Protocol Testing
Test equipment vendors are addressing the need to ensure signal fidelity, as well as protocol-level interoperability. There are also myriad second-tier test-and-measurement suppliers of serial data analysis tools. Enumerating the capabilities and features of these products is beyond the scope of this article, however a look at some significant products will give you a feel for their capabilities.
Obviously, the troika of major oscilloscope makers – Agilent Technologies, LeCroy, and Tektronix – is emphasizing serial data analysis. For its part, Agilent has an evolving E2960 Series of equipment that comprises a protocol test platform for PCI Express (PCIe), based on an x8, x4 and x1 protocol analyzer and protocol exerciser. The instrumentation literally creates a test environment for debug and bring-up of PCIe based designs, as well as server and chipset validation.
Agilent E2960B Series for PCIe 2.0 integrates analysis and exerciser tools, along with non-intrusive probing. The instrumentation accommodates x1lane through x16 lanes, along with an LTSSM (Link Training and Status State Machine) exerciser that processes PCIe 1.0 signals as well as the faster PCIe 2.0.
With its unique midbus probing scheme, Agilent E2960B equipment is based on the company's predecessor N2X platform. An E2969A PCIe protocol test card also helps verify and ensure compliance with the specs defined by the PCI SIG (the test card is a collaborative development between Agilent and Intel).
As a set of protocol test tools, the E2969A is designed primarily for validating functional compliance of chips, add-in cards, and systems. In operation, the card is plugged into the device-under-test, with a connection to Agilent's E2960A Protocol Analyzer.
User-Controlled Traffic
In use, the exerciser sends, and responds to, user-controlled test traffic. This traffic is administered by means of an API that Agilent calls its Command Application Programming Interface. The CAPI lets you stress all data paths in a system, to test corner cases and overall behavior, by inserting errors and protocol violations on physical, data link, and transactional layers.
Though it sounds daunting, the CAPI operates through a rather friendly graphical user interface (GUI). Alternatively, you can use a Tool Command Language interface. The choice gives you quick test setups with the GUI or more comprehensive test coverage with the API. Regardless of how you decide to use it, the analyzer ensures non-intrusive monitoring of PCIe traffic between either an add-in card and a system, or between a protocol exerciser and a system.
Rounding it out is Agilent's midbus probe. It lets you perform measurement in parts of a system, such as a chip-to-chip I/O path, which might otherwise be inaccessible due to lack of a slot or a connector. You can connect an E2960A protocol analyzer to a PCIe design either through a slot/interposer probe board or through the company E2941A midbus probe.
The midbus probe provides a direct connection from the probe to the targets on a board, without the use of an adapter. This simplifies validation of a high-speed design that might be tight on space or that requires minimal disruption of signal routing. The so-called soft-touch probe uses special subminiature spring-pins to ensure reliable connections to signal pads, sidestepping the need for cleaning or special surface finishes of probe pads.
As another top-tier test-and-measurement company, LeCroy also offers both oscilloscope and protocol analyzer technologies. Its SDA family of real-time serial data analyzers, for example, includes models with up to 18 GHz of analog bandwidth, along with 60 Gsample/s sampling rates. A deep-memory SDA 18000 instrument can store up to 150 million points in memory. For cost-conscious users, LeCroy SDA 9000, SDA 11000, and SDA 13000 versions are slated for mid-speed serial data analysis.
The DSP-based SDA analyzers (the DSP is used for equalization compensation, not bandwidth extension) is replete with capabilities that otherwise might require two or three separate instruments. For example, most serial data measurement and jitter analysis tools are based on sampling oscilloscopes, but they're only useful if you're monitoring well-behaved signals or small sequences of repetitive bit patterns. In contrast, the LeCroy SDA 18000 can make signal integrity measurements, and do jitter and waveform analysis on almost any waveform. An SDA 18000 can capture data streams with multiple points per transition edge, with digitizing resolution as low as 16 ps/point.
Real-Time Serial Measurement
The 4-channel SDA 18000 bandwidth supports real-time measurements of most ultra-fast serial data standards, such as 10-Gbit/s Ethernet or Fibre Channel 8.5. That capability lets the instrument do thing such as embedded clock recovery, where it can phase-lock to a clock to ensure precise jitter measurement.
Similarly, a LeCroy SDA can capture of up to 12 million Unit Intervals (UI) in a single acquisition, letting you measure low-frequency effects such as spread-spectrum clocking or switching-supply noise.
LeCroy SDA instruments also make use of a Windows-hosted software architecture the company calls X-Stream. Optimized for large waveforms, X-Stream pipelines data at 10 Gbit/s into acquisition memory, where they are packetized into segments small enough to work in processor-cache memory. The scheme eliminates constant external memory fetches.
Under X-Stream, all oscilloscope operation, acquisition, and analysis takes place in one application under Win XP Pro to provide a breakdown of jitter composition, even with non-repeating patterns or live data.
X-Stream also lets you switch back to an individual bit pattern or the specific location where a mask violation may have occurred. Eye patterns can also show mask violations to the bit, and eye pattern measurements can be acquired on up to 12 million consecutive bits. Consecutive-bit eye pattern analysis even lets you measure waveshapes of individual bits that might violate a compliance mask.
LeCroy serial data analyzers are also equipped with so-called Q-Scale analysis and plot-view. Q-Scale is an alternative method of breaking down jitter components that extrapolates jitter histogram tails, thus improving accuracy, stability, and measurement convergence time. A Q-Scale plot also provides a visual intuitive representation of jitter breakdown, as opposed to a conventional bathtub curve.
Making S-Parameter Measurements
As a LeCroy and Agilent arch-competitor, Tektronix offers an integrated tool set. Its DSA8200 Digital Serial Analyzer, essentially a sampling oscilloscope, serves for TDR/TDT (time-domain reflectometry/time-domain transmission) verification, and can additionally make S-parameter, jitter, noise, and BER measurements.
Losses and distortion caused by a channel must be equalized in a design's receiver, in order for signals to be recognizable. The effects of loss and crosstalk must also be fully verified, in both the time-domain and frequency-domains. Like Agilent stress testing, Tek equalization and channel-emulation techniques let you see the effect of the channel on the eye pattern, and observe realistic signals at the inputs of receivers.
Fitted with remote-sampling electrical modules and Tektronix IConnect software, a DSA8200 includes 50-GHz TDR (time-domain-reflectometry) and electrical modules that Tek claims represent the most significant performance advancement in TDR test technology in 20 years. The fact that a DSA8200 can analyze signal paths and interconnects in both time and frequency domains should go a long way to understand the effects of loss and crosstalk in real-world high-speed serial I/O paths.
Considering the microwave-domain speeds of the highest-speed serial standards, S-parameter and impedance measurements are a good way to characterize transmission lines and terminations. The Tek DSA8200 can make repeatable S-parameter measurements at speeds out to 12.5 Gbit/s.
Note that the conventional alternative to a TDR has been the vector network analyzer. A VNA is a specialized frequency-domain instrument that typically takes its toll in tricky setup and calibration routines. VNAs are expensive, too. If you go the VNA TDR and S-parameter route, the routines for serial data network analysis will likely take hours to set up and execute. In contrast, a DSA8200 mainframe can do the task in minutes.
Tek supplies four plug-in modules for its DSA8200. These offer combinations of bandwidth and noise specs to optimally address different high bit-rate differential signaling types. The modules feature channel-to-channel and module-to-module de-skew of ±250 ps, user-selectable bandwidth, and small form-factor remote samplers. Each channel also includes an integrated cable with a remote sampler that brings the TDR head very close to a device-under-test, minimizing parasitic effects of probes, cables, and fixtures.
The Tek 80E10 Dual Channel True Differential TDR module, for example, exhibits a 12 ps incident, and 15 ps reflected, TDR risetime spec, as well as 50 GHz of bandwidth and a low 600 µV rms noise spec. It can do independent step generation for true differential measurements, driving signals in both positive and negative directions at the same time.
Other modules for the Tektronix DSA8200 include the 80E08, a dual-channel TDR electrical module with 18 ps incident and 20 ps reflected risetimes, and a 300 µV rms noise spec (at 30-GHz bandwidth). Two dual-channel low-noise electrical modules, the 80E09 and 80E07, feature 450-µV noise specs (at 60 GHz), and 300-µV noise (at 30 GHz), respectively.
Signal Integrity Software
Key to operation is Tek IConnect signal integrity analysis software. It helps measure reflections, insertion and return losses, and displays eye diagrams. It's also responsible for making jitter, crosstalk, reflection, and ringing measurements. IConnect handles signal-path integrity analysis, does impedance characterization, makes S-parameter measurements of insertion and return loss, handles eye-diagram compliance tests, and can perform fault isolation.
A command line interface helps with automation of multi-port S-parameter measurements, and reduces calibration and test times. IConnect also processes record lengths up to 1-million points, and the instrument million-point memory provides high S-parameter frequency domain resolution.
Tektronix 80SJNB software for this system also addresses initial signal acquisition as, quite often, capturing a signal through a physical fixture can result in distortion. Tek 80SJNB provides a fixture de-embedding feature that lets you remove the effects of a fixture from a measurement. Tek claims this capability can spell the difference between a passing design and a failing one, because the impact of fixtures on signal fidelity is typically large.
Jitter and noise measurements in Tek 80SJNB add improvements to the algorithms used for extraction of a waveform, and its vertical and horizontal impairment parameters. What's more, a spread-spectrum clocking capability is offered. Using the DSA8200 or TDS/CSA8200 mainframes, plus an 82A04 phase reference module, the 80SJNB not only measures signals under the presence of spread-spectrum clocking, it also measures the clocking parameters.
The 80SJNB package can also separate jitter caused by noise impairments versus jitter-based components. The 80SJNB also makes BER eye plots. While measurements on narrow parts of the signal are now common (jitter at the crossing, noise at the cursor), the 80SJNB captures whole signals and then presents the accumulation of all impairments, with measurement results shown at different decision thresholds or timing points. That's possible because the 80SJNB package always stores a whole 3-D shape and all the waveforms behind it.
Automatic Measurement
Speaking of software, a company that specializes in software for oscilloscopes from most vendors is Amherst Systems Associates. ASA M1 OT (oscilloscope tool) with AutoMeasure software can automatically make many measurements, and even detect the type of oscilloscope it's used with, and which scope inputs have signals.
M1 OT can also detect and scale signals, and determine what standards a signal might fit within. ASA intelligent measurement process operates without pressing a button. AutoMeasure does the equivalent of more than dozens of mouse clicks. The toolkit can analyze up to thousands of acquisitions in order to locate an instance of undesired behavior. It can also help analyze how parameters vary over very long periods.
This software can save you money, too. The price of vendor-supplied oscilloscope software is typically high, and most applications are proprietary and operate on one or two platforms from one vendor. In contrast, ASA software is universal and multi-platform applicable. It can run on just about any Windows-equipped scope, or on your PC. M1 OT Virtual Deep Memory Toolkit can also leverage whatever memory your storage scope already has, without memory upgrades.
Significantly, ASA patented approach to analyzing jitter and timing in both clock and data-streams is considered by some designers as a de facto standard for how high-precision timing should be measured, regardless of whether you're analyzing signals in DDR/DDR2 memories, or serial communications standards. It will also analyze waveforms associated with phase-locked loops, and fast oscillators and clocks.
ASA supports Agilent digital storage scopes, including the 81000, 5485x, 5484x, 54845/46, and 5483x Series, and supports Tektronix TDS7000, TDS6000, and TDS5000 scopes. The software also works with LeCroy WaveMaster/SDA, WavePro, WaveRunner, and WaveSurfer scopes, and Yokogawa oscilloscopes.
Isolated Sub-Population Analysis
ASA's latest v6 (version 6) M1 tool, priced from about $1000 to $80,000, includes a patent-pending technology called Isolated Sub-Population Analysis. ISPA can help debug signal behaviors that repeat, and those that can be associated with events or behaviors on other signals.
ISPA is a back-end process that takes anomalies discovered by M1 Hidden Anomaly Location technology, as well as other event-isolating capabilities of M1 OT, and determines what the root causes of the anomalies are. In operation, ISPA creates an analysis population comprised only of events of interest, disregarding normal events that comprise the majority of the waveform present in the isolated sub-population. The ISP can then be explored and analyzed separately using the M1 collection of debug tools.
In Conclusion
As you might suspect, the products from the four vendors discussed above represent the tip of a large and multifaceted iceberg. Other suppliers of tools for serial data analysis, high-speed digital communications testing, arbitrary waveform and pulse generation, protocol testing, bit error rate testing, and related test-and-measurement approaches, include National Instruments, Wavecrest, Keithley Instruments, Stanford Research Systems, Pico Technology, Instek, Kikusui, Rohde and Schwarz, EXFO, Aeroflex, Anritsu, and Spirent Communications, to name a few.
- Alex Mendelsohn
