Signal Integrity: The Key to Successful High-Speed Circuit Design
A conversation with Signal Integrity Evangelist Eric Bogatin.
As system speeds continue to rise and new product design cycles shrink, the ability to design circuits that have a high probability of meeting system specifications with the first iteration has become a top priority. Signal integrity (SI) analysis has become an integral part of the circuit design process to achieve that goal.
Years ago, a physical prototype could be built to determine if a proposed design would work. Today’s greatly escalating bandwidth and sensitivity to a host of circuit parameters make that approach too costly, time consuming, and wildly inaccurate. Establishing SI of a 56 Gb/s channel requires a combination of advanced test equipment, simulation software, as well as experienced engineers capable of correctly interpreting the results. Using this process, designers can quickly identify problems and make adjustments in real time to optimize performance and meet objectives.
Eric Bogatin has been an outspoken advocate of signal integrity techniques for 41 years and is a respected expert in advanced SI technology. Eric received his BS in physics from MIT and PhD in physics from the University of Arizona. He is currently a Fellow and dean of the Signal Integrity Academy with Teledyne LeCroy, and a professor at the University of Colorado, Boulder, in the Electrical, Computer, and Energy Engineering department. He is also the editor of Signal Integrity Journal.
I had an opportunity to interview Eric before he traveled to Europe to spread the Gospel of SI.
How do you define signal integrity?
Eric: Signal integrity are all those issues that arise because the interconnects are not transparent.
When did SI become recognized as an important design criterion?
Eric: In principle, interconnects have been an issue since the transatlantic cable failed in 1850 due to too much dispersion. This was an SI problem which Lord Kelvin solved using an equalizer circuit, for which he was given the Lordship by Queen Victoria. Bell Labs had been dealing with SI issues in long haul telephone cables since the early 1900s.
But the topic of SI in electronics was really an issue when the first IC-based computer systems were introduced in the early 1980s. There were the bleeding-edge groups doing really high- speed designs, like IBM, DEC, Sun Microsystems, and Cray, who were pioneers in this field as early as 1980. It became a thing in the industry in the early 1990s when HP began their High-Speed Digital Symposium. This put a focus on SI and the term became more widely known in the industry.
At what data rates does SI analysis become essential?
Eric: Of course, it is the rise time, not the data rate that influences SI problems. But, to put a number on it, the interconnects and their design become an issue at roughly 50 MHz clock rates. If you do not pay attention to terminations and routing and using return planes, SI problems may cause a product to not work.
Is channel simulation part of the SI analysis?
Eric: Yes. It is an integral part of pre-layout analysis. This is used to create the design rules for the product. Then system level simulation is done post layout to verify the final design.
What are the objectives of SI analysis?
Eric: Analysis will not design your product for you. What it will do is catch problems before they get too far in the design cycle. The last place you want to find a problem is when your customer turns your product on and it does not work. Simulation and analysis are risk reduction.
How is SI measured? Eye diagrams, S-parameters, BER, Smith Charts, etc.
Eric: Yes, all of the above. System performance of a channel is measured by the bit error ratio. An eye diagram is a rough indication of the quality of the received signal. S-parameters, scope traces, and component characterization can be used to help debug your system and validate a design.
What are the factors that impact SI in a high-speed channel?
Eric: There are generally six families of problems in high-speed channels:
- Reflection noise from impedance discontinuities
- Rise time degradation from losses
- Crosstalk from channel to channel
- Mode conversion
- Power delivery network (PDN) noise
- Radiated emissions.
Each of these problems have more than one root cause. Multiple solutions are available to reduce the problems.
What tools or equipment are used to quantify SI? Are they expensive?
Eric: As the bandwidth of the signals goes up, the cost of the instruments goes up, more than linearly. Many field programmable gate arrays (FPGAs) and application-specific integrated circuits (ASICs) are starting to incorporate on-die measurement IP that will aid in recording received eye diagrams after the CDR and slicer. This makes debugging easier to do without high-performance scopes.
Has SI analysis become integrated into the design of all high-speed channels?
Eric: There are always some companies which just follow the guidelines in reference platforms, but these companies are making commodity systems. The real differentiators are when custom features are added, which require SI analysis to get it right the first time.
It only takes a major product slipping a schedule because of an SI problem found too late for a company to realize that $100k invested in a simulation tool would have saved $1M of profits lost due to missing a market window.
Are there specialists that perform SI analysis?
Eric: There are a few companies that will do SI analysis as a service, and some consultants, but the companies which really want to rise above the rest will have their own team of SI experts. This should be a core competency in all high-end OEMs and suppliers to the high-speed industry.
Do EE students typically take a course in SI?
Eric: Unfortunately, SI is not taught at many schools. Few faculty are knowledgeable, and it is not seen as a glamorous field. RF and wireless, and terahertz optical interconnects are what researchers are attracted to, not teaching SI principles. A few schools, like CU Boulder, offer a professional master’s program in SI.
At what stage in the design process is SI analysis performed?
Eric: As early as possible. The earlier in the design cycle SI issues can be identified and eliminated, the lower the risk and the lower the development costs.
How does SI analysis compare with Channel Operating Margin (COM)? Is it complementary?
Eric: COM is a performance metric that gives some level of confidence that the system composed of the interconnects and the Tx/Rx (transmitter/receiver) technology will meet the compliance spec. If the channel meets an acceptable COM level, like 3 dB, then there is confidence the channel may work. You still run the risk of the system failing for some other reason.
If the COM metric is low, like 2 dB, it tells you nothing about the possible root cause. This is where SI analysis comes in — to do a more thorough evaluation if the system will meet the performance spec and then to find the root cause of problems so they can be fixed.
Which industry standards define SI?
Eric: All the high-speed protocols have specs that define performance in terms of signal integrity concepts. Many specs are defined in terms of interconnect compliance to S-parameter masks.
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