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 for the constantly increasing bandwidth demand. Four-level pulse amplitude modulation (PAM-4) signaling is a leading contender for implementing the 56G lane data rate which will enable 400G links for 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 extremely signal loss at high frequencies. The alternative signaling technique, PAM-4, is used to transmit at 28Gbaud, but with 4 amplitude levels (where each symbol represents 2 bits), effectively delivering 56Gbps throughput.
Conventional impairments such as jitter, noise, channel loss, and inter-symbol interference (ISI) have more complicated expression with PAM-4. 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 received)
- 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.
Keysight’s Advanced Design System (ADS) offers a design space with a channel simulator, accommodating not only lumped-element models but also the distributed transmission line, S-parameter, and EM models that are essential to model high-speed PCB traces and determine ultralow #BER contours in seconds not days.
The ADS #channel simulation enables a comparison of PAM-4 versus NRZ technology. This example demonstrates the trade-off of price vs performance in #PCB design. Cheaper PCB materials with more loss and no-back-drilling 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, if the resonances are higher in frequency than the main spectral content of the #PAM-4 signal.
The system designer attempting to compare NRZ to PAM-4 trade-offs needs to use PAM-4 IBIS-AMI models from SerDes vendors within their channel simulation. 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
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Where to find more information:
- Download application note: PAM-4 Design Challenges and the Implications on Test
- Contact us for PAM-4 Simulation Models for TX/RX and a FREE trial license for ADS
- Get to know our 400G Design and Test Solutions
For more information on PAM-4 solutions from Keysight Technologies, refer to http://www.keysight.com/find/pam4