Standardization for next generation data communications links operating at 400 Gb/s continues to make steady progress. With the basic architectures defined, the task of setting link budgets and subsequent transmitter and receiver specifications is also moving forward. Making the leap from 100 Gb/s to 400 Gb/s is not trivial, as technology is pushing against physical limits, while at the same time the need to create cost effective solutions constrains the solutions designers can choose from. Several of the optical systems being designed have adopted advanced modulation schemes in the form of PAM-4, with 4 signal levels, leaving behind the traditional “on-off” non-return-to-zero (NRZ) schemes used in 1, 10 and 100 Gb/s solutions. Use of PAM-4 signaling leads to some substantial and important changes in how optical transmitters and receivers are tested.
IEEE 802.3bs is a draft Ethernet standard that defines 400 Gb/s links over optical channels with spans of 100, 500, 2000 and 10000 meters. The 2000 and 10000 meter links will each use one single-mode fiber, with 8 transmitters, each at a unique wavelength. Transmitters operate at 26 GBaud (switching at a 26 GHz rate), and will use PAM-4 modulation, with each of the possible four amplitude ‘symbol’ levels representing 2 bits. The 500 meter link uses four single-mode fibers, with 4 transmitters transmitting at 53 GBaud, also using PAM-4 modulation for an aggregate 400 Gb/s rate.
TDECQ (transmitter dispersion and eye closure quaternary) has been developed for PAM-4 transmitters as a metric that is similar to traditional transmitter dispersion penalty (TDP) measurements. It is intended to determine the additional power that is required for the transmitter being tested to achieve the symbol error ratio (SER) that is achieved when using an ideal reference transmitter. Two important differences exist between TDP and TDECQ. Rather than use a physical reference transmitter, a virtual reference transmitter is mathematically created based on the measured OMA of the transmitter under test. Also, equalization will be employed to overcome the significant impairments of the overall system. In addition to a virtual reference transmitter, a virtual equalizer is used in the TDECQ test to emulate operation of the actual communications system.
Keysight provides TDECQ solutions that are fast, accurate, and easy to use meeting both R&D and manufacturing test needs. Both the 86100 DCA-X and 1092 DCA-M have TDECQ analysis capability at 26 and 53 GBaud rates.
See this Keysight solution live at ECOC in Gothenburg.