The Future Of Signal And Power Integrity Designs

Istvan Novak from Samtec and Engineer of the Year at DesignCon 2020 sat down with Design News to answer technical and show questions from over the last 20 years.

The winner of the DesignCon 2020 award was Istvan Novak, Principal Signal and Power Integrity Engineer at Samtec. Aside from his impressive career as an engineer, Novak has been a long time participant at DesignCon. In the following interview, Design News talks with Novak about past and future trends in Signal Integrity (SI) and Power Integrity (PI), as well as what the award means to him.

Design News:  How has the signal integrity (SI) industry and technology changed in the last 20 years?

Istvan Novak: One change is the tremendous increase in data transfers, which has meant faster CPU clock speeds, data rates and cable signals. The bandwidth requirement is also exploding. Perhaps more importantly, the semiconductor industry has continued to change. Twenty years ago, Moore’s Law was in full force. Now it is significantly slowing down. I won’t call it dead yet. We still see improvement. However, the demand for bandwidth did not slow down. Instead, it actually continued to go up in terms of percentage rate. People have found other ways of satisfying increasing bandwidth. One example is by using different modulation techniques for to improve signal integrity such as pulse amplitude modulation (PAM) for stage 4. This technique is now excepted at the very high end of the data speed range. PAM-4 is a modulation technique whereby 4 distinct pulse amplitudes are used to convey the information.

Image Source: Keysight / PAM-4

Design News: Have power integrity (PI) challenges been addressed at the same pace as ones for signal integrity (SI)?

Istvan Novak: There have been major differences between SI and PI over the last 20 years. SI issues evolved from the electromagnetic compatibility (EMC) discipline sometime in late 80s to early 90s. In contrast, PI didn’t become an issue until 10 to 15 years later. What this tells us is that SI has had more time to mature. Today, there are a lot of SI standards and guidance available for high-speed signal designers. This is in very stark contrast to PI designs. Today, PI is probably in the same state or maturity level as SI was in the mid- to late-90s.

I have second thoughts as to whether PI will ever be as developed, as mature, as SI. The reason is that PI is definitely more complex than SI, which has come a long way. Back in the early 90s, high-speed signal integrity design was considered black magic. The problem was that people didn’t approach things systematically to understand what happens and why. Once the underlying physics were understood, the subject was no longer considered black magic. Technical works like Howard Johnson's very popular book on Signal Integrity helped a great deal.

The reason why we may not expect our understanding of PI to even catch up to where SI is today is that the PI problem definition is far more convoluted. In other words, it cannot be isolated in such a nice way as it was for SI.

Design News: Why is that the case? Why is PI more complex, more convoluted than SI?

Istvan Novak: For high-speed data signal integrity, we don’t typically worry about what happens to the signal when it goes into the silicon (chip). Most of the signal measurement and other checking for SI happens in the passive interconnects like cables, sockets and interfaces. And the passive interconnects can be treated in isolation from the rest of the system. They can be treated in segments.

It’s not that easy to break PI into smaller, easier to handle pieces in order to understand it. Consider a high-power and high-speed CPU. If you turn over a chip or the board, you’ll see a number of high-speed I/O pins with one dedicated return pin. You’ll also have power pins  that are not very isolated or as numerous. Further, the ground plane is not separated between the high current and the high-speed sections. We like to keep the power planes intact.  With high frequencies and high-speed signals, we don’t necessarily need to consider how the return signals spread over a large distance or plane because they will not.

This again is in stark contrast to power integrity (PI), where power related noise has a way bigger range of interaction from high frequencies all the way down to DC.  This noise may interfere with the high-speed signal. My point is that we cannot easily, physically separate blocks of power. That is probably one of the reasons why standards or any general guidance are harder to come by for PI design.

Design News: Will improvements in models and simulation tools help us to better model and simulate more complex PI issues?

Istvan Novak: I believe it absolutely will help. But I’m not sure if underlying limitations will change because designs are driven by competitiveness. Would it be possible to design a system were the power can be compartmentalized and segmented very nicely? Yes, it is technically doable. It would be just much more complex. While it might be easier to design systems with segmented power elements, it would also require bigger, probably heavier and more numerous components. This is why is wouldn’t be competitive in the marketplace today.

This is why people try to push the limit on PI design, to allow all of these interactions I mentioned earlier to take place. Using detailed simulation and measurements, these detailed interactions can be taken into account. But it takes a lot of time to simulate and measure these interactions.

Design News: What activities are critical to ensure designs meet both SI and PI requirements?

Istvan Novak: Simulation and measurement validation are critical design activities. But equally important is understanding. These three activities are needed to ensure a good design and engineered approach. However, very often we may see people trusting tools blindly. They assume that the tools will give the correct answer to what they thought they asked from the tool. Such a believe can be extremely misleading. Even developers with experience and lots of support from the tool developers can be misled.  It takes a lot of time to get a decent correlation to the measurement. But even if the correlations are good we must ask ourselves: “Do we understand why all that is happening? Can we explain to ourselves?”

It is true that, when I start my car and drive it, I don’t necessarily need to understand how the car is working on the inside. That is because the details have been worked out by the automotive designers. But the people who designed the car still need a very good understanding of what is happening and why.

Design News: In this age of reference designs and intellectual property, some would argue that designers don’t need to understand things like SI and PI for their designs to work.

Istvan Novak: The world is going toward simplifying the process in which, for instance, a big vendor marketing a new chip may say; “You don’t need to do SI. Here is a reference design - just copy it and you will be happy.” And that is a working solution, if the reference design is done correctly. If so, it saves a lot of time as people don’t have to recreate all of the groundwork to understand the SI and PI details. However, anyone who relies on this solution for multiple generations of system design should agree that it carries a big risk.

First of all, the users who rely too heavily on reference designs may become depended or at the mercy of the silicon vendor. If anything goes wrong such as a slight change in design or requirements or even an honest mistake, then not understanding SI and PI design might cause big problems. Companies that lack or don’t want to invest in design resources and rely heavily on reference designs are creating a very strong dependence on others.

Again, I’m not against it, I’m just pointing it out that we need to understand that a dependency can be hard to break.

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