View source for Square wave

{{Use dmy dates|date=July 2023}}
{{short description|Type of non-sinusoidal waveform}}
{{About|the waveform|the type of ocean surface wave|Cross sea}}
{{refimprove|date=December 2019}}

{{Infobox mathematical function
| name = Square wave
| image = Waveforms.svg
| imagesize = 400px
| imagealt = Sine, square, triangle, and sawtooth waveforms
| caption = [[Sine wave|Sine]], square, [[triangle wave|triangle]], and [[sawtooth wave|sawtooth]] waveforms
| general_definition = <math>x(t) = 4 \left\lfloor t \right\rfloor - 2 \left\lfloor 2 t \right\rfloor + 1, 2 t \notin \mathbb{Z}</math>
| fields_of_application = Electronics, synthesizers
| domain = <math>\mathbb{R} \setminus \left\{ \tfrac{n}{2} \right\}, n \in \mathbb{Z}</math>
| codomain = <math>\left\{ -1,1 \right\}</math>
| parity = Odd
| period = 1
| antiderivative = [[Triangle wave]]
| fourier_series = <math>x(t) = \frac{4}{\pi} \sum_{k=1}^{\infty} \frac{1}{2 k - 1} \sin \left( 2 \pi \left(2 k - 1 \right) t \right)</math>
}}

{{Listen|filename=220 Hz anti-aliased square wave.ogg|title=Square wave sound sample|description=5 seconds of square wave at 220&nbsp;Hz|format=[[Ogg]]}}

A '''square wave''' is  a [[non-sinusoidal waveform|non-sinusoidal periodic waveform]] in which the amplitude alternates at a steady [[frequency]] between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions between minimum and maximum are instantaneous. 

The square wave is a special case of a [[pulse wave]] which allows arbitrary durations at minimum and maximum amplitudes. The ratio of the high period to the total period of a pulse wave is called the [[duty cycle]]. A true square wave has a 50% duty cycle (equal high and low periods).

Square waves are often encountered in [[electronics]] and [[signal processing]], particularly [[digital electronics]] and [[digital signal processing]]. Its [[stochastic]] counterpart is a [[two-state trajectory]].

== Origin and uses ==

Square waves are universally encountered in digital switching circuits and are naturally generated by binary (two-level) logic devices. They are used as timing references or "[[clock signal]]s", because their fast transitions are suitable for triggering [[synchronous logic]] circuits at precisely determined intervals. However, as the frequency-domain graph shows, square waves contain a wide range of harmonics; these can generate [[electromagnetic radiation]] or pulses of current that interfere with other nearby circuits, causing [[noise]] or errors. To avoid this problem in very sensitive circuits such as precision [[analog-to-digital converter]]s, [[sine wave]]s are used instead of square waves as timing references.

In musical terms, they are often described as sounding hollow, and are therefore used as the basis for [[wind instrument]] sounds created using [[subtractive synthesis]]. They also make up the "beeping" alerts used in many household, commercial, and industrial contexts. Additionally, the distortion effect used on [[electric guitar]]s clips the outermost regions of the waveform, causing it to increasingly resemble a square wave as more distortion is applied.

Simple two-level [[Rademacher function]]s are square waves.

== Definitions ==

The square wave in mathematics has many definitions, which are equivalent except at the discontinuities:

It can be defined as simply the [[sign function]] of a sinusoid:
<math display="block">\begin{align}
   x(t) &= \sgn\left(\sin \frac{2\pi t}{T}\right) = \sgn(\sin 2\pi ft) \\
   v(t) &= \sgn\left(\cos \frac{2\pi t}{T}\right) = \sgn(\cos 2\pi ft),
 \end{align}</math>
which will be 1 when the sinusoid is positive, −1 when the sinusoid is negative, and 0 at the discontinuities.  Here, ''T'' is the [[Period (physics)|period]] of the square wave and ''f'' is its frequency, which are related by the equation ''f'' = 1/''T''.

A square wave can also be defined with respect to the [[Heaviside step function]] ''u''(''t'') or the [[rectangular function]] Π(''t''):
<math display="block">\begin{align}
 x(t) &= 2\left[\sum_{n=-\infty}^\infty \Pi\left(\frac{2(t - nT)}{T} - \frac{1}{2}\right)\right] - 1 \\
   &= 2\sum_{n=-\infty}^\infty \left[u \left(\frac{t}{T} - n\right) - u \left(\frac{t}{T} - n - \frac{1}{2} \right) \right] - 1.
 \end{align}</math>

A square wave can also be generated using the [[floor function]] directly:
<math display="block">x(t) = 2\left(2\lfloor ft\rfloor - \lfloor 2 ft\rfloor \right) + 1</math>
and indirectly:
<math display="block">x(t) = \left(-1\right)^{\lfloor 2ft \rfloor}.</math>

Using the fourier series (below) one can show that the floor function may be written in trigonometric form<ref>{{cite web |title=Partial sum formula |url=https://www.wolframalpha.com/input?i=%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%5Cfrac%7B%5Csin%5Cleft%28%5Cleft%282n-1%5Cright%29x%5Cright%29%7D%7B2n-1%7D |website=www.wolframalpha.com |access-date=9 July 2023 |archive-url=https://web.archive.org/web/20230122100923/https://www.wolframalpha.com/input?i=%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%5Cfrac%7B%5Csin%5Cleft%28%5Cleft%282n-1%5Cright%29x%5Cright%29%7D%7B2n-1%7D |archive-date=January 22, 2023 |language=en |url-status=live}}</ref>
<math display="block">\frac{2}{\pi}\arctan\left(\tan\left(\frac{\pi ft}{2}\right)\right)+\frac{2}{\pi}\arctan\left(\cot\left(\frac{\pi ft}{2}\right)\right)</math>

== Fourier analysis {{anchor|Fourier_Analysis}} ==
[[File:SquareWaveFourierArrows.gif|thumb|upright=1.35|The six arrows represent the first six terms of the Fourier series of a square wave. The two circles at the bottom represent the exact square wave (blue) and its Fourier-series approximation (purple).]]
[[File:Spectrum square oscillation.jpg|thumb|upright=1.6|right|(Odd) harmonics of a 1000 Hz square wave]]
[[File:Fourier Series-Square wave 3 H.png|thumb|Graph showing the first 3 terms of the Fourier series of a square wave]]

Using [[Fourier series|Fourier expansion]] with cycle frequency {{math|''f''}} over time {{math|''t''}}, an ideal square wave with an amplitude of 1 can be represented as an infinite sum of sinusoidal waves:
<math display="block">\begin{align}
x(t) &= \frac{4}{\pi} \sum_{k=1}^\infty \frac{\sin\left(2\pi(2k - 1)ft\right)}{2k - 1} \\ 
     &= \frac{4}{\pi} \left(\sin(\omega t) + \frac{1}{3} \sin(3 \omega t) + \frac{1}{5} \sin(5 \omega  t) + \ldots\right),
     &\text{where }\omega=2\pi f.
\end{align}</math>

{{Listen|filename=Additive_220Hz_Square_Wave.wav|title=Additive square demo|description=220&nbsp;Hz square wave created by harmonics added every second over sine wave}}

The ideal square wave contains only components of odd-integer [[harmonic]] frequencies (of the form {{math|2π(2''k'' − 1)''f''}}).

A curiosity of the convergence of the [[Fourier series]] representation of the square wave is the [[Gibbs phenomenon]]. [[Ringing artifacts]] in non-ideal square waves can be shown to be related to this phenomenon. The Gibbs phenomenon can be prevented by the use of [[sigma approximation|σ-approximation]], which uses the [[Lanczos sigma factor]]s to help the sequence converge more smoothly.

An ideal mathematical square wave changes between the high and the low state instantaneously, and without under- or over-shooting. This is impossible to achieve in physical systems, as it would require infinite [[Bandwidth (signal processing)|bandwidth]].

[[File:Fourier_series_for_square_wave.gif|thumb|upright=1.6|right|Animation of the additive synthesis of a square wave with an increasing number of harmonics]]

Square waves in physical systems have only finite bandwidth and often exhibit [[Ringing (signal)|ringing]] effects similar to those of the Gibbs phenomenon or ripple effects similar to those of the σ-approximation.

For a reasonable approximation to the square-wave shape, at least the fundamental and third harmonic need to be present, with the fifth harmonic being desirable. These bandwidth requirements are important in digital electronics, where finite-bandwidth analog approximations to square-wave-like waveforms are used. (The ringing transients are an important electronic consideration here, as they may go beyond the electrical rating limits of a circuit or cause a badly positioned threshold to be crossed multiple times.)

== Characteristics of imperfect square waves ==

As already mentioned, an ideal square wave has instantaneous transitions between the high and low levels. In practice, this is never achieved because of physical limitations of the system that generates the waveform. The times taken for the signal to rise from the low level to the high level and back again are called the ''[[rise time]]'' and the ''[[fall time]]'' respectively.

If the system is [[overdamped]], then the waveform may never actually reach the theoretical high and low levels, and if the system is underdamped, it will oscillate about the high and low levels before settling down. In these cases, the rise and fall times are measured between specified intermediate levels, such as 5% and 95%, or 10% and 90%. The [[Bandwidth (signal processing)|bandwidth]] of a system is related to the transition times of the waveform; there are formulas allowing one to be determined approximately from the other.

== See also ==
* [[List of periodic functions]]
* [[Rectangular function]]
* [[Pulse wave]]
* [[Sine wave]]
* [[Triangle wave]]
* [[Sawtooth wave]]
* [[Waveform]]
* [[Sound]]
* [[Multivibrator]]
* [[Ronchi ruling]], a square-wave stripe target used in imaging.
* [[Cross sea]]
* [[Clarinet]], a musical instrument that produces odd overtones approximating a square wave.

==References==
{{reflist}}

==External links==
* [https://www.geogebra.org/m/wUanseCs Fourier decomposition of a square wave] Interactive demo of square wave synthesis using sine waves, from GeoGebra site.
* [http://sara.ng/apps/square-wave Square Wave Approximated by Sines] Interactive demo of square wave synthesis using sine waves.
* [http://www.electric1.es/armonicos/armonicosOC.html Flash applets] Square wave.

{{Waveforms}}

[[Category:Waveforms]]
[[Category:Fourier series]]