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JUNE 2023 I DESIGN007 MAGAZINE 45 e increased data rates also require more complex modulation schemes for data encod- ing, such as non-return-to-zero (NRZ) to pulse amplitude modulation 4-level (PAM4) encoding. ese schemes use multiple levels or phases of signals to encode more bits per symbol, which increases the spectral efficiency and data rate of the channel. However, these schemes also increase the complexity and sen- sitivity of the signal processing and recovery circuits, which require careful PCB design to ensure proper signal quality and synchroniza- tion. NRZ is a modulation technique that has two voltage levels to represent logic 0 and logic 1. While PAM4 uses four voltage levels to represent four combinations of 2-bit logic: 11, 10, 01, and 00 (Figure 1). Impedance is the key factor that controls the stability of a design. It is the core issue of the signal integrity methodology. e imped- ance should be simulated by a field solver to obtain accurate values of impedance for each signal layer of the substrate. e impedance of the trace is extremely important, as any mis- match along the transmission path will result in a reduction in the quality of the signal and possible radiation of noise. For perfect transfer of energy, the impedance at the source must equal the impedance at the load. However, this is not naturally the case and terminations are generally required at fast edge rates to limit ringing. If this noise is not constrained at the source, then it will be cou- pled into nearby victim traces (crosstalk) and Figure 1: NRZ eye vs. PAM4 eye. (Source: Xilinx)