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Jitter is an unwanted variation of one or more characteristics of a periodic signal in electronics and telecommunications. Jitter may be seen in characteristics such as the interval between successive pulses, or the amplitude, frequency, or phase of successive cycles. Jitter is a significant factor in the design of almost all communications links (e.g. USB, PCI-e, SATA, OC-48). In clock recovery applications it is called timing jitter.

Jitter can apply to a number of signal qualities (e.g. amplitude, phase, pulse width or pulse position), and can be quantified in the same terms as all time-varying signals (e.g. RMS, or peak-to-peak displacement). Also like other time-varying signals, jitter can be expressed in terms of spectral density (frequency content). Jitter period is the interval between two times of maximum effect (or between two times of minimum effect) of a jitter characteristic, for a jitter that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse. Generally, very low jitter frequency is not of interest in designing systems, and the low-frequency cutoff for jitter is typically specified at 1 Hz.

Due to additional sector level addressing added in the Yellow Book (CD standard), CD-ROM data discs are not subject to seek jitter.

A jitter meter is a testing instrument for measuring clock jitter values, and is used in manufacturing DVD and CD-ROM discs.

For clock jitter, there are three commonly used metrics: absolute jitter, period jitter, and cycle to cycle jitter.

Cycle-to-cycle jitter is the difference in length of any two adjacent clock periods. Accordingly, it can be thought of as the discrete-time derivative of period jitter. It can be important for some types of clock generation circuitry used in microprocessors and RAM interfaces. dh All of these jitter metrics are really measures of a single time-dependent quantity, and hence are related by derivatives as described above. Since they have different generation mechanisms, different circuit effects, and different measurement methodology, it is still useful to quantify them separately.

In the telecommunications world, the unit used for the above types of jitter is usually the UI (or Unit Interval) which quantifies the jitter in terms of a fraction of the ideal period of the clock. This unit is useful because it scales with clock frequency and thus allows relatively slow interconnects such as T1 to be compared to higher-speed internet backbone links such as OC-192. Absolute units such as picoseconds are more common in microprocessor applications. Units of degrees and radians are also used.

If jitter has a Gaussian distribution, it is usually quantified using the standard deviation of this distribution (aka. RMS). Often, jitter distribution is significantly non-Gaussian. This can occur if the jitter is caused by external sources such as power supply noise. In these cases, peak-to-peak measurements are more useful. Many efforts have been made to meaningfully quantify distributions that are neither Gaussian nor have meaningful peaks (which is the case in all real jitter). All have shortcomings but most tend to be good enough for the purposes of engineering work. Note that typically, the reference point for jitter is defined such that the mean jitter is 0.

In networking, in particular IP networks such as the Internet, jitter can refer to the variation (statistical dispersion) in the delay of the packets.

Random Jitter, also called Gaussian jitter, is unpredictable electronic timing noise. Random jitter typically follows a Gaussian distribution or Normal distribution. It is believed to follow this pattern because most noise or jitter in a electrical circuit is caused by thermal noise, which does have a Gaussian distribution. Another reason for random jitter to have a distribution like this is due to the Central limit theorem. The central limit theorem states that composite effect of many uncorrelated noise sources, regardless of the distributions, approaches a Gaussian distribution. One of the main differences between random and deterministic jitter is that deterministic jitter is bounded and random jitter is unbounded.

Some systems use sophisticated delay-optimal de-jitter buffers which are capable of adapting the buffering delay to changing network jitter characteristics. These are known as adaptive de-jitter buffers and the adaptation logic is based on the jitter estimates computed from the arrival characteristics of the media packets. Adaptive de-jittering involves introducing discontinuities in the media play-out which may appear offensive to the listener or viewer. Adaptive de-jittering is usually carried out for audio play-outs which feature a VAD/DTX encoded audio, that allows the lengths of the silence periods to be adjusted, thus minimizing the perceptual impact of the adaptation.

- Phase noise
- Buffer (telecommunication)
- Dither
- Deterministic jitter
- Drift
- Wander
- Pulse (signal processing)

- * Wolaver, Dan H. 1991. Phase-Locked Loop Circuit Design, Prentice Hall, ISBN 0-13-662743-9, pages 211-237
- * Trischitta, Patrick R. and Varma, Eve L. 1989. Jitter in Digital Transmission Systems, Artech ISBN 089006248X

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- Understanding and Characterizing Timing Jitter a primer from Tektronix
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- Jitter in Packet Voice Networks

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Last updated on Friday October 03, 2008 at 05:30:33 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Friday October 03, 2008 at 05:30:33 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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