The Components of Total Jitter

Figure 1: An overview of the jitter hierarchy,
or "jitter tree," showing the various elements
that make up total jitter

In an earlier post, we began looking at the topic of jitter, a topic of keen interest to anyone working with high-speed serial communications or the components of such a system, including transmitters, receivers, and data channels. To gain an understanding of jitter, an important first step is getting to know a little about the various categories that comprise total jitter.

Referring to Figure 1, total jitter (Tj) is composed of two broad categories of jitter: random and deterministic. Random jitter (Rj) is what it sounds like: it's timing variation, or noise, that cannot be predicted because it has no discernible pattern to it. It's also termed Gaussian jitter, because it typically follows a Gaussian (or normal) distribution. A good example of random noise is the static you hear when your radio is tuned between stations. Most random jitter is the product of thermal noise, which itself has a Gaussian distribution.

The other broad category of jitter is deterministic jitter (Dj). The main difference between random and deterministic jitter is that the former is unbounded while the latter has determinable bounds or limits. Deterministic jitter, unlike random jitter, is predictable and reproducible. It has a peak-to-peak value that is bounded and observable. Deterministic jitter can either be correlated to the data stream (data-dependent jitter) or uncorrelated to the data stream (bounded uncorrelated jitter).

Bounded uncorrelated jitter breaks down further into periodic jitter (Pj) and other bounded uncorrelated jitter (OBUj). Periodic jitter is also called sinusoidal jitter in that it takes on a sine-like shape. Its source is most typically interference from signals related to the data pattern, ground bounce, or power-supply variations. These external sources of deterministic jitter couple into the data-transmission system from adjacent circuitry. OBUj is jitter that is neither Gaussian or periodic, stemming from sources such as crosstalk from non-repeating data (live traffic), high rate frequency modulation of the Pj component, power-supply switching noise, EMI, and simultaneous switching noise.

Data-dependent jitter consists of intersymbol interference (ISI) and duty-cycle distortion (DCD). All three subcategories are timing errors that vary with the data pattern. Their primary sources are 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.

Intersymbol interference is the most common form of DDj. It is usually caused by bandwidth limitations of transmission lines. It affects single bits surrounded by the bit of the opposite state.

Duty-cycle distortion jitter is caused when certain bit states have different durations; i.e. a logical one is always longer than a logical zero or vice versa. DCD jitter is caused by bias setting and by an insufficient VCC supply of a component.

There's a brief rundown of the broad categories of jitter. In later posts, we'll look at some of the ways to perform measurement and characterization of jitter.