Jitter

Jitter [ dʒɪtɚ ] (English for " fluctuation " or " fluctuation " ) refers to the time clock jitter in the transmission of digital signals, a slight variation in the accuracy of the transfer clock (English clock). Jitter is undesirable as an interference in the normal case. More generally, jitter in transmission technology, an abrupt and undesired change in the signal characteristics. This may affect both amplitude and frequency and phase position. The jitter is the first derivative of a delay ( Delay Data Sheet ). The spectral representation of the temporal variations is called phase noise. Jitter is not to be confused with quantization errors.

In the network technology, the variance of the duration of data packets is referred to as jitter as well. This effect is particularly important for multimedia applications on the Internet (such as Internet radio and Internet telephony ) is annoying as this packets may arrive too late or too early to be issued in time with can. The effect is reduced by a so-called jitter buffer, a special " data buffer ", but at the price of additional runtime, what bothers especially in dialog applications. This effect also plays a role in process control. Critical process information must be shipped and received within a certain time. If the jitter is too large, the timely arrival of the critical process information is not guaranteed.

Assessment

For the evaluation of jitter in the form of measurements, various methods are available. In the field of digital signal processing, such as the digital audio in the context of AES-3 or digital video signals within the Serial Digital Interface (SDI ), the time jitter is expressed as a relative size in unit intervals (UI). A UI corresponds to the duration of a symbol. In a binary transfer is shown the time for the transmission of a bit, as shown in the adjacent diagram for an exemplary binary sequence " 01001 ". The transitions between two consecutive different bits are represented by a slurred signal edge in light blue stylized. The jitter effect that the actual time signal waveform of the ideal waveform, which is shown as dark blue line deviates in the range of the signal edge.

A large jitter causes an amplified -symbol interference and consequently an increased bit error rate, which may also be represented in a reduction of the horizontal opening of the eye diagram. The deviation from the ideal timing of the edge signal can be expressed in addition to the information relating to the symbol rate specified in the UI as an absolute time. Common absolute values ​​Aj as or as a peak -to-peak shown in the diagram, have femtosecond (fs ) to a few 100 ps ( picoseconds ) in the case of serial digital transmissions in the mega to gigabit some 100. Slower transmissions are depending on the process, even under circumstances absolute jitter down to the microsecond range allowed.

The deviations whose spectral representation is referred to as phase noise can be divided into periodic and deterministic and random jitter components. The periodic components can be described by a fundamental, referred to in the chart with the duration Tj because of her pose the most time deflection. Your superimposed, higher spectral components with lower amplitude and random jitter, which are pronounced differently depending on the cause.

The jitter frequency of the fundamental frequency f j is given as:

To receive digital data streams, and the determination of the sampling time at the receiver is a clock recovery necessary, using different forms, inter alia phase-locked loops. Those control loops can slowly progressive, ie low frequency spectral components of jitter by re-adjustment of the local oscillator offset directly while higher-frequency jitter components are suppressed by the low-pass behavior of the loop filter and thus can lead to scanning errors.

For the numerical evaluation, it is therefore necessary to divide the jitter in its spectral components and to evaluate them separately or set allowable limits for the individual frequency ranges, depending on the transmission method. For example, the spectral jitter components that are smaller than 10 Hz is generally referred to as wander.

The names of the parent jitter Shares are not uniformly chosen in the literature and in the individual transfer process. Thus, for example within the framework of digital video transmission (SDI ) under timing jitter those spectral components between 10 Hz and 1 kHz (for SD -SDI, in the standard SMPTE 259M ) or between 10 Hz and 100 kHz (HD -SDI, in the standard SMPTE 292M ) understood. This jitter components can be compensated directly in the rule or by the control loops. Spectral shares also are referred to as alignment jitter, since they can lead to scanning errors directly and will not be compensated by the phase-locked loops.

Species

Jitter is in deterministic jitter (DJ) and random jitter divided (English random jitter, RJ). In transmission systems, both parts come in different strength weightings at superimposed. The deterministic jitter, in contrast to the random jitter normal distribution to, is always limited in amplitude and is described by its maximum occurring peak -to-peak deviations. It can be determined among other things by a corresponding symbol sequences and is divided into the following units:

• Periodic jitter components: Cause which are typically inject periodic external interference in the transmission system.
• Data-dependent jitter components: these proportions depend on the transmitted data sequences and are caused by the symbol crosstalk.
• Amount of jitter due to unequal pulse widths ( engl. duty cycle jitter): Cause are different slew rates at the rising or falling edge.

Of these shares of the deterministic jitter of random jitter is to be distinguished, which has a normal distribution and its cause, among others, in the thermal noise, non-uniform doping of impurities into the semiconductor material used, and has other random disturbances such as cosmic radiation. Random jitter is described by its standard deviation.

Total jitter

The total or total jitter TJ is a combination consisting of deterministic jitter (DJ) and random jitter (RJ) in the form:

The weighting factor is determined by the allowable bit error rate ( BER). Typical values ​​for the bit error rate, such as in Ethernet, values ​​such as 10-12 are with. Other values ​​are summarized in the adjacent table.

Digital Audio Systems

Another example of jitter are errors that can occur when converting analog signals to digital signals. In the scanning (English sampling) a fixed period is used, for example, at 22.67 microseconds 44.1 their respective amplitude values ​​are read out in the range of audio signals.

Also in digital audio systems according to the AES3 standard jitter according to the spectral distribution in

• Low-frequency interface jitter and
• Higher-frequency sampling jitter

Divided. Sampling jitter is produced in digital audio systems, inter alia, for the analog -to-digital converters, the asynchronous sample rate converters and digital -to-analog converters.

Mathematical definitions

Periodic Jitter:

It is the period of the first oscillation according to the trigger event and the ideal period.

Cycle -to -Cycle Jitter:

It is determined, the maximum deviation from one period to the next.

Accumulated jitter

The accumulated jitter starting at a trigger event is related to the event (eg, a rising edge of a clock signal ) of the jitter. The longer the bars in the future the greater the shift will be when the jitter is not evenly distributed.