In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significant, and usually undesired, factor in the design of almost all communications links. Jitter can be quantified in the same terms as all time-varying signals, e.g., root mean square (RMS), or peak-to-peak displacement. Also, like other time-varying signals, jitter can be expressed in terms of spectral density. Jitter period is the interval between two times of maximum effect (or minimum effect) of a signal characteristic that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse. ITU-T G.810 classifies deviation lower frequencies below 10 Hz as wander and higher frequencies at or above 10 Hz as jitter. Jitter may be caused by electromagnetic interference and crosstalk with carriers of other signals. Jitter can cause a display monitor to flicker, affect the performance of processors in personal computers, introduce clicks or other undesired effects in audio signals, and cause loss of transmitted data between network devices. The amount of tolerable jitter depends on the affected application.
Metrics For clock jitter, there are three commonly used metrics:
Absolute jitter The absolute difference in the position of a clock's edge from where it would ideally be. Maximum time interval error (MTIE) Maximum error committed by a clock under test in measuring a time interval for a given period of time. Period jitter (a.k.a. cycle jitter) The difference between any one clock period and the ideal or average clock period. Period jitter tends to be important in synchronous circuitry such as digital state machines, where the error-free operation of the circuitry is limited by the shortest possible clock period (average period less maximum cycle jitter), and the performance of the circuitry is set 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 The difference in duration of any two adjacent clock periods. It can be important for some types of clock generation circuitry used in microprocessors and RAM interfaces. In telecommunications, the unit used for the above types of jitter is usually the unit interval (UI) which quantifies the jitter in terms of a fraction of the transmission unit period. 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. This translates to an RMS measurement for a zero-mean distribution. 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 may be more useful. Many efforts have been made to meaningfully quantify distributions that are neither Gaussian nor have a meaningful peak level. All have shortcomings but most tend to be good enough for the purposes of engineering work. In computer networking, jitter can refer to packet delay variation, the variation (statistical dispersion) in the delay of the packets.
Types One of the main differences between random and deterministic jitter is that deterministic jitter is bounded and random jitter is unbounded.
Random jitter Random jitter, also called Gaussian jitter, is unpredictable electronic timing noise. Random jitter typically follows a normal distribution due to being caused by thermal noise in an electrical circuit.
Deterministic jitter Deterministic jitter is a type of clock or data signal jitter that is predictable and reproducible. The peak-to-peak value of this jitter is bounded, and the bounds can easily be observed and predicted. Deterministic jitter has a known non-normal distribution. Deterministic jitter can either be correlated to the data stream (data-dependent jitter) or uncorrelated to the data stream (bounded uncorrelated jitter). Examples of data-dependent jitter are duty-cycle dependent jitter (also known as duty-cycle distortion) and intersymbol interference.
Total jitter
Total jitter (T) is the combination of random jitter (R) and deterministic jitter (D) and is computed in the context to a required bit error rate (BER) for the system:
T = Dpeak-to-peak + 2nRrms, in which the value of n is based on the BER required of the link. A common BER used in communication standards such as Ethernet is 10−12.
Examples
Sampling jitter In analog-to-digital and digital-to-analog conversion of signals, the sampling is normally assumed to be periodic with a fixed period—the time between every two samples is the same. If there is jitter present on the clock signal to the analog-to-digital converter or a digital-to-analog converter, the time between samples varies and instantaneous signal error arises. The error is proportional to the slew rate of the desired signal and the absolute value of the clock error. The effect of jitter on the signal depends on the nature of the jitter. Random jitter tends to add broadband noise while deterministic jitter tends to add errant spectral components, birdys. In some conditions, less than a nanosecond of jitter can reduce the effective bit resolution of a converter with a Nyquist frequency of 22 kHz to 14 bits. Sampling jitter is an important consideration in high-frequency signal conversion, or where the clock signal is especially prone to interference. In digital antenna arrays ADC and DAC jitters are the important factors determining the direction of arrival estimation accuracy and the depth of jammers suppression.
… excerpt ends here. Continue reading the full article.
