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An Extended Comparison of 25 Gbps NRZ & PAM-4 Modulation Used in Legacy & Premium Backplane Channels
Keywords: 25 Gbps, Backplane, PAM-4
Within standards bodies such as the IEEE 802.3 Ethernet Working Group and the Optical Interconnect Forum (OIF), there is much debate on how to achieve 25 Gbps transmission per differential pair across high-density backplane channels of up to 1 meter. One possible method for achieving this is to construct high-quality backplane links utilizing low-loss dielectrics, smooth copper, and low-reflection, low-crosstalk connectors. These channels are required for 25 Gbps binary NRZ signals but are more expensive than legacy backplane systems built for 10 Gbps operation. An alternative method that has been suggested is to use PAM-4 signaling at 12.5 Gbaud to allow successful 25 Gbps signal transmission across cheaper legacy backplane systems. Before considering PAM-4, however, one must clearly outline where PAM-4 signaling at 12.5 Gbaud has performance advantages over NRZ signaling at 25 Gbaud. This paper will provide a detailed and reliable reference for engineers to use when considering when it is appropriate to use 25 Gbaud NRZ signaling and when it is appropriate to use 12.5 Gbaud PAM-4 signaling for successful high-density backplane operation. To complete a broad study of 25 Gbps NRZ vs. PAM-4 electrical performance, a range of backplane channel insertion loss and crosstalk levels will be included in the paper. Various insertion loss levels will be studied by including multiple lengths (1.0 m & 0.75 m), dielectrics (Improved FR4 & Megtron6), and connectors (Z-PACK TinMan & STRADA Whisper). Various crosstalk levels will be studied by including multiple connectors (Z-PACK TinMan & STRADA Whisper) under various pinout configurations. Proven time-domain simulation methods will be used to complete 25 Gbaud NRZ vs. 12.5 Gbaud PAM-4 comparisons. Package and chip parasitics will be inlcuded in the channel models, and simulations will be completed for a given driver rise time, jitter level, and equalization scheme.
Chad Morgan, Principal Engineer
TE Connectivity
Middletown, PA

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