Electronic Design
Scopes Conquer Debug With Bandwidth, Smarts

Scopes Conquer Debug With Bandwidth, Smarts

For most designers and test engineers, the oscilloscope, as it has been for decades, remains the go-to tool on the bench for debugging circuits of all descriptions. Scopes have become true all-purpose tools, sporting extremely high bandwidths, huge memories, and low noise floors that enable them to get to the bottom of the most elusive glitches. At the same time, test vendors have made it easy to set up and navigate routine, repetitive tasks such as jitter measurements and compliance testing with pre-packaged test routines.

In this report, we’ll look at some of the ways in which scopes are being used today and round up the test vendors’ latest and greatest introductions. Among them, you may find the scope that will serve your bench needs for months and years to come.

What’s Driving Test Requirements?
Several design trends are driving oscilloscope vendors toward higher performance. Chief among them is system complexity, which itself is driven by the growing adoption of today’s next-generation serial communication standards. These communication protocols have their own particular test requirements (see “Look Third-Generation Serial Data Link Testing In The Eye,” Sept. 9. But oscilloscopes are an important part of the mix in compliance testing.

The integration of multiple technologies further impacts system complexity, such as RF combined with lower-speed buses used for control lines. Designers may not have the requisite domain expertise to deal with these various elements of the overall system design. Thus, it’s incumbent upon the instruments themselves to encapsulate this expertise.

There are four key challenges in the system design and validation space, according to Randy White, technical marketing manager at Tektronix. One is what White terms data qualification, which really boils down to interpretation of test data.

For example, your system powers up but doesn’t function properly. Is this due to a signal integrity issue or a state-machine problem? Lots of variables are involved, and your scope needs to be able to help you sort those out quickly.

When it comes to the mixed-signal arena, double-data-rate (DDR) memory has emerged as a leading design challenge. DDR memory is ubiquitous, found in everything from servers to automobiles to toys. “Being able to isolate the read data from the write data and figure out which glitch caused a miss is critical,” says White.

A third challenge centers around system state visibility. Consider a scenario in which power-supply noise coupling onto the memory bus is causing a state machine error (Fig. 1). Switching noise from the power supply then causes a memory read issue, which in turn creates an overflow state. “Because of system visibility, we could tell the problem stemmed from power-supply coupling,” says White.

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Even simulation can miss logic states and/or events such as an unexpected logic state combination due to an asynchronous input to an FPGA, a state machine malfunction, or timing errors caused by asynchronous events relative to the internal first in, first out (FIFO) clock. In these and other instances, oscilloscopes are the key to uncovering the root causes, and that’s a matter of system state visibility.

A fourth key challenge is in timing correlation between the analog and digital domains. Here, another illustrative example is debugging of an HDMI video subsystem. HDMI systems will generally use a control bus. In this instance, it was an I2C bus, and probing the data line with a mixed-signal oscilloscope enabled the capture of both the analog and digital domains with a single probe.

Tektronix’s MSO70000 scopes can capture the signal from the digital side and internally multiplex that signal into the analog side of the scope and give the user both views. This testing revealed a crosstalk-related glitch that was shown to be stemming from an adjacent line (Fig. 2). Slowing down the edge rates on that line with the addition of some shielding restored proper clocking to the I2C line.

The Sky’s The Limit At The Top
For years now, oscilloscope vendors have sought to outpace each other’s equipment in terms of specs of merit such as bandwidth and noise floor. As recently as this past spring, Agilent Technologies held the bandwidth crown for real-time scopes with its Infiniium 90000 X-Series with models ranging up to 32 GHz.

But LeCroy Corp. has smashed through that barrier by launching its WaveMaster 8Zi-A, which it touts as the world’s highest-performance four-channel oscilloscope (Fig. 3). The scopes are upgradeable from 4 to 45 GHz in bandwidth.

First, with the 820Zi-A model, you’ll get 20-GHz bandwidth on four input channels. (The earlier model 820Zi delivered 20 GHz on two channels and 16 GHz on four channels.) The 845Zi-A model also delivers 30 GHz of bandwidth on two channels.

Overall, the instruments provide a maximum 45 GHz of bandwidth combined with a sampling rate of 120 Gsamples/s. That’s combined with 768 Mpoints of analysis memory. The sampling-rate breakdown is 120 Gsamples/s at 45 GHz, 80 Gsamples/s for bandwidths of 25 to 30 GHz, and 40 Gsamples/s on all four channels at a 20-GHz bandwidth.

The scopes’ improved performance is attributable in part to a second-generation silicon-germanium (SiGe) chipset that not only provides the increased bandwidth, but also improves the earlier scopes’ signal-to-noise performance by about 25% (see “Anatomy Of A Scope Front End”).

The new scopes’ time-interval error (TIE) jitter noise floor is just 125 fsRMS, and stability for >10s acquisitions is 175 fsRMS. The new chipset comprises a track-and-hold chip and a monolithic 40-Gsample/s analog-to-digital converter (ADC), which LeCroy calls the world’s fastest single-component ADC.

As with LeCroy’s previous WaveMaster scopes, the 820Zi-A uses the company’s digital bandwidth interleave (DBI) technology to achieve the high bandwidths. Input signals are divided into two frequency bands. The dc to 20-GHz portion is routed through one digitizer, while the 20- to 40-GHz range is digitized after being downconverted. The two signal paths are post-processed, mixed back up in frequency digitally, and recombined. The result is double the bandwidth by using two channels. In the 45-GHz 845Zi-A, the interleaving of three channels takes DBI to another level (Fig. 4).

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This is the secret to the scope’s inherent upgradeability; it’s accomplished simply by the addition of a high-speed RF deck. This amounts to investment protection, says Mike Schnecker, LeCroy’s business development manager for vertical markets.

According to Schnecker, the DBI technology brings inherent advantages. “For one thing, we don’t need silicon that operates at the high speed. This lets us stay ahead of the bandwidth curve,” he says. Overall, Schnecker notes, the recombined bands that were divided using the DBI technology provide slightly better noise performance than an equivalent full-band amplifier could achieve.

Schnecker also says that the WaveMaster 8Zi-A scopes are significantly faster than the current high-end offerings from Agilent and Tektronix in performing demanding analysis tasks. This is due to the LeCroy scopes’ deep analysis memory and quad-core analysis processing engine, which runs at an effective CPU speed of 12 GHz. For example, an eye-pattern diagram of 100 unit intervals/s can be completed in just 284 unit intervals/s, or about half a second.

The WaveMaster 8Zi-A scopes aren’t inexpensive, starting at $68,490 for a basic 4-GHz model and topping out at $304,000 for a fully loaded 45-GHz version.

So where is LeCroy heading with this technology? The answer lies in the company’s LabMaster family of high-speed, multichannel oscilloscope modules. LabMaster provides up to five synchronized channels with 45 GHz of bandwidth, up to 10 synchronized channels with 30 GHz of bandwidth, and up to 20 synchronized channels of up to 20 GHz of bandwidth.

This new offering is aimed at designers of emerging high-speed multi-lane data communications systems. LeCroy says more customers are dealing with multi-lane serial data architectures and will need this kind of channel count and bandwidth for those systems.

Second Place: Not So Shabby
Despite having been knocked off its throne, Agilent Technologies’ Infiniium 90000 X-Series oscilloscopes are still formidable instruments. They sport real-time bandwidths of up to 32 GHz, which promises to meet the needs of engineers working with emerging wireline communication standards, high-speed serial data links such as USB, SAS, or PCI Express, or high-energy physics.

Alongside their high bandwidths, the series offers up to 2 Gpoints of memory and a maximum sampling rate of 80 Gsamples/s. There are 10 models in the series with bandwidths of 16, 20, 25, 28, and 32 GHz.

Accuracy in jitter measurements at high speeds requires a scope that offers both high bandwidth and a low noise floor. The 90000 X-Series scopes offer an industry-low jitter measurement floor of about 180 fs as well as the lowest noise floor (2 mV at 50 mV/division, 32 GHz).

One element of the “secret sauce” that gives these scopes their capabilities is Agilent’s investment in a proprietary indium-phosphide (InP) process technology optimized for RF and high-performance scopes. The scopes’ front-end chips, which are fabricated on that process, are packaged in a multichip module where they’re embedded inside a substrate for improved shielding and grounding. Packaging the front end in this fashion also minimizes wirebond lengths, which in turn minimizes inductances and helps to hold down the noise floor.

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The front end includes two new preamplifiers rated to 32 GHz as well as two edge-trigger chips with about 22 GHz of edge-trigger bandwidth, lending the scopes nicely to radar applications. There’s also a new 32-GHz sampling chip that requires no digital bandwidth interleaving or DSP boosting to reach the rated bandwidth. Both of those techniques introduce increased noise density, cause measurement inaccuracies, and result in nonlinear frequency response. In contrast, not only does this front end deliver superior pulse distortion control and significant margins in speed and fidelity, it also offers headroom for higher speeds in the future.

So what does all this mean in terms of measurement capability and accuracy? For one thing, with all of that true analog bandwidth, the 90000 X-Series scopes can make measurements that lesser instruments can miss, such as the fall time on a 13.5-ps edge (Fig. 5). Moreover, the scopes’ low noise floor lends them greater measurement accuracy at equivalent bandwidths compared with competitive instruments. A lower noise floor allows for the capture of additional harmonic signal content.

Without a suitable probing system, your scope’s bandwidth doesn’t matter. Agilent has rolled out the InfiniiMax III 30-GHz probing system for the 90000 X-Series scopes, accompanied by a full suite of accessories that allow the probe to work in a number of different environments, including 2.92/3.5-mm pitches, solder-in scenarios, and browsing.

Calibration software performs a full ac calibration of the probing system. The scope pulses out a signal with a fast edge and automatically performs time-domain reflectometry on the entire probing system to the tip of the browser. The probe amplifiers carry their own individual S-parameter files preloaded on EEPROM, ensuring the greatest possible measurement accuracy.

To help designers get the most out of these scopes. Agilent offers a comprehensive lineup of application-specific measurement packages. A broad range of jitter, triggering, analysis, and display tools is included, as well as pre-built compliance-testing software put together with the aid of Agilent engineers who serve on the standards committees in question. The packages provide support for emerging high-speed serial-bus standards including the QuickPath interconnect, Fibre Channel, SAS 12G, and 10-Gbit/s Ethernet.

One example of an application-specific measurement software package, InfiniiSim for De-embedding, combines measurements with transmission-line models to view simulated scope measurements at any location along a signal path. Additionally, equalization is becoming an integral part of high-speed serial technologies. The 90000 X-Series scopes support this with its SDE software, which provides full DFE, FFE, and CTLE analysis. The software lets users employ their own tap values or let the instrument find the optimal tap values for a given signal.

Lastly, these scopes stand ready for emerging standards, proprietary standards, or whatever may come along in the future. Agilent’s User-Defined Application software provides a framework for the development of automated compliance testing on proprietary buses or on emerging standards that have yet to solidify.

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Prices start at $131,000 for a two-channel, 16-GHz model and range up to $286,000 for the high-end version. Limited shipments began in July.

Sampling Scopes Improving
Moving over to the realm of sampling oscilloscopes, a sticky problem that almost every engineer runs into is the need to capture a waveform at a point in a circuit that is physically inaccessible. The test-equipment industry’s answer to this problem is known as “de-embedding,” a process by which the instrument removes the effects of fixtures and probes on the measurement.

The latest instrument that features embedding/de-embedding capability, Agilent’s 86100D DCA-X, is a wideband sampling oscilloscope that sports great flexibility in addition to high accuracy (Fig. 6). The scope’s analog bandwidth of 18 GHz to more than 90 GHz combines with low noise and ultra-low intrinsic jitter to make it possible to capture the nuances of a device’s or circuit’s true performance.

The scope’s highlights include a new front-panel layout, a new CPU (Intel Core Duo 2 at 3 GHz), a new power supply, and a new distribution board. But the new feature that makes the most impact on usability and functionality is a graphical user interface (GUI) that Agilent calls FlexDCA. Coupled with the CPU, the GUI enables the integrated embedding/de-embedding functionality, which springboards from the Infiniisim feature on Agilent’s high-end Infiniium 90000 X-series scopes.

Even as the DCA-X scope enables designers to make measurements with de-embedding, it remains 100% backward compatible with all previous DCA modules and completely code-compatible with the earlier 86100C scope. The latest version offers a high-channel-count architecture with support for up to 16 parallel measurement channels (eight differential pairs).

Several hardware enhancements have been incorporated into the scope to boost usability and flexibility. One of these is a mappable vertical control, which controls the gain/offset of any channel and/or math function. Meanwhile, a user-definable knob, another feature imported from the Infiniium 90000 X-series scopes, can be assigned to a commonly used analog control such as data rate or phase-locked loop (PLL) bandwidth.

In addition, the scope sports a user-definiable multipurpose button. For example, users can save a waveform, print a report or screenshot, or run a macro with the press of one button, among other things.

The key to the scope’s de-embedding functionality is an optional waveform-transformation toolset, the 86100D-SIM InfiniiSim-DCA. This toolset provides de-embedding, embedding, and virtual-probing capabilities that help users characterize high-speed digital designs more thoroughly and with improved margins.

With the toolset option installed on the DCA-X mainframe, users can remove the full effect of fixtures, cables, and channels from measurements. It’s also possible to emulate a signal at the output of a cable or channel.

The instrument accurately characterizes fixtures, devices, and cables, providing calibrated S-parameter files in Touchstone format. From there, it’s a simple matter to have the instrument de-embed or embed these elements of the device under test (DUT), displaying a simulated signal that removes (or adds) the effect of the extraneous elements. After measurements are taken, the simulated signal can be displayed alongside the actual measured signal to compare results. Corrected jitter and amplitude results are also provided.

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Pricing for the 86100D DCA-X scope starts at $21,500 with four-week delivery. The 86100D-SIM InfiniiSim-DCA waveform-transformation software starts at $5000 with 10-week delivery.

A New Player Emerges
Already a competitive field with Agilent Technologies, LeCroy, and Tektronix, the oscilloscope market has grown more crowded with the recent entrance of Rohde & Schwarz. The company, long known for its focus on the wireless and aerospace/defense arenas, has launched two oscilloscope families that span bandwidths from 500 MHz to 2 GHz.

At the lower end is the RTM family of universal oscilloscopes intended for everyday measurement tasks. These instruments offer a 500-MHz bandwidth, a sampling rate of 5 Gsamples/s, and memory depth of up to 8 Msamples.

Available in two- and four-channel models, the RTM scopes provide accurate signal display, excellent time resolution even for long sequences, and tools for fast signal analysis. Maximum input sensitivity is 1 mV/div with no bandwidth limitation or software-based zooming, which translates to excellent vertical resolution.

As the RTM scopes are meant for quick benchtop measurements, ease of use was a prime goal for their design. To this end, they offer color-coded control elements, flat menu structures, and keys dedicated to frequently used functions, making them quick and easy to operate.

A sharp, high-resolution 8.4-in. display makes even miniscule signal details visible. Further, analysis tools are available at the push of a button, such as the QuickMeas function, which displays the key measurement values for a currently active signal on the waveform, including positive and negative peak voltages, rise and fall times, and mean voltage.

At the higher end of Rohde & Schwarz’s oscilloscope range, the RTO digital family includes two- and four-channel models with bandwidths of 1 and 2 GHz and a maximum sampling rate of 10 Gsamples/s (Fig. 7). A key feature of the RTO scopes is their ability to continuously capture and analyze 1 million waveforms/s, which makes even rare glitches readily viewable. Moreover, this acquisition rate is accessible without having to place the scope into a special fast-acquisition mode.

Perhaps most important in the RTO family’s battery of features is a digital trigger system, which the company claims as the first of its kind in a digital scope. “We digitize first and then triggering is done on the same data that’s part of the acquisition,” says Chris Eriksen, marketing manager for the oscilloscopes.

With a purely digital triggering architecture, the trigger and captured data share a common signal path and common time base. This results in exceptionally low triggering jitter and exact assignment of the trigger to the signal. Additionally, the digital trigger rearms immediately after a trigger event. Thus, the trigger-rearming delay typical of an analog trigger is eliminated, which means that subsequent signal faults won’t be missed.

To ensure high accuracy, the RTO scopes are built around a single-core ADC sitting behind each channel. In achieving operation at 10 Gsamples/s, the converters perform no interleaving, so there’s no interleaving byproducts or noise associated with the interleaving process.

Finally, the scopes’ user interface has been built from the ground up. Features such as semitransparent dialog boxes, movable measurement windows, configurable toolbars, and preview icons with live waveforms make the scopes easy to use and well balanced between usability and portability.

Pricing for the RTM family of 500-MHZ scopes starts at $10,220 for a four-channel model. The RTO family 1-GHz instruments start at $20,550, also for four channels.

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