jitter

Jitter is an unwanted variation of one or more signal characteristics 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).

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.

CD

In the context of digital audio extraction from Compact Discs, jitter causes extracted audio samples to be doubled-up or skipped entirely. The problem occurs because the Philips CD specification doesn't require block-accurate addressing. As a result, the extraction process will restart a few samples early or late, resulting in doubled or omitted samples. These glitches often sound like tiny repeating clicks during playback. An approach that has produced good results is to do jitter correction in software. This involves performing overlapping reads, and then sliding the data around to find overlaps at the edges. Most extraction programs will perform jitter correction.

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

Phase jitter metrics

An Eye diagram that is being stressed by jitter.

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 between 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.

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.

Jitter Types

Random Jitter

Dark blue is less than one standard deviation from the mean. For the normal distribution, this accounts for about 68% of the set (dark blue) while two standard deviations from the mean (medium and dark blue) account for about 95% and three standard deviations (light, medium, and dark blue) account for about 99.7%.

Random Jitter, also called Gaussian jitter, is electronic timing noise that is unable to be predicted. An example of what random jitter is would be the white noise heard over the radio or seen on a TV screen. In theory random jitter should be random but it has been discovered that it 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 jitter and deterministic jitter is that random jitter is the opposite of deterministic in that deterministic jitter is bounded and random is unbounded.

Deterministic Jitter

Deterministic jitter is clock timing jitter or data signal jitter that is predictable and recreatable. The peak-to-peak value of this jitter is bounded, and the bounds can easily be observed and predicted. Deterministic Jitter is a type of jitter and under this type are different categories. Periodic Jitter, Data-Dependent Jitter, and Duty-Cycle Dependent Jitter are types of Deterministic Jitter.

Jitter Testing

Jitter analysis has become more important in recent years due to high speed electronic components and the need for every thing to work just right. The jitter testing field has grown and with it new tools to test a motherboard's jitter output and receiving component's jitter tolerance. New serial bus architectures are very susceptible to jitter because of the high clock speeds. The eye openings are about 160 picoseconds in some new architectures. This is extremely small compared to parallel bus architectures that have an eye opening of 1000 picoseconds for a very fast architecture. Jitter affects the clock edges and parallel bus architectures there is room for some jitter but in serial buses the eye is so small the jitter is now a major factor.

Measuring Jitter

Jitter measuring is the analysis of the amount of jitter that is occurring on a clock or a data stream. Engineers measure the jitter so that they can find out the breaking point of the component. Motherboard manufacturers measure the jitter that is coming off of the board to make sure it is a reasonable amount. If it is too high, components attached to it will not work properly.

Jitter Injection

Engineers inject jitter into electronic components to test the jitter tolerance of the component. There are standards for serial bus architecture because it is more of a problem than in anything else, so the serial buses must comply with the standards in order to be produced.

Jitter Standards

The standards regarding jitter for different application include jitter tolerance, jitter transfer function and jitter generation, where the required values for the three are different among different applications.

Jitter Tools

FuturePlus's Jitterlyzer - measures, analyzes and injects jitter
Agilent's BERT solutions - measures and analyzes jitter
BertScope products - measures and analyzes jitter
[Shunra.com] simulate lan jitter

Jitter prevention

Anti-jitter circuits

Anti-jitter circuits (AJCs) are a class of electronic circuits designed to reduce the level of jitter in a regular pulse signal. AJCs operate by re-timing the output pulses so they align more closely to an idealised pulse signal. They are widely used in clock and data recovery circuits in digital communications, as well as for data sampling systems such as the analog-to-digital converter and digital-to-analog converter. Examples of anti-jitter circuits include phase-locked loop and delay-locked loop. Inside digital to analog converters jitter causes unwanted high-frequency distortions. In this case it can be suppressed with high fidelity clock signal usage.

Jitter buffers

Jitter buffers or de-jitter buffers are used to counter jitter introduced by packet switched networks so that a continuous playout of audio (or video) transmitted over the network can be ensured. The maximum jitter that can be countered by a de-jitter buffer is equal to the buffering delay introduced before starting the play-out of the mediastream.

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.

Dejitterizer

A dejitterizer is a device that reduces jitter in a digital signal. A dejitterizer usually consists of an elastic buffer in which the signal is temporarily stored and then retransmitted at a rate based on the average rate of the incoming signal. A dejitterizer is usually ineffective in dealing with low-frequency jitter, such as waiting-time jitter.

See also

• Buffer (telecommunication)
• Dither
• Delay jitter
• Deterministic jitter
• Drift
• Wander
• FuturePlus
• Agilent
• Tektronix

References

External links

• Fibre Channel MJSQ (Methodology of Jitter Specification & Quality) Information
• Jitter in VoIP - Causes, solutions and recommended values
• Jitter Buffer
• Definition of Jitter in a QoS Testing Methodology
• An Introduction to Jitter in Communications Systems
• Jitter Specifications Made Easy A Heuristic Discussion of Fibre Channel and Gigabit Ethernet Methods
• Understanding and Characterizing Timing Jitter a primer from Tektronix
• Jitter Master Competence Center & Forum from Agilent Technologies Jitter Forum and multimedia expert explanations
• Jitter analysis tool from FuturePlus
• [1] Network simulation tools M.Vaknin

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