What is bit jitter, and what are its component jitters?

Bit jitter can be a problem. A digital data stream is composed of a series of rapidly changing “ones” and “zeros.” Bit jitter can make it difficult to tell the difference and result in data errors.

This article begins by defining jitter, then looks at its component jitters, compares bit jitter with clock jitter, considers how jitter can be measured using an eye diagram, and closes by looking at acceptable levels of jitter.

Bit jitter is the small variation in the timing of individual bits in a data stream. It’s manifested in how early or late a signal transition occurs compared with the time it should transition. Communication errors can happen when jitter becomes so large that the signal appears on the wrong side of the transition threshold at the sampling point. That can cause the bit to be misinterpreted and cause a transmission error (Figure 1).

There are two general types of jitters; bounded jitter, also called deterministic jitter, and unbounded jitter, also called random jitter. Unbounded jitter sources don’t reach a maximum or minimum deviation within any time interval. It can be caused by sources like thermal noise, trace width variations, and so on.

There are two types of deterministic jitters: periodic jitter and data-driven jitter. Periodic jitter has a fixed frequency, and sources can include power supply noise and crosstalk between data lines. Data-driven jitter is related to transitions in the data stream. For example, a stream of alternating bits like 01010101 would have a different characteristic compared with a transition that follows a long series of the same bit, like 000011000011.

Bit jitter vs clock jitter

Clock jitter refers to the timing variations in the clock signal. Clock jitter can be one cause of bit jitter. But clock jitter can cause larger problems with the timing of the entire data stream.

Applications like 5G telephone require highly precise clocks to ensure reliable communication. In these systems, jitter is measured in picoseconds and characterized by its RMS value. Like bit jitter, clock jitter has several component jitters:

Random, or intrinsic, jitter is inherent in a system and is usually caused by thermal noise, trace width variations, and so on.
Deterministic jitter refers to repeatable variations in clock timing can be caused by power supply ripple or crosstalk.
Phase jitter also affects the timing of data transitions and is the time domain view of the phase noise that is measured in the frequency domain of a clock.

What does jitter look like?

One way jitter can be seen is by using an oscilloscope to produce an eye diagram that superimposes the rising and falling edges of a bitstream over time. Variations can be visualized and quantified that can lead to bit error rates.

The horizontal spread of the signal transitions at the crossing point of the eye can be used to measure jitter. Most oscilloscopes can automatically generate a histogram based on the distribution of the edge timings. The width of the histogram illustrates the jitter level (Figure 2).

What is an acceptable jitter?

Jitter can be controlled, but not usually eliminated. The acceptable level of jitter depends on the application. There are no standards that govern the acceptable level of jitter. 5G communication systems need very low levels of jitter often in the range from 1ps down to tens of fs, other guidelines include:

Voice over internet protocol (VoIP) calls typically require jitter to be less than 150 ms.
Video calls have different requirements. Usually, jitter needs to be under 30 ms, but if the video does not include rapid motion, jitter up to 200 ms may be acceptable.
On-line gaming and live streaming often require that jitter be less than 50 ms.

Summary

Bit jitter is the small variation in the timing of individual bits in a data stream and can result in data errors. Bit jitter can be bounded (deterministic) or unbounded (random). The two types have different causes, but both can be viewed and measured using an oscilloscope. The level of acceptable jitter is strongly application dependent and can range from a few fs up to tens of ms.