Martin Rowe -March 30, 2015
Last week at OFC 2015, the OIF (Optical Internetworking Forum) demonstrated two 50 Gbps transmissions using both PAM4 and NRZ formats. Over the past year, PAM4 has emerged as what appears to be the modulation format of choice for many systems, although NRZ will still have its place.
PAM4, the topic of the Jitter Panel at DesignCon 2015, looks to become the modulation format for LR (long reach) and MR (medium reach) optical links. In particular, PAM4 looks to take over from NRZ for electrical links that lead up to an optical module and across backplanes. For XSR (extra-short reach) applications, NRZ is likely to live on in applications where signal-to-noise ratio is important such as within an optical module or in memory buses. The two videos below show demonstrations from the OIF booth.
In the first video, Scott Sommers of Molex shows a 50 Gbps PAM4 signal traveling over a 0.54 m Molex backplane. The signal is generated by an (AWG) arbitrary waveform generator from Keysight Technologies. The demo uses the AWG because silicon to generate the PAM4 signal isn’t yet available.
Silicon that can generate a 56 Gbps data stream using NRZ is available, and Jeff Twombly of Credo Semiconductor demonstrated it in the OIF booth. In this demonstration, a Credo 56G NRZ SerDes drove three demonstrations: a CEI-56G-VSR-NRZ channel, a CEI-56G-MR/LR-NRZ backplane and a CEI-56G-MR-NRZ passive copper cable. The video shows the signal passing through a 1 m length of copper cable.