Serial standards keep test engineers on edge...

Ransom Stephens- February 9, 2012

…the cutting edge, that is.

Clocks are embedded. Measurements of the jitter alphabet soup never seem to agree. Good old logic buses have become waveguides. Bit-error-rate contour measurements look like tie-dye designs. And the eye is as closed as the box office in Green Bay the day after the Giants visited.

In the last decade, HSS (high-speed-serial) specifications have taken us on a thrill ride. Emerging multigigabit-per-second specifications like the 3rd generations of PCI Express, USB, and SAS/SATA, plus the 100-Gbit Ethernet locomotive, require test engineers to learn many new tricks. But the cool thing about being a test engineer is that whether you’re responsible for 50 test stations on a manufacturing floor, designing test equipment that has to stay half a step ahead of the technology being tested, or a conference-call-sitting contributor to the spec itself, you’re always ahead of the game. It’s the kind of work where everyone feels they have an advantage–RF/microwave engineers, applied mathematicians, physicists, and digital engineers–and every background has something special to contribute to the challenge, and the challenges change every day with every spec.

It’s easy to think of a conducting trace on a PCB (printed circuit board) as a signal highway from one package to another, but at multigigabit-per-second rates, the FR-4 (flame retardant Type-4) media is a dielectric permeated with electromagnetic fields. The complex waveguide traits of PCB traces can’t be ignored.

“The root causes of digital communications problems like maintaining waveform integrity, jitter, crosstalk, and so on are better understood through classic microwave analysis tools involving spectral analysis, phase noise analysis, and network analysis,” said Agilent Technologies’ Greg LeCheminant.

Along with time-domain oscilloscope analyses, an interconnect’s impulse response, whether it’s a cable or a trace, needs to be understood in the differential S-parameter frequency domain. LeCheminant added, “You don’t need to talk at the level of Maxwell’s equations, but microwave fundamentals like transmission-line theory and being comfortable thinking in the frequency domain will go a long way.”

Along with some microwave and radio frequency expertise, HSS test engineers analyzing signals with closed eyes need to understand both linear and nonlinear equalization schemes, and that requires hands-on knowledge of DSP (digital-signal-processing) filters.

As specifications become more cryptic and test challenges grow, some companies outsource their compliance testing. Tom Purdy from Granite River Labs, a Silicon Valley compliance and characterization firm, explained: “Many companies designing products that utilize these high-speed interfaces are outsourcing testing because they don’t have the financial and human resources to do it all in house. While $100k-plus for a high-bandwidth digital oscilloscope may be justifiable as a critical general-purpose tool, the additional $200k to $300k needed to purchase a data generator with jitter capability, a network analyzer, and other test equipment is often outside the budget.”

For management, compliance testing has become an optimization problem. Finding balance between the need for in-house test expertise and the cost of test equipment necessary for compliance is unique for every company. Return on investment for a $300,000 bit-error-rate tester can be calculated on paper, but a test engineer who can bob from digital to microwave and weave from DSP to fiber optics makes the difference between delivering a product and not.

For test engineers, on the other hand, grinding through the next specification, figuring out how to use equipment already in the lab to perform or approximate new compliance tests, and learning new skills as data rates climb are the challenges that make going to work worthwhile.