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PowerPedia:Jitter

Lasted edited by Andrew Munsey, updated on June 14, 2016 at 10:10 pm.

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Jitter, in There was an error working with the wiki: Code[2] characteristics or electrical impulse, such as the interval between successive electrical pulses, the amplitude of successive cycles, or the There was an error working with the wiki: Code[3] of successive cycles.

Introduction

Jitter is a significant factor in the design of almost all communications links (e.g. There was an error working with the wiki: Code[4], or peak-to-peak displacement). Also like other time-varying signals, jitter can be expressed in terms of spectral density (frequency content). Generally, very low frequency jitter is not of interest in designing systems, and the low-frequency There was an error working with the wiki: Code[15] for jitter is typically specified at 1 Hz. A jitter meter is a testing instrument for measuring jitter values and is used in manufacturing There was an error working with the wiki: Code[16] and There was an error working with the wiki: Code[17] drives.

Phase Jitter Metrics

For There was an error working with the wiki: Code[5] jitter, there are three commonly used metrics: absolute jitter, period jitter, and cycle to cycle jitter.

Absolute jitter is the absolute difference in the position of a clock's edge from where it would ideally be if the clock's frequency was perfectly constant. The absolute jitter metric is important in systems where a large number of clock sources are trying to pass data to one another (eg. There was an error working with the wiki: Code[6]).

Period jitter (aka cycle jitter) is the difference between any one clock period and the ideal clock period. Accordingly, it can be thought of as the discrete-time derivative of absolute jitter. Period jitter tends to be important in synchronous circuitry like digital state machines where the error-free operation of the circuitry is limitted by the shortest possible clock period, and the performance of the circuitry is limitted by the average clock period. Hence, synchronous circuitry benefits from minimizing period jitter, so that the shortest clock period approaches the average clock period.

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 There was an error working with the wiki: Code[7] 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 (ie. is random), 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 There was an error working with the wiki: Code[8] jitter is 0.

In There was an error working with the wiki: Code[9], in particular There was an error working with the wiki: Code[10] There was an error working with the wiki: Code[11] such as the There was an error working with the wiki: Code[18], jitter can refer to the variation (There was an error working with the wiki: Code[19]) in the delay of the There was an error working with the wiki: Code[20]s (because of There was an error working with the wiki: Code[21]s' internal queues behaviour in certain circumstances, There was an error working with the wiki: Code[22] changes, etc).

Jitter prevention

Anti-jitter circuits

Anti-jitter circuits (AJCs) are a class of There was an error working with the wiki: Code[12]. 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 There was an error working with the wiki: Code[23], as well as for data sampling systems such as the There was an error working with the wiki: Code[24] and There was an error working with the wiki: Code[25]. Examples of anti-jitter circuits include There was an error working with the wiki: Code[26] and There was an error working with the wiki: Code[27]. 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 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 There was an error working with the wiki: Code[13] 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.

Related

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References and external links

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Fibre Channel MJSQ (Methodology of Jitter Specification & Quality) Information

Jitter Buffer

An Introduction to Jitter in Communications Systems

Jitter Specifications Made Easy A Heuristic Discussion of Fibre Channel and Gigabit Ethernet Methods

Jitter Test Solutions Be A Jitter Master With Agilent Technologies

Understanding and Characterizing Timing Jitter a primer from Tektronix

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

See also

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