What’s jitter and what can I do about it (half 1 of two)?

Measuring and quantifying jitter are the primary steps towards controlling it.

Jitter is a measure of the timing efficiency of a digital information stream akin to Ethernet, USB, PCIe, or HDMI. It defines when information transitions happen in relation to a perfect waveform. Extra jitter can result in signal-integrity issues that end in excessive bit-error charges and poor communications-channel efficiency. Observing jitter and assigning numbers to it are the primary steps in limiting it to acceptable ranges.

What instrument do I take advantage of to measure jitter?
Jitter is principally a time-domain phenomenon, and the oscilloscope is often the go-to device for jitter measurements. Be aware, although, that jitter is expounded to section noise, and you should use a frequency-domain instrument akin to a spectrum analyzer or phase-noise analyzer to measure some points of jitter akin to its part frequencies, however not all. Frequency-domain devices can not, say, straight measure timing variations between successive cycles.

You'll be able to, nonetheless, make these measurements within the time area. In the event you take a look at a serial information stream utilizing an oscilloscope, you will notice the acquainted eye diagram (Determine 1), the place every eye makes up one unit interval (UI). A watch diagram is made up of successive bits overlayed on high of each other. That allows you to see the time and amplitude variations amongst bits. Amplitude does have an effect on jitter. A sign with a smaller amplitude will make its transition from excessive to low (or low to excessive) quicker than a sign of better amplitude.

What can I study jitter from this display?
You can't be taught an excessive amount of from this single snapshot. We will, although, see that successive transitions deviate from the best, so we all know jitter is current. We will estimate the peak-to-peak jitter, as indicated by the arrows. If the horizontal time-base is about to twenty ps/div, then the peak-to-peak jitter seems to be about 10 ps.

How can we be taught extra about jitter?
Makers of high-end oscilloscopes present jitter-measurement instruments for his or her devices. For instance, Keysight supplies D9020JITA jitter and phase-noise evaluation software program; Rohde & Schwarz gives its RTP-K12 jitter-measurement software program; Tektronix supplies its DPOJET eye-diagram, jitter, noise, and timing-analysis package deal; and Teledyne LeCroy gives its SDAIII-CompleteLinQ jitter and eye-diagram evaluation instruments. These instruments can derive a number of details about jitter that’s not instantly observable from one oscilloscope show.

How do these instruments work?
Let me present a easy handbook instance of what these instruments can do routinely. Take into account the highest waveform in Determine 2, the place the black sq. wave represents the best transition time. You'll be able to see that every pink rising and falling edge deviates from the best transition time. This deviation, a part of jitter, is known as the time interval error (TIE). Let’s zoom in on the rising edge (Determine 2, backside). The earliest transition happens 7.5 ps earlier than the best transition time, and the most recent happens 7.5 ps after, so our measurement exhibits that peak-to-peak jitter is 15 ps.

Is peak-to-peak jitter the banner spec for jitter?
It’s only one measure of jitter. In the event you look carefully at Determine 2, you will notice 28 edges, and solely two of them contribute to peak-to-peak jitter. There would possibly nicely be helpful data within the others. Step one in extracting this data is to compile the 28 information factors for every transition right into a desk (Desk 1).

Desk 1. Transition timing

Partly 2 of this two-part sequence, I’ll present what varieties of data might be extracted from this set of 28 information factors. Bear in mind, although, that the automated instruments from the oscilloscope makers course of hundreds or tens of hundreds of information factors at a time, not simply a few dozen. Additionally partially 2, I’ll talk about the jitter hierarchy, together with complete jitter (Tj), random jitter (Rj), deterministic jitter (Dj), data-dependent jitter (DDj), and periodic (or sinusoidal) jitter (Pj). Lastly, I'll counsel what you possibly can and can't do to alleviate jitter issues.

The place can I be taught extra proper now?
Keysight has an software notice on jitter fundamentals, and Tektronix has a primer on characterizing jitter. Rohde & Schwarz has a white paper on the best way to debug jitter issues, whereas a white paper from Teledyne LeCroy explains how the corporate’s jitter instruments carry out calculations. Skyworks has a jitter dictionary with measurement suggestions. Lastly, EEWorld has articles on eye diagrams and on utilizing an oscilloscope to measure jitter.