What is Signal Integrity?

Great question.  Now that I’ve devoted well over 30 years to the field, I’ll venture an answer.  My simplistic definition has always been “the analog side of digital”.  I still like that description, because in practice the binary world of “ones” and “zeroes” only exists in theory.  And, hopefully, in system memory - assuming the interface’s Signal Integrity (SI) was done correctly.

My answer to the question will describe Signal Integrity from angles not typically articulated.  My statements are best understood against the backdrop of my series Signal Integrity, In Practice, which provides a detailed description of what Signal Integrity is.  My hope is that the series equips you to not only understand and succeed performing SI, but also empowers you to carry the practice into new and uncharted territory.  In practice, that is non-optional.  As the face of SI is always changing, one aspect of what SI “is” is participating in defining how it is done.  Stay in Signal Integrity for more than a decade and you will see what I mean.  The field has always provided ample room for contribution, and the larger SI community assumes collaboration as a given.

Textbook Signal Integrity
In my full-day class I start with the dictionary’s definition of Signal Integrity:  a “signal” is “an electrical impulse transmitted or received”, and “integrity” is “an unimpaired condition”.  Putting it together, the practice of SI involves getting an electrical impulse to travel from a Tx to an Rx unimpaired.  As that definition is overly simplistic, we can then spend the rest of the day discussing how to figure out what impairments are problematic, and how such impairments can be predicted, understood, and corrected.  In many ways, SI is all about the impairments.

I like SI textbooks, and buy them regularly.  Because the world of SI is small most of the books were written by friends or acquaintances, and I expect to write one too.  But the problem with SI is that it keeps re-inventing itself, rendering textbooks quickly out-of-date.  It’s not that Maxwell’s Equations and most of SI’s equations change, it’s just that – just like the technological world that SI attempts to tame – the practice of SI keeps changing.  Truth be told, this is what I like about SI:  to survive in the craft you have to be nimble and creative, as I’ll explain in more detail below.

How has Signal Integrity Changed?
As frequencies have increased 4 orders of magnitude throughout my career, perhaps a better question is how could SI not change?  And frequency is not the only thing that has changed; we’ve also seen orders of magnitude transitions in integration, materials, and measurement equipment.  These forces have influenced change, and are inter-related.  In the same time frame, items that have remained fairly constant are PCB fabrication dimensions and computer architecture.  As a result, we are still moving signals between the same things (e.g., CPU, memory, IO) amidst roughly the same dimensions (few things resist change more than PCB tooling).

The most obvious SI changes are the interface transitions from common-clock to source-synchronous to serial links.  Said another way, the “clock” (i.e., the signal that defines when the others are valid) has transitioned from traveling “to” the data to “next to” the data to “inside” the data.  These changes brought our required precision from nanoseconds to hundreds of picoseconds to tens of picoseconds.  As these transitions are adequately described elsewhere (textbooks that are becoming out of date), I will not belabor this point here.  Thankfully, models, simulators and measurement equipment have kept pace – always keeping us in the ~3% accuracy realm.  As changed happened, it has been interesting to watch the interplay between measurement and simulation; measurement being more bandwidth-limited than the simulation, while simulation’s lack of limitation helped foreshadow the future while simultaneously leaning on measurement for self-correction.  Say what?  Great fun.

What Signal Integrity is NOT
As SI emerged as a design task in the 1990s it unfortunately became shrouded in mystery.  This was further perpetuated by the seminal 1993 SI book that subtitled itself “A Handbook of Black Magic”, a myth even the authors quickly dispel in their preface.  I still wish they hadn’t likened our craft to “black magic”.  If there is any mystique associated with SI it is around how it is done, which I’ve already established to be dynamic.  Signal Integrity is not black magic. 

In reality the principles and physics that govern SI are not mysterious.  The engineer willing to step beyond the tidy “digital” world quickly discovers the many SI principles and equations that existed before digital electronics began.  The problem is modern electronics engineers are constrained by either their job description or inclinations, relegating digital “fuzziness” to the SI engineer.  And we haven’t always given clear and timely answers, which has further fueled the mysterious side of SI.  Indeed, our favorite answers are not the binary “yes” or “no” but rather “it depends”.  I guess that’s our analog expression of how we view the digital world.

So what is signal integrity?  Looking at SI at the highest level we see three things:  Signal Integrity is essential, a differentiator, and a practice that requires creativity.

Signal Integrity is Essential
I still remember the day the CEO of Intel passed down guidance to the divisions to invest in Signal Integrity.  In the 1990s the key metric for each new processor – even from the consumer’s viewpoint – was frequency.  As increasing frequency demanded faster edge rates a new wave of SI problems emerged that had to be contained; SSO, crosstalk, ringing, non-monotonicity, flight times, and meta-stability were affecting system performance, schedules, and volume production ramps.  SI had become essential.

During this time, it was interesting to watch the responsibility of SI jump from system to IC companies.  Integration moved an increasing amount of system design inside the ICs, and IC companies discovered their products could not succeed in the market unless they credibly resolved SI issues – both inside and outside their components.  And so IC companies began publishing “design guidelines” and “reference designs” to detail the boundaries in which reliable signal integrity could be achieved.  We began calling this required design space “the box”.  Make the box too small and system companies could not design in the product.  Make the box too big and SI was lost.  The burden of achieving reliable SI within a realizable design space became the burden of silicon design, and this is why – even to this day – many of the best SI engineers work for IC companies.

The requirement to hand-off a reliable design space (i.e., “the box”) blurred the practice of SI.  While EDA vendors may assert that every system design must perform SI simulation before production sign-off, in practice this is not the sense in which SI is essential.  What is essential is that SI must be resolved at some point in the design flow, and it may be done by the IC company, the system company, or both.  System companies using components from world-class IC vendors and staying within the box often choose not to simulate.  However, if every system company stayed within the box there is no differentiation in product design and the market is flooded with too many instances of the same thing.  Find a way to implement different form factors, higher frequency, or increased functionality and a product stands out among the others.  But these ideas require robust SI analyses to engineer beyond the box.  As such, Signal Integrity is a great way to differentiate a product.

Signal Integrity is a Differentiator
While the IC company finds Signal Integrity an essential part of design, the systems company that invests in Signal Integrity finds it to be a significant differentiator.  Systems companies use SI to cut costs (pages 23-24), improve performance, add design and manufacturing margin (pages 20-23), and – most importantly – to expand the design space (“box”) defined by their IC vendors and develop a better product than their competitors using the exact same components.  I think this is where SI shines the brightest, and adds the most value.

It’s no secret that “the box” defined by IC companies is overly constrained and slanted in their favor.  And good for them; they need to ensure the system company with no SI capability succeeds implementing their product.  And so many design engineers willingly stretch a component’s rules and guidelines, knowing they are guard-banded and not as authoritative as they may seem.  (When I worked at IC companies, we often quipped “I used to think datasheets were perfect, and then I wrote one”.)  And yet where is the edge?  How do I understand, and even quantify, the risk I am taking in my journey outside the box?  Better yet, is there a way to re-think the assumptions and engineer these components into an expanded or entirely new application?  For those ready to think creatively, Signal Integrity provides the answers.

Signal Integrity Requires Creativity
If you step into SI you will consistently be asked to craft clear answers from insufficient data based on ill-defined measures of success.  Does that require resilience and creativity?  You bet.  It’s not that our simulators can’t provide answers to 14 decimal places; indeed they do, but that is not an indication of precision.  And so the process of learning to what extent SI’s data can be trusted is a journey unto itself.  The models are never perfect, simulators and measurement tools can easily lead you astray (or, as Dr. Mike says, “pick your pocket”), and – even if you learn how to use all that to derive a valid answer – it is often unclear what to compare it to.  That’s a great job description, huh?

I believe my first career as a musician and artist helped me here.  In music you learn to create something that makes sense from a palette of notes, sounds, harmonies and rhythm.  You can combine it all without skill and frustrate those around you, or you can blend them into something that feels right.  And so it is with SI.  Artistic creativity is required because – even though it is an “engineering discipline” – the practice of Signal Integrity and its deliverables are not as measurably precise as say a schematic or a layout.  Those tasks have a well-defined and literal set of inputs and outputs; there’s no fuzziness to success because everything is either connected or it’s not.  While I admire the attempts of SI tool wizards and textbooks to present tasks as steps 1, 2, 3, I find the analogy of viewing SI as an attempt to paint more consistent.  Will your painting come out right?  …will people understand it?  …and, most importantly, will the product “work”?

The practice of Signal Integrity requires the creative application of a baseline set of skills.  It’s imperative to learn the principles and equations that govern SI as, using the painting analogy, these are your colors.  It’s also imperative to become skilled at using models and simulators, these are your brushes.  The project is your canvas.  The challenge is to combine it all creatively to produce something that makes sense and “works”.  My advice to engineers climbing the SI learning curve continues to be “hang onto your creativity” (2:25 to 3:10). 

And so there you have it:  Signal Integrity – as it has emerged in the realm of digital electronics – has proven itself to be an essential and creative practice that offers product differentiation.

In Conclusion
High-speed digital Signal Integrity has worked its way into every corner of the electronics industry.  Whether you’re a PCB Fabricator, Test & Measurement Vendor, Contract Manufacturer, Component Vendor, Standards Organization, or involved in any type of digital product design, some amount of fluency in SI is required.  Signal Integrity touches everyone, and is no longer a fringe skill set.

Tune into my larger series on Signal Integrity, In Practice to learn more about what SI “is” in terms of specific skills and design tasks.  In the series I have endeavored to make the complex simple, boiling SI down to its most essential aspects.  I trust it will be helpful, and look forward to your contribution as together we answer the question:  What is Signal Integrity?