What Is Jitter?
Jitter as defined by NIST as the “short term phase variation of the significant instants of a digital signal from their ideal positions in time”. We imagine jitter as a change in the position of a wave-forms rising or falling edge from where it should be (and quite is meant by ‘where it should be’ leads to the different types of jitter that a designer may have to consider – more on this later). The term Jitter is reserved for changes in the signals phase above 10Hz – below 10Hz the term Wander applies. As an oscillator’s signal gets multiplied and filtered, the jitter gets multiplied and filtered as well, and a system’s timing budget can quickly disappear. It is here that we see just how problematic jitter can be and as a consequence is has become a necessity for designers to possess a good understanding of timing jitter and how it will affect their system design.
What causes Jitter?
Jitter can be the result of intrinsic noise within the oscillator itself that causes modulation of the signals phase or amplitude, or other disturbances in the system, such as power supply noise (as we have previously covered), thermal noise, vibration, crosstalk or interference from other components and many other factors.
How do we categorize Jitter?
In most discussions, jitter is separated into one of two main categories: Deterministic Jitter and Random Jitter (DJ and RJ). The figure below shoes a quick analysis of jitter including a brief introduction to the different components of DJ and RJ.
Random jitter is an always present phenomenon that cannot always be predicted. The random jitter experienced by a device is a combination of multiple minor factors, including thermal noise, trace width variations, shot noise, flicker, etc. Random jitter is a broadband stochastic Gaussian process that is sometimes referred to as intrinsic noise because it is always present. Random jitter has a normal probability distribution function (PDF) that is unbounded, and cannot maintain a well-defined peak-to-peak value. Instead it is commonly described by its standard deviation. Random jitter is also independent from other sources of jitter, in that its presence does not magnify the effects of other sources of jitter.
Deterministic jitter, sometimes referred to as bounded jitter, is often defined such that if all components of a system were known, then you could accurately predict how much jitter will be observed at each transitional edge. Since deterministic jitter is composed of all other non-random forms of jitter, it does not follow a general distribution function. There is a finite amount of non-random jitter sources, and therefore we can deduce that it has a PDF that is bounded. This allows us to characterize deterministic jitter by its peak-to-peak value (a quantifiable value).
Deterministic jitter can be further broken down into two subcategories: Periodic Jitter and Data-Dependent Jitter. Periodic jitter includes any jitter at a fixed frequency or period. It is easy to measure accurately and appears in the frequency spectrum as distinct peaks. Some good examples of periodic jitter are power supply noise and crosstalk from neighboring data lines. Data-dependent jitter encompasses all jitter whose magnitude is affected by changes in a signal’s duty cycle or clock edges. For example, in a data stream the transition between a 0 and 1 of alternating bits (01010101) is going to be different compared to a transition that follows a long string of identical bits (00011001). As this type of behavior is not present in clocks and oscillators, this form of deterministic jitter is considered a non-factor.
There are many ways to categorize jitter, and while it is important to understand what type of jitter you are observing, it is equally, if not more so, important to be able to measure the different types of jitter so that efforts can be made to filter/remove them and reduce the overall Bit Error Rate (BER) of a system.