The second-generation (Gen2) versions of the SATA, USB, PCI, and double data-rate (DDR) standards all have seen wide market adoption. Meanwhile, the quest for faster computing, storage, video, and connectivity has spurred progress toward the third generation (Gen3), and even the fourth, of all these standards. Making characterization measurements is fundamental to a robust design of high-speed serial links. But what can an engineer do when there is very little signal left to look at?
Despite the prevalence of these high-speed serial-link standards in system design, the industry, driven by severe cost constraints, persists in its reliance on FR-4 PCB material that leads to signal degradation and closed (or nonexistent) eyes at the receiver. Even as system speeds are rising, system integrators are not universally embracing the concept of signal equalization to offset the deleterious effects of FR-4 on their serial-link signals. For example, the USB 3.0 specification shows an eye-diagram template at the output of the equalizer, not at the input to the receiver. With this trend, what does this mean for designers who are accustomed to making signal integrity measurements on far-end eye diagrams?
Test-equipment vendors are keeping busy these days in their efforts to educate designers and system integrators with respect to the test requirements for these next-generation serial links. According to Jit Lim, senior technologist for high-speed signal analysis at Tektronix, many designers are aware of the need for equalization techniques but are not using them. “Most people understand the concept of equalization but the big difference today is they have to use the technique,” says Lim. One reason for that is the afore-mentioned closed eye pattern at the receive end of the link. Another is that the standards require the use of an equalization filter when making eye-pattern measurements. “PCI Gen2 actually had a requirement for equalization before making measurements but most integrators ignored it,” says Lim. But for the Gen3 serial standards, equalization is a strict requirement. “The rule of thumb is that most standards in the 5 Gb/s range or higher require it.”
Most of these standards are buttressed by compliance testing at the system level; designers must arrange for their systems to be tested for compliance at a third-party laboratory, where they either pass or fail. That testing is performed with equalization. The difference, says Lim, is in whether the design team has done its own pre-compliance testing with equalization before the third-party test suite is run. “One challenge I have is to educate designers to invest in proper tools to ensure the device passes those pre-compliance tests,” says Lim.
This becomes a particular issue for designers who are, for example, moving up from USB 2.0 to USB 3.0 (or SuperSpeed USB). The far slower speeds, and consequently greater margin for error, could lull designers into a false sense of security as they embark on a USB 3.0 upgrade to their next system design. “Margins with USB 3.0 are much smaller,” says Lim. “It’s important that they perform proper pre-compliance tests.”
Moreover, because the final compliance testing is of a pass/fail nature, a product that passes final compliance tests may have done so very marginally. That leaves designers open to the risk of shipping a “standards-compliant” product that may exhibit erratic behavior in certain circumstances. “The compliance testing is done at ambient temperature,” explains Lim. “What happens if the system is operated at temperatures significantly warmer or colder?” Designers who perform pre-compliance testing can investigate these scenarios and build in enough margin to account for them.
So what is the solution to the issues surrounding pre-compliance testing? One element of it is the creation of a supply of S parameter models for impaired channels. That aspect falls largely to test equipment vendors. Once models are in hand, they’re used in serial-link data analysis, a process that requires a waveform and uses that to de-embed the effect of the impaired channel to open up the eye diagram at the far end of the link.
Another aspect of the solution is dealing with the complexities of performing the pre-compliance testing. Test vendors such as Tektronix provide packages in which the hardware and software are combined to create a one-button measurement suite. Users receive a pass/fail result for the serial link under test; the system also prints out complete measurement results.