Daniel Bock, Ph.D. and David Raschko recently presented a paper at the SWTest 2021 Conference. Their presentation – The Digital Revolution: NRZ to PAM4 – won best overall presentation of the conference.

With increasing demand to process more data and pass large amounts of data through servers, cellular devices, and even within the computer for the highest performing video cards, the need for more complex digital processing is becoming greater than ever. This presentation shows some of the impacts of NRZ (Non-Return to Zero) communications mode compared to PAM4 (4-level Pulse-Amplitude Modulation) on wafer test through example probe cards and describes the changing test requirements.

The presentation covers all that is new with PAM4, including the timeline and why it’s needed. They review the current status quo for digital test – NRZ – and the sensitivity of NRZ to insertion and return loss. They address how digital test is best managed and share performance, including testerization (direct connection to tester channel), loopback test (signal from DUT TX to RX), and loopback tap (a combination of high speed on loopback, and low speed on tester path).

The requirements for PAM4 versus NRZ indicate that probe card specifications should be more stringent than that for NRZ probe cards. Loopbacks with a tap to the tester are possible, but require care in design and may have limitations. At the end of the day, our products are ready for the challenges being presented by the move to PAM4 thanks to custom solutions, including a next-generation MT Series Probe for high-speed digital test, available for evaluation by the end of 2021.

You can download the presentation for review here (PDF).