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The usual way of signaling through PCB interconnects is a two-level pulse, an encoding of 1s and 0s or bits, named NRZ (non-return-to-zero) or PAM-2 line code type. Increasing the data rate with the NRZ code type presents some obstacles. For a 28 Gbps NRZ signal, the bit time is about 35.7 ps with the main spectral lobe below 28 GHz. For a 56 Gbps NRZ signal, the bit time is about 17. 86 ps, with the main spectral lobe below 56 GHz.
One can feel the problem already: Getting PCB interconnect analysis and measurements up to 56 GHz and beyond is very challenging, to say the least. In addition, the expected attenuation (dielectric, conductor and roughness losses) would also be an obstacle for 56 GHz NRZ. To reduce the bandwidth of the signal, pulse amplitude modulation with four levels (PAM-4) is being used more frequently on production boards.
Instead of single bits, symbols 00, 01, 10, and 11 are coded by four levels of the pulse and the symbol time is twice as large as the bit time for NRZ signal with the same data rate; that is about 35.7 ps for 56 Gbps PAM-4—the same as for 28 Gbps NRZ! If we know how to design interconnects that correlate with the measurements for 28 Gbps NRZ, is it going to be a free lunch to move to 56 Gbps NRZ?
To read this entire article, which appeared in the August 2018 issue of Design007 Magazine, click here.
