Electronic Design

Taking The Fear Out Of The Jitter Bug

Although the semiconductor industry is seduced by high-performance, multi-gigabit interface technologies like Serial ATA and PCI Express, abandoning our comfort zone of lower-speed signaling and approaching these enigmatic high-speed serial (HSS) interface signals with confidence is not easy.

At first glance, these new HSS interfaces seem alien. In the past, it was easy to recognize problems like power-supply ground loop noise by the voltage ripples on a signal. But the influence of complex jitter on the miniscule unit interval periods of emerging HSS signaling now challenges even seasoned test engineers who are unfamiliar with the detailed composition and origins of jitter--random jitter, deterministic jitter, periodic jitter, and bounded uncorrelated jitter. Ensuring device interoperability in real-world systems is certainly a daunting challenge when HSS jitter behavior is so mysterious.

Fortunately, recent advances in jitter measurement instrumentation now make characterizing jitter as easy as learning the jitterbug. Finding the right jitter analyzer will help you break jitter components down one step at a time, making your HSS I/O characterization job a whole lot easier and your boss much less nervous about meeting your volume production launch deadline. The good news is that advances in jitter analysis tools that run on the latest real-time digitizers, sampling oscilloscopes, and time interval analyzers go a long way toward demystifying and simplifying the understanding of the nature and origin of noise in your HSS signals.

For example, like visual step-by-step dance instructions that guide you unambiguously through each dance move, a real-time digitizer is a great confidence builder when analyzing jitter. With digitizing rates typically in the 20-Gsample/s range, real-time digitizers capture entire waveforms of multi-gigabit data streams without skipping a single cycle. The latest post-processing software tools enable real-time digitizers to provide a straightforward picture of edge timing behavior on a consecutive cycle basis, albeit only a short-term jitter behavior, as memory size limits the sampling window to only a few milliseconds of signal capture time. Most useful is the real-time digitizer's ability to precisely measure duty cycle distortion and very high-frequency jitter.

Some engineers prefer longer-term reliability assurance, such as repeating your dance lessons over and over until you finally "get it." For them, undersampling oscilloscopes and time interval analyzers capture signal timing information over many repetitive loops of a data bit pattern to get a statistical feel for how edge timing jitters and wanders over long patterns and longer periods of time.

The latest and greatest in undersampling jitter analysis oscilloscopes, for instance, can sample data streams as long as 1012 bits, extracting edge timing from the sampled waveforms to compute many of the elusive components of jitter, such as low-frequency timing drift and data-dependent jitter.

Unlike oscilloscopes, however, which store vast amounts of unnecessary measurements not used in jitter analysis, time interval analyzers (TIA) capture only the edge timing measurements that directly relate to the jitter analysis of interest. This reduces test times from minutes down to seconds. With that extra time saved using a TIA, you can perform more measurements in a day--or take longer coffee breaks.

But the really exciting thing is that the newest continuous time interval analyzers (CTIA), which are of "continuous time-stamping" nature, can perform complete jitter decomposition and analysis in just hundreds of milliseconds. They do not need pattern sync arming, making it practical to test for jitter even in high-volume production, with minimal test time overhead.

So despite the intimidating mystique of today's emerging high-speed serial interfaces, the bottom line is that, at long last, our fears of characterizing these enigmatic HSS signals can be allayed by the reassuring lineup of sophisticated yet simple to use jitter analysis instruments that are now appearing on the market.

Shall we dance?

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