Product category:
Design and Development Hardware
News Release from: Tektronix | Subject: iView
Edited by the Electronicstalk Editorial
Team on 24 May 2001
Confronting signal integrity in digital
design
Signal integrity is a broad topic, one that impacts many electronic design disciplines, as Dave Ireland of Tektronix explains.
Signal integrity is a broad topic, one that impacts many electronic design disciplines But until a few years ago, it wasn't much of a problem for digital designers
This article was originally published on Electronicstalk on 24 May 2001 at 8.00am (UK)
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They could rely on their logic designs to act like the Boolean circuits they were.
Noisy, indeterminate signals were something that occurred in high-speed designs - something for RF designers to worry about.
Digital systems switched slowly and signals stabilised predictably.
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Market forces in the PC, networking, and telecommunications realms have changed everything, following a curve on which the pace of change itself is accelerating.
Processor clock rates have multiplied by orders of magnitude.
Computer applications such as 3D graphics, video, and server I/O demand vast bandwidth.
Much of today's telecommunications equipment is digitally based, and similarly requires massive bandwidth.
So too does digital high-definition TV.
At the functional circuit level, the term 'bandwidth' means speed - high data rates, narrow pulses, and fast rise/fall times.
Integrated circuit processes have evolved, generation by generation, to meet the market's speed demands.
The current crop of microprocessor devices handles data at rates up to 2, 3 and even 5Gbyte/s, while some memory devices use 400MHz clocks as well as data signals with 200ps rise times.
Importantly, speed increases have trickled down to the common IC devices used in automobiles, VCRs, and machine controllers, to name just a few applications.
A processor running at a 20MHz clock rate may well have signals with rise times similar to those of an 800MHz processor.
We have crossed a performance threshold that means, in effect, almost every design is a high-speed design.
Without some precautionary measures, high-speed problems can creep into otherwise conventional digital designs.
If a circuit is experiencing intermittent failures, or if it encounters errors at voltage and temperature extremes, the chances are there are some hidden signal integrity problems.
These can affect time to market, product reliability, EMI compliance, and more.
It's time to face signal integrity problems head on.
Let's look at some of the specific causes of signal degradation in today's digital designs.
Why are these problems so much more prevalent today than in years past? The answer is, again, speed.
In the 'slow old days', maintaining acceptable digital signal quality meant paying attention to details like clock distribution, signal path design, noise margins, loading effects, transmission line effects, bus termination, decoupling, and power distribution.
All of these rules still apply, but - Bus cycle times are up to a thousand times faster than they were 20 years ago! Transactions that once took microseconds are now measured in nanoseconds.
To achieve this improvement, edge speeds too have accelerated: they are up to 100 times faster than those of two decades ago.
This is all well and good; however, certain physical realities have kept circuit board technology from keeping up the pace.
The propagation time of inter-chip buses has remained almost unchanged over the decades.
Geometries have shrunk, certainly, but there is still a need to provide circuit board real estate for IC devices, connectors, passive components and of course the bus traces themselves.
This real estate adds up to distance, and distance means time - the enemy of speed.
The ratio of device rise time to circuit board propagation delay has changed by about two orders of magnitude over the last 20 years.
Even a system that doesn't use the latest 1GHz processor may be populated with, say, 50MHz devices whose rise time pushes the limits of printed circuit trace technology.
It's important to remember that the edge speed (rise time) of a digital signal can carry much higher frequency components than its repetition rate might imply.
For this reason, some designers deliberately seek IC devices with relatively 'slow' rise times.
The lumped circuit model has always been the basis of most calculations used to predict signal behaviour in a circuit.
But, when edge speeds are more than four to six times faster than the signal path delay, the simple lumped model no longer applies.
Circuit board traces just six inches long become transmission lines when driven with signals exhibiting edge rates below four to six nanoseconds, irrespective of the cycle rate.
In effect, new signal paths are created.
These intangible connections aren't on the schematics, but nevertheless provide a means for signals to influence one another in unpredictable ways.
At the same time, the intended signal paths don't work the way they are supposed to.
Ground planes and power planes, like the signal traces described above, become inductive and act like transmission lines; power supply decoupling is far less effective.
EMI goes up as faster edge speeds produce shorter wavelengths relative to the bus length.
Crosstalk increases.
In addition, fast edge speeds require generally higher currents to produce them.
Higher currents tend to cause ground bounce, especially on wide buses in which many signals transition at once.
Moreover, higher current increases the amount of radiated magnetic energy and, with it, crosstalk.
What do all these characteristics have in common? They are classic analogue phenomena.
To solve signal integrity problems, digital designers need to step into the analogue domain.
And to make that step, they need tools that can show them how the digital and analogue worlds interact.
The long-established solution for digital design verification and troubleshooting work is the logic analyser.
There is simply no better tool for capturing and analysing timing relationships, logic state activity, and software execution steps.
A logic analyser such as the Tektronix TLA600 or TLA700 Series can capture many thousands of cycles of bus activity and display waveforms and code symbols for each.
Equally important, the logic analyser can trigger on numerous conditions, including digital errors of various types.
The logic analyser is a tool that can help you quickly pinpoint digital faults in your design.
But these digital errors often have their roots in analogue signal integrity problems.
To track down the cause of the digital fault, it's often necessary to turn to an oscilloscope, which can display waveform details, edges, and noise; can detect and display transients; and can help you precisely measure timing relationships such as setup and hold times.
Clearly the solution to digital signal integrity problems requires both logic analyser and oscilloscope functions.
For years, engineers have used the two discrete instruments in tandem, first detecting a digital fault with the logic analyser, then using their knowledge of that fault to set up the oscilloscope's trigger to capture the same event.
Of course, this approach requires a consistent, repetitive event.
If the problem is intermittent or aperiodic, then it is very difficult to capture reliably.
And the two waveforms don't co-exist on the same display, complicating the effort to interpret their timing relationships.
A workable solution emerged in the form of cross-triggering features built into compatible logic analysers and oscilloscopes.
While this simplified the simultaneous capture problem (by ensuring that the DSO responded to the logic analyser trigger), it did not address the need to display both waveforms on a single screen.
The need for an effective integrated logic analyser/oscilloscope solution has not gone unnoticed by instrumentation vendors.
Several alternatives have emerged over the past few years, each with its strengths.
Each solution allows users to trigger on a digital problem (that is, detect it) and capture the related analogue waveform information simultaneously.
The alternatives differ in their cost and flexibility, but all of them offer the ability to display, on one screen, time-correlated waveforms from the two domains - analogue and digital.
There are three basic architectures that segment the logic analyser and oscilloscope functions in different ways: the modular logic analyser with integral plug-in DSO modules; the nonmodular, monolithic logic analyser/DSO combination; and the fully integrated pairing of discrete logic analyser and DSO instruments.
The first truly integrated, high-performance logic-analyser/DSO solution was the Tektronix TLA700 Series, a modular logic analyser family with plug-in DSO modules.
The instrument is an uncompromised logic analyser built on a mainframe concept that allows either digital or analogue modules to plug into the system.
The available DSO is a powerful 2- or 4-channel unit with 500 MHz or 1GHz bandwidth.
They provide a full array of oscilloscope functions, and is specifically designed to accept a trigger command from the TLA700 Series logic modules via internal high-speed signals.
The oscilloscope module's acquisition data is stored in the module's own 15,000-sample memory, and is automatically time-correlated with the logic analyser data captured when a trigger occurs.
The results from both instruments are displayed on the TLA700 screen.
The modular logic analyser delivers the valued benefit of configurability.
On the digital side, it gives users the opportunity to configure as many digital channels as they need (into the hundreds), and to handle growing numbers of buses and device pins.
Oscilloscope capability, too, is configurable in terms of bandwidth - up to 1GHz maximum at 5Gsample/s - and channel count.
The modular system requires no special cabling between the logic analyser and the oscilloscope.
Modular solutions have their limitations.
High-performance oscilloscopes or advanced features are not always readily available, restricting a user's ability to use the integrated solution to its maximum performance potential.
In addition, some designers may require a separate oscilloscope to enable individual oscilloscope-based test needs, driving up the total investment in benchtop instrumentation.
Fixed configuration tools offer innate simplicity and the convenience of a compact, self-contained benchtop instrument.
Within their performance limits, they are a cost-effective way to bring digital and analogue waveforms into the same environment.
They can offer the same efficient level of integration as the modular system described above.
Fixed-configuration systems, however, are bound by the specifications of the two built-in 'instruments' they encompass.
The target application must not exceed either the digital pin count or the analogue bandwidth of the integrated instruments.
Some advanced features are not available in the built-in oscilloscope, which means a separate instrument must be purchased for many oscilloscope measurements.
The fixed-configuration tool is limited to the capabilities it was built with.
As the target application evolves (inevitably requiring higher speeds, or more channels, or both), the instrument can't evolve with it.
In addition, when using this configuration, the oscilloscope is not available for use by other team members.
Still, the fixed logic analyser/oscilloscope may be the lowest-cost answer if the target requirements are within its reach and expected to remain stable for the life of the instrument.
Many digital design applications require a solution that can deliver an optimum balance of logic analyser and oscilloscope features today, and grow with changing measurement needs.
Most leading instrument makers already have a broad range of logic analyser and oscilloscope models, reflecting diverse performance and price levels.
Why not develop a solution that lets users pick and choose the exact combination of digital and analogue attributes they need? If users have the means to quickly set up, acquire, transfer, and time-correlate oscilloscope waveforms on a logic analyser, they will have the best of both worlds.
That is the rationale behind the latest class of solutions to reach the market.
The integrated instrument pairing is the most open of the three approaches discussed here.
Importantly, the term 'pairing' doesn't imply matching a specific logic analyser with a specific oscilloscope model.
In a sense, the integration itself is the solution, while the instruments are the flexible components you choose according to your application needs.
The instrument pairing architecture is built around a software and interconnect package that seamlessly integrates proven logic analysers and oscilloscopes, creating an interoperable tool set for digital development and troubleshooting.
The Tektronix Integrated View (iView) capability integrates the company's TLA600 or TLA700 Series logic analysers with external oscilloscopes in the TDS3000 Series, as well as the high-performance TDS7000 Series and TDS684C/694C oscilloscopes.
The solution makes it possible to transport oscilloscope waveforms to the logic analyser display and automatically time-correlate them.
The two types of waveforms can even be superimposed if desired.
With the paired instrument approach, users can match virtually any digital performance level to an equally broad analogue performance range.
For example, designers developing a server with the fastest, most complex processors and buses will want high channel count, bandwidth, and memory depth on the digital side, and exceptional acquisition (sample) rate on the analogue side.
These engineers might combine a TLA700 logic analyser with a 4GHz TDS7404 Series oscilloscope.
In contrast, a design team working on an automotive drive-by-wire controller may require fewer digital channels and generally lower bandwidth.
They might choose a smaller TLA700 Series logic analyser configuration or its self-contained equivalent, the TLA600 Series.
They might then match this digital toolset with a cost-effective 500 MHz TDS3000 Series digital phosphor oscilloscope.
It's worthwhile to note that the paired instrument approach overcomes the record-length limitations that exist in the other configurations.
In the past, logic analysers' record length has far exceeded that of any oscilloscope that could be integrated.
With the much broader choice of models available for the paired instrument user, the oscilloscope can be chosen to better match the amount of 'time' the logic analyser can record.
High-frequency design concepts are no longer something the digital hardware engineer can afford to ignore.
With today's digital devices producing subnanosecond rise times and clock rates exceeding 1GHz, the digital designer is as likely to run into signal integrity problems as is the RF engineer.
At speeds in the GHz range, circuits begin to exhibit increasing crosstalk, noise, jitter, EMI, and more.
These signal integrity problems can be traced to the growing gap between conventional circuit board performance and present-day IC clock and edge rates.
Troubleshooting signal integrity problems requires careful attention to the analogue characteristics of digital signals, and this in turn calls for an integrated measurement solution that can capture and display both domains in one time correlated view.
Today, engineers can choose from several solutions, all of which integrate the features of a logic analyser with those of an oscilloscope.
The choice depends on many variables: the required mix of analogue and digital capabilities, cost, anticipated need to expand analogue or digital functionality, and more.
Increasingly, designers are looking for flexible solutions that they can configure for their specific bandwidth, channel count, and record length needs.
The integrated instrument pairing is a promising new solution to this requirement. Request a free brochure from Tektronix ...
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