Digital/Mixed-Signal Scopes Address The Data Dilemma

March 29, 2012
With the proliferation of high-speed serial buses, designers need ever more capable debug tools at the ready. Today's digital and mixed-signal scopes have evolved into multi-functional debugging machines built to give deep insight into digital signals.
Throughout the long history of oscilloscopes, one thing has remained constant. The scopes of the moment have always reflected the state of the art in design, both in terms of their own construction and in terms of their capabilities.

Scopes are always among the most sophisticated electronic systems of their day. The revered analog scopes of decades ago were works of art when you opened them up. They also were built like tanks. Plenty of them are still in use, and there’s still a thriving market for these instruments and parts for repair.

Also, scopes have managed to stay ahead of the curve when it comes to having the dynamic range, bandwidth, and low noise floor it takes to visualize bleeding-edge signal content. It’s a testament to the men and women who design and build them. It’s no mean feat to design a circuit that not only can keep up with today’s high-speed buses, but also disassemble and display their traffic with great fidelity.

Whereas scopes were once about signal visualization, purely and simply, modern digital scopes are far more capable than that. There is always a tight correspondence between the design issues that complicate matters in the consumer market at any given time and what the oscilloscopes of the day can do.

The Data Dilemma

Today’s trend is toward greater amounts of data, driven by data-intensive applications like streaming audio and video on tablets and smart phones. “It’s not always the application that drives the trend, but rather the technology behind it,” says Gina Bonini, technical marketing manager at Tektronix. “As more inexpensive memory is designed into systems, those systems can handle larger files, and those files have to be transferred around somehow.

Thus, designers are moving steadily toward higher-speed serial buses in their system designs, even in lower-end applications. Likewise, servers are gravitating toward high-speed buses. This drives the integration of buses like USB 2.0 and Ethernet into more consumer applications.

According to recent surveys conducted by Tektronix of its customers, 36% are integrating USB 2.0, while 23% are building in Ethernet functionality. Meanwhile, 34% are using the I2C bus. This tells us that the higher-speed buses are now as pervasive as the lower-speed buses. Fast buses moving large files mean the need for a scope that’s up to the task of debugging those buses.

Over the past five years, oscilloscopes have changed dramatically with regard to their ability to dig into buses for debug. That, in turn, has caused an explosion in the amount of memory (or record length) in modern scopes to facilitate examination of bus protocols.

With today’s digital scopes, you can look at the bus protocol and the data being transferred in great detail. As soon as you identify issues, you can begin debugging immediately with your go-to debug tool. Scopes can decode and trigger on various buses, and they can deploy a large set of digital debugging triggers, or parametric triggers, to find signal-integrity problems as well. Issues from runts to glitches to sample/hold violations are all easy to find.

There are various ways of looking at scopes, of course. The obvious and sexiest trend is toward higher bandwidths. LeCroy’s LabMaster 10Zi, with its 60-GHz real-time bandwidth, is the current leader. But Agilent may have some news on this front in the near future. (For details, sign up for the free webcast, “See The Future Of High-Performance Real-Time Oscilloscopes,” scheduled for April 11 at 1 p.m. ET.)

In scopes, “fast” has come to mean high bandwidth. But the term “fast” has changed over the years, according to Mike Schnecker, business development manager at Rohde & Schwarz.

“Six or seven years ago, the ‘fastest’ scope meant the one that was most like an analog scope,” says Schnecker. “Fast” was the speed with which you could acquire and display a waveform. “That was the hangover from the analog era,” says Schnecker. And, the vintage analog scopes certainly do still have their devotees (see “Old Scopes Versus New: New Isn’t Necessarily Better”).

There are two broad, basic applications for oscilloscopes. One is traditional design and debug. A circuit doesn’t work as expected, or perhaps at all, and the scope is the tool of choice for figuring out why.

The other, verification of compliance with various standard data-communication protocols, has become a major agent of change in oscilloscopes. This trend really started about 10 years ago, and it’s now highly developed with software for automation of many measurements.

Hardware Evolution

You’d expect to have large record lengths, big displays, protocol debugging, and other bells and whistles in the high-end, megabuck oscilloscopes. But you don’t have to blow your equipment budget to get a highly capable instrument these days. In fact, scopes have evolved to the point that they often embody more than one instrument in a single enclosure.

Consider Agilent’s recent additions to its InfiniiVision 3000 X-Series. Launched last year for general-purpose engineering applications as well as the educational market, these scopes packed the power of four instruments in one box (Fig. 1). They offer the functionality of an oscilloscope, logic analyzer, protocol analyzer, and function generator (which was subsequently upgraded to an arbitrary function generator).

1. Agilent’s InfiniiVision X-Series oscilloscopes (left) provide an oscilloscope, logic timing analyzer, protocol analyzer, and WaveGen function generator all in one instrument with a footprint that’s only 5.57 in. deep.

Joining the 3000 X-Series lineup are four 1-GHz models (two- and four-channel versions) that start at under $10,000. Bandwidths range from 100 MHz to 1 GHz. And the scopes no longer emulate four instruments. Thanks to the addition of digital voltmeter (DVM) functionality, they are now a five-in-one instrument package. A $75 option (DSOXDVM) equips the scopes with the essential functions of a DVM.

A look into the DVM’s architecture reveals a common scope input, which feeds two functional blocks. One is the counter circuits and triggering resources, which the DVM leverages for frequency calculations. For voltage measurements, the scope input feeds an analog-to-digital converter (ADC), which in turn feeds both scope operations and a DVM buffer. After some additional calculations, the value is sent to the display.

Pricing starts at $9950 for the DSOX3102A, a 1-GHz model with two analog channels, and ranges to $15,500 for the MSOX3104A, a four-channel version that adds 16 digital channels. Additionally, upgrade kits are available to bring 500-MHz models up to the 1-GHz bandwidth.

In a very similar vein, Tektronix’s MSO/DPO4000B mixed-signal scopes also are 1-GHz instruments (Fig. 2). Tek’s target is the embedded-system test and debug market, which faces all of the challenges outlined above and then some.

2. The Tektonix MSO/DPO4000B series (below) sports six new models, including two-channel models with 20-Mpoint record lengths and two- and four-channel versions with 5-Mpoint record length

Like the Agilent scopes described above, Tek’s MSO/DPO4000B models deliver bandwidths from 100 MHz to 1 GHz, enabling designers who are debugging high-speed buses such as USB 2.0 and Ethernet to gain more channels and longer record lengths.

The series comprises six models, including two-channel models with 20-Mpoint record lengths and two- and four-channel versions with 5-Mpoint record lengths. The latter models, such as the DPO4102B-L, drop the price to under $10,000. Channel counts include 16 digital channels and either two or four analog channels.

Each scope comes with one TPP1000 1-GHz passive probe per analog channel. A sister series, the MSO/DPO3000 scopes, offers an upgrade path to higher bandwidth levels. Users can start with a 100-MHz model and upgrade as required to as much as 500 MHz. Prices start at $3380.

Besides the upgradable bandwidths, the MSO/DPO3000 series oscilloscopes now support the MIL-STD-1553 and FlexRay serial buses, which are popular in the aerospace and automotive industries, respectively. This further broadens the scopes’ support for communications protocols, which already includes I2C, SPI, CAN, LIN, RS232/422/485/UART, and I2S/LJ/RJ/TDM.

Aiming At Debug

Rohde & Schwarz, a venerable manufacturer of test equipment in the communications market, is a relative newcomer in the oscilloscope arena. Its approach to scopes has been to try and make some improvements in the scope’s utility for debugging applications.

“We felt the debug market wasn’t being well served,” says Rohde & Schwarz business development manager Mike Schnecker. “Scopes were adding capabilities but weren’t getting faster in the traditional sense of update rates.”

The approach Rohde & Schwarz chose to improve the debug capabilities in its relatively new scope line was to place an ASIC between the ADC and display buffer memory. As a result, the company’s RTO series of scopes sports a maximum acquisition rate of 1 million waveforms/s (Fig. 3).

3. Rohde & Schwarz’s RTO series instruments were designed with improvements in debugging capabilities.

Moreover, Rohde’s designers went beyond simply having the ASIC perform acquisition processing for the display. “We built in histograms and mask testing,” says Schnecker. Having these capabilities in digital hardware enables the scope to perform these functions in real time. Thus, engineers can probe a circuit for eye-pattern analysis, capturing hundreds of millions of samples against the mask and gaining a good deal of insight into the circuit’s behavior.

Another improvement Rohde & Schwarz deemed critical was better noise performance in the scopes’ front end. With designers looking at fast-switching, small signals, scope noise becomes a more significant factor in the accuracy of measurements. “Noise is driven by the ADC itself. That’s the weak link,” says Schnecker. Also, the amplifiers ahead of the ADC must be low noise and broad banded.

Rohde’s designers have tried to address the sensitivity of the front end to achieve high gain at high bandwidths as well. In debugging, this permits the use of an electromagnetic interference (EMI) probe to determine where the emitters are in a circuit. The scopes’ frequency analysis capabilities then can determine where they are in the spectrum.

The Swiss Army Scope

Instruments such as Agilent’s InfiniiVision 3000 X-Series scopes exemplify a fast-growing trend toward multi-functional instruments that take the place of several benchtop units, saving space and cost. With these faster serial buses moving quickly into the design mainstream, design teams not only need scopes with higher bandwidths, they also need functionality such as triggering, decoding, and the ability to search on packet information.

“When an engineer has a problem with his or her design, he takes it back to the bench where he has a variety of gear to choose from, including protocol analyzers, scopes, DMMs, and so on,” says Tektronix’s Randy White. “But more often than not, it’s the scope that gets used. The engineer wants information with which to make decisions about what’s going on in the system, and he usually wants it presented in the most simple form. Seeing the signal is as simple as you can get.”

As modern scopes grew more capable, their manufacturers began using them as the platform to add the functionality of these other instruments. “For example, we added simplified logic-analyzer functions when the MSO scopes were launched,” says Gina Bonini, Tek’s technical marketing manager.

With as many as 16 digital channels and four analog channels, today’s oscilloscopes can examine these high-speed buses from a logic analyzer’s perspective. Because that functionality is built into the scope, the same instrument then can be used to troubleshoot problems.

“It’s having that correlation, being able to look at the ones and zeroes in the digital domain, and then looking at the analog domain to see the physical shape of the signal and the runts and glitches causing the digital errors,” says Bonini.

Addressing Wireless Everywhere

Another recent series that bends the boundaries of traditional instruments is Tek’s MDO4000 line of mixed-domain scopes, which made quite a splash upon their introduction last year (Fig. 4). In the case of the MDO4000, the emphasis is less on high-speed serial/parallel buses and more on the growing ubiquity of wireless functionality.

4. Tektronix’s MDO4000 series scopes combine an oscilloscope and spectrum analyzer for true time-correlated, mixed-domain signal capture and analysis.

As discussed earlier, consumer electronics and embedded systems are moving around great amounts of data, and not only on internal buses but also to and from the outside world. One of the predominant pathways for that data is wireless networks. Thus, there’s a growing need for multi-function instruments that encompass the frequency domain.

According to Tek’s customer surveys, more than 60% of engineers use both a spectrum analyzer and scope. The MDO4000’s addition of a spectrum analyzer is a leap forward in scope functionality. The important aspect of that leap is cross-domain correlation between the frequency and analog domains.

As it is both a scope and a spectrum analyzer, all in a package that is just 5.8 inches deep, Tek claims the MDO4000 instruments are the world’s first mixed-domain oscilloscopes. They capture time-correlated analog, digital, and RF signals, letting users see how the RF spectrum changes over time.

The instrument’s built-in spectrum analyzer offers a 3-GHz or 6-GHz RF port, which addresses the vast majority of wireless test needs that embedded system designers would encounter. It augments that with a capture bandwidth of at least 1 GHz at all center frequencies, which is considerably broader than typical spectrum analyzers.

With the scope’s ability to display all signals in time-correlated fashion, designers can see all of their analog, digital, and RF signals accurately correlated with each other. This is a boon to troubleshooting embedded systems. The scopes offer up to 21 channels (four analog, 16 digital, one RF) with analog bandwidths of 500 MHz or 1 GHz.

For spectral analysis, the instrument provides both automated markers for quick, easy identification of signal peaks as well as manual markers for measurement of non-peak areas of interest. A spectrogram display provides visualization of slowly changing RF phenomena (Fig. 5). The spectrum-analyzer portion of the instrument brings a full suite of built-in analysis tools.

5. The MDO4000 instrument’s spectrogram display enables visualization of slowly changing RF phenomena.

Based on the MSO4000B series, the scope offers a maximum waveform capture rate of 50,000 waveforms/s. There are more than 125 trigger combinations, including the ability to trigger on serial packet content. The scope’s MagnaVu high-speed digital acquisition delivers 60.6-ps resolution. Low-capacitance passive probing offers twice the bandwidth (1 GHz) and half the loading (4 pF) of typical passive probes.

On the analysis side, the scope is pre-loaded with 41 automated measurement setups and advanced waveform math functions. Serial-bus analysis packages are available as options. With these analysis features and the integrated spectrum analyzer hardware, the MDO4000 makes short work of mixed-domain tasks such as system-level debug of wireless-enabled designs, timing analysis for mixed-signal circuits, and tracking down sources of noise and/or interference.

A somewhat more modest entry into the multiple-instrument category is B&K Precision’s 2540B-GEN and 2542B-GEN digital scopes, which integrate function/arbitrary-waveform generation. The scopes, with bandwidths of 60 MHz and 100 MHz, respectively, sample at 1 Gsample/s. They offer good memory depth at up to 2.4 million points and a built-in local-area networking (LAN) interface, which enables front-panel emulation of the scope and AWG for full Web functionality.

Make Scopes The Experts

Another trend that has made oscilloscopes more functional and easier to use is the proliferation of application-specific software that scope makers build into the instruments. “An overriding trend in scopes is toward applications,” says Dave Rishavy, business segment manager for high-performance and sampling scopes at Agilent Technologies.

With the overlay of a task-centric program that automatically sets up the instrument for the task at hand, users no longer have to be the repository of expertise on how to make given measurements. This is especially important given that not every test engineer makes every measurement all that often.

With many design teams relying so heavily on standard protocols, it makes a lot of sense for the instruments to be the expert on a given measurement. “Designers want to use a common USB PHY (physical layer) on both ends of a link, because the part costs are low and they can get to market faster,” explains Rishavy. “There’s fewer engineering resources out there, and people want off-the-shelf solutions.”

Thus, the application software makes the scope itself the expert on any particular topic. It could be eye diagrams, jitter measurements, compliance with any of a host of protocols, power measurements, or double-data rate (DDR) measurements. “Whatever the topic is, the application on the scope brings industry knowledge into the scope,” says Rishavy. This is the big meta trend that has altered our perspective and design decisions.

Rishavy says that Agilent was among the first of the scope makers to take the application-specific approach, building a framework for applications in its scopes that allows deft transitions from task to task. “We have 34 different applications, all with the same look and feel,” says Rishavy. User-interface consistency is a key consideration, as many engineers must cross from domain to domain in the course of a test routine.

Likewise, Tektronix offers 41 automated measurements in the MSO/DPO4000B series, as well as waveform histograms and fast-Fourier-transform (FFT) analysis. There is specialized application support for serial bus analysis, power supply design, limit and mask testing, and video design and development.

Moving Up The Protocol Stack

The prevalence of application-specific software has led Agilent in the direction of building a structure around protocols. “Allowing customers to go up in the protocol stack is another way it has impacted our structure and our software design approach,” says Rishavy.

Additionally, application-specific software has been Agilent’s impetus for embracing multiple functionality in its instruments. “Applications have, in a way, driven integration of other measurement types into the scopes,” says Rishavy. “That’s why we have mixed-signal capabilities across most of our product portfolio. We’ve also integrated arbitrary waveform generators and digital voltmeters across parts of it.”

When you consider how oscilloscopes work, you see how it can help with debugging the PHY. “The scope converts a signal into a trace, takes the timing information and 8 bits of vertical data, and turns it over to processing algorithms that let you move up in the stack,” says Rishavy.

This approach doesn’t completely replace a protocol analyzer, but it can help you to zoom in on a repetitive problem. You can capture a sequence and then look at that data from a transaction or protocol layer perspective. Digging one level down provides you with markers that bridge between the physical and analog representations, exposing the root cause of issues.

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