# jitter fundamentals

Random Jitter (RJ)

Jitter that is not bounded and can be described by a Gaussian probability distribution. Random jitter is characterized by its standard deviation (rms) value.

The principal source is Gaussian (white) electrical noise within system components. Electrical noise interacts with the slew rate of signals to produce timing errors at the switching points.

Random

RJ measurement method. A probability distribution based on the difference in time between an actual clock edge and its ideal (intended) position.

Although two measurements of the same source, random and cycle to cycle jitter are not equivalent. Cycle to cycle jitter has frequency dependant terms and, compared to random jitter measurements, will accentuate high frequency jitter sources while rejecting low frequency sources. The random jitter measurement is independent of frequency.

Cycle to Cycle

Adjacent Cycle

RJ measurement method. A probability distribution based on the difference in the period measured between one clock cycle and an adjacent cycle.

Deterministic Jitter (DJ)

Jitter with a non-Gaussian probability density function. Always bounded in amplitude and with specific causes. DJ is characterized by its bounded, peak-to-peak, value.

Sources are generally related to imperfections in the behavior of a device or transmission media but also may also be due to EMI, crosstalk, grounding problems.

Duty Cycle Distortion

Pulse Width Distortion

Pulse Skew

DJ component. Deviation in duty cycle value from the ideal (intended) value. In many serial data systems this equates to a deviation in bit time between a 1 bit and a 0 bit. May also defined as the difference in propagation delay between low to high and high to low delay times.

Source is commonly timing differences between rising and falling edges within a system. May also be caused by ground shifts in single ended systems.

Data Dependant Jitter

Pattern Jitter

Inter-Symbol Interference

DJ component. Timing errors that vary with the data pattern used. Data dependant and Pattern jitter are used to describe the effect of jitter in the time domain. Inter-Symbol Interference is more commonly applied to frequency domain measurements i.e., the spreading of a signal peak as would be seen on a spectrum analyzer.

Primary source is component and system bandwidth limitations. Higher frequency signals have less time to settle than lower frequency ones. This leads to changes in the start conditions for transitions at different frequencies and produces timing errors dependent on the data pattern being applied.

Sinusoidal Jitter

Periodic Jitter

DJ component. Jitter that has a sinusoidal (or periodic) form and is related to (correlates to) the data pattern.

Source is interference from signals that are related to the data pattern. Ground bounce and other power supply variations are common causes although the levels of sinusoidal jitter normally encountered are very low.

Uncorrelated bounded Jitter

DJ component. Jitter that is bounded in amplitude and uncorrelated (to the data pattern).

Commonly sinusoidal in nature, source is interference from other signal sources either within the system or external to it. Sources include EMI, capacitive and inductive coupling and power supply switching noise.

Total Jitter (TJ)

The summation (or convolution) of deterministic and random jitter. Total jitter is the peak to peak value obtained.

TJ = DJ + n × RJ where n = number of standard deviations corresponding to the required BER. This summation is usually applied due to it's simplicity although this method over-estimates the actual BER since the maximum RJ errors will not always coincide with maximum DJ error. A probabilistic (convolved) summation of the two jitter types would produce a more accurate solution though the application of this would require knowledge of the DJ modulation waveform.

Mapping Jitter

System level jitter component of DJ type. Jitter due to mapping of data from one transmission standard to another when bit stuffing has occurred during the mapping process.

Gaps are left in the recovered signal after de-mapping. Phase locked loops (PLLs) are used to smooth the resulting gaps but a certain amount of jitter remains.

Pointer Jitter

System level jitter component of DJ type. Jitter resulting from the application of a SONET signal containing defined sequences of pointer activity to a demultiplexer.

Wander Jitter

System level jitter component of DJ type. Low frequency timing errors less than 10Hz in frequency (SONET).

Principal source is system temperature variations.

Jitter Transfer

Jitter Gain

Ratio of jitter on output signal to jitter applied on input signal.

Used to quantify the jitter accumulation performance of data retiming devices: regenerators, PLLs.

Jitter Tolerance

Amount of input jitter a receiver must tolerate without violating system BER specifications.

Can be split into Random Jitter Tolerance and Deterministic Jitter Tolerance.

Unit Interval (UI)

Time period equivalent to 1 bit time in a serial data stream.

Reciprocal of baud rate. Jitter specifications are often quoted in multiples of UI.

via An Introduction to Jitter in Communications Systems – Maxim.

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