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Digital Revolution: PAM4 Wafer Test
As data rates continue to increase, the difficulty of increasing clock rates for improved bitstreams becomes more challenging with ever-increasing loss. As of today, Non-Return-to-Zero (NRZ) is the standard for digital encoding, but this begins to run into problems with loss and the collapse of the digital eye beyond 40 Gbps. To counter this, Pulse Amplitude Modulation 4-Level (PAM4) is beginning to emerge in digital communications which allows for the same fundamental frequency to be used as NRZ while doubling the data rate by introducing two additional amplitude levels to the traditional 2-level encoding. This paper will address the implications of this change for wafer test and will explore the sensitivity of PAM4 to different types of loss along with how this loss can be countered in the probe card.
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