "canonical" representative for a class of signals, lượt thích is for all sinusoidal signals, then the phase shift called simply the initial phase of .

Therefore, when two periodic signals have the same frequency, they are always in phase, or always out of phase. Physically, this situation commonly occurs, for many reasons. For example, the two signals may be a periodic soundwave recorded by two microphones at separate locations. Or, conversely, they may be periodic soundwaves created by two separate speakers from the same electrical signal, and recorded by a single microphone. They may be a radio signal that reaches the receiving antenna in a straight line, and a copy of it that was reflected off a large building nearby.

A well-known example of phase difference is the length of shadows seen at different points of Earth. To a first approximation, if is the length seen at time at one spot, and is the length seen at the same time at a longitude 30° west of that point, then the phase difference between the two signals will be 30° (assuming that, in each signal, each period starts when the shadow is shortest).

### For sinusoids with same frequency

For sinusoidal signals (and a few other waveforms, lượt thích square or symmetric triangular), a phase shift of 180° is equivalent vĩ đại a phase shift of 0° with negation of the amplitude. When two signals with these waveforms, same period, and opposite phases are added together, the sum is either identically zero, or is a sinusoidal signal with the same period and phase, whose amplitude is the difference of the original amplitudes.

The phase shift of the co-sine function relative vĩ đại the sine function is +90°. It follows that, for two sinusoidal signals and with same frequency and amplitudes and , and has phase shift +90° relative vĩ đại , the sum is a sinusoidal signal with the same frequency, with amplitude and phase shift from , such that

and .

A real-world example of a sonic phase difference occurs in the warble of a Native American flute. The amplitude of different harmonic components of same long-held note on the flute come into dominance at different points in the phase cycle. The phase difference between the different harmonics can be observed on a spectrogram of the sound of a warbling flute.[4]

## Phase comparison

Phase comparison is a comparison of the phase of two waveforms, usually of the same nominal frequency. In time and frequency, the purpose of a phase comparison is generally vĩ đại determine the frequency offset (difference between signal cycles) with respect vĩ đại a reference.[3]

A phase comparison can be made by connecting two signals vĩ đại a two-channel oscilloscope. The oscilloscope will display two sine signals, as shown in the graphic vĩ đại the right. In the adjacent image, the top sine signal is the test frequency, and the bottom sine signal represents a signal from the reference.

If the two frequencies were exactly the same, their phase relationship would not change and both would appear vĩ đại be stationary on the oscilloscope display. Since the two frequencies are not exactly the same, the reference appears vĩ đại be stationary and the test signal moves. By measuring the rate of motion of the test signal the offset between frequencies can be determined.

Vertical lines have been drawn through the points where each sine signal passes through zero. The bottom of the figure shows bars whose width represents the phase difference between the signals. In this case the phase difference is increasing, indicating that the test signal is lower in frequency than vãn the reference.[3]

## Formula for phase of an oscillation or a periodic signal

The phase of an oscillation or signal refers vĩ đại a sinusoidal function such as the following:

where , , and are constant parameters called the amplitude, frequency, and phase of the sinusoid. These signals are periodic with period , and they are identical except for a displacement of along the axis. The term phase can refer vĩ đại several different things:

## Absolute phase

Absolute phase refers vĩ đại the phase of a waveform relative vĩ đại some standard (strictly speaking, phase is always relative). To the extent that this standard is accepted by all parties, one can speak of an absolute phase in a particular field of application.

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