During the 1940s when aerospace engineers were attempting to break the sound barrier for the first time, they were confronting a slew of new technical issues that had never been dealt with before, and in some cases never seen before. In subsonic flight airflow was predictable and well understood. In crossing the sound barrier, they were confronted with new physical effects and issues that had to be resolved. Today there is a similar challenge facing designers of chips and systems that communicate at data rates of 56G and 112G. Circuits operating at Millimeter Wavelength (mmWave) frequencies are not unlike aircraft flying at supersonic speeds. New behaviors including reduced margins, increased noise and electromagnetic effects can dominate the performance of these circuits.
Designing SerDes for 56G and 112G is complex and challenging. In fact, there are only a few companies with working silicon. eSilicon is one of them and made an announcement at DesignCon this year that highlights another huge challenge for these designs – how to test them and measure their performance. What is necessary to test these new high speed circuits is a collaboration of companies that can all contribute essential pieces of the puzzle. eSilicon worked with Wild River Technology to design and build a test board that allows de-embedding up to and beyond 70GHz. Also involved were Keysight’s Advanced Design System and Samtec’s Bulls-Eye Test Point System.
eSilicon’s 56G PAM4 & NRZ DSP based 7nm SerDes was used to drive the channels in the test system. Without proper de-embedding the actual circuit operation cannot be accurately characterized. PAM4 introduces multi-level signaling, which decreases the noise margin, making characterization and validation of the SerDes more difficult. For eSilicon this marks an important milestone along the way to having a fully qualified, production ready 56G SerDes.
Central to this project is the upcoming IEEE P370 standard which addresses the quality of measured S-parameters for PCBs and related interconnect. Wild River has expertise in this area. Their founder and chief technologist, Al Neves, has contributed a great deal of time and effort to this developing standard. Within the standard there are three groups: test fixture, de-embedding, and S-parameter quality.
Prior to the announcement I had a chance to talk to Al Neves. He said they did a number of things to ensure their success. One of them was to create three teams to work on the problem to validate the results. He took inspiration from the Apollo program, where independent teams were given the assignment of calculating the launch trajectories. If they all agreed then the likelihood of success improved, as witnessed by their overall operational record.
Another important factor that Al identified was their tool selection. Projects like this live or die by their analysis tools. They used both ANSYS HFSS and Simeor THz software, among others. Al said they push their tools pretty hard and often have to consult with the vendors to get fixes and confirm portions of the methodology.
eSilicon now plans to move to the next step with their 56G SerDes by building a test socket suited for 70GHz. While no test pilots’ lives are at stake, the risks are not insignificant. This stuff is hard, but some companies have the right stuff. The eSilicon website has more information on this announcement and other aspects of their 56G/112G SerDes project.