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PAM-4 Simulation and Design of Next Generation High-Speed Digital Links

As people use more applications on their phones, tablets, computers, and Internet of Things (IoT) devices, the network needed to deliver the data is constantly being upgraded. Four-level Pulse Amplitude Modulation (PAM-4) signaling is a leading contender for implementing the 56G lane data rate which will enable 400G links and fuel the next upgrade in network bandwidth.

PAM-4 is gaining traction for high-speed SerDes links over an electrical backplane, especially for designs attempting to deliver greater than 56Gbps throughput. Doubling data rates with traditional non-return to zero (NRZ) signaling is technically challenging due to the extraordinary signal loss at high frequencies. The alternative signaling technique, PAM-4, is to transmit at 28Gbaud, but with 4 amplitude levels (where each level represents 2 bits), effectively delivering 56Gbps throughput.

 

Advanced Design System (ADS) Channel Simulation
PAM-4 Simulated Eye Diagrams

 

System Design Challenges

Conventional impairments such as jitter, noise, channel loss, and inter-symbol interference (ISI) have an unexpected behavior in a PAM-4 context. In addition to this, receiver architectures for PAM-4 introduce new concepts for system designers such as: 

  • 3 Slicer outputs with time-varying voltage thresholds (for deciding which amplitude level has been recieved)
  • Individual Slicer Timing Skew (each Slicer's decision point can be offset in time from the other two)
  • Multi-tap Decision Feedback Equalization (DFE)
  • Clock and Data Recovery

The complex interaction of these new concepts influences specific design trade-offs for PAM-4.

How to send data at 56Gbps

The ADS channel simulation enables a comparison of PAM-4 versus NRZ technology.
This example demonstrates the concept of price vs performance in PCB design. Cheaper
PCB materials with more loss and no-backdrilling of vias are more likely to exhibit
resonances at higher frequencies. This channel may not support NRZ to 56Gbps, but
will support PAM-4 more easily, assuming that the resonances are higher in frequency
than the main spectral content of the PAM-4 signal.

PAM4 Eye Diagrams

The Channel Simulation Solution

For the system designer attempting to compare NRZ to PAM-4 trade-offs, an immediate need exists to use PAM-4 IBIS-AMI models from SerDes vendors within their Channel Simulation. Through Keysight Technologies' continuing leadership in the IBIS Open Forum, Keysight EEsof EDA now offers support for the new IBIS v6.1 specification. Developed in collaboration with the industry's leading PAM-4 SerDes IC vendors, the ADS Channel Simulator provides a trusted bit-by-bit simulation engine for PAM-4.

Where to find more information:

For more information on PAM-4 solutions from Keysight Technologies, refer to https://www.keysight.com/find/pam4