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TDECQ (Transmitter Dispersion and Eye Closure Quaternary) is the primary metric defining the performance of an optical transmitter used in a PAM4-based Ethernet link. It is found in IEEE 802.3bs (400 Gb/s optical), IEEE 802.3cd (50 and 100 Gb/s optical), Fiber Channel PI-7 (64GFC), and is being leveraged into other PAM4-based optical standards. TDECQ is intended to be the equivalent of transmitter dispersion penalty (TDP) metrics developed in earlier NRZ-based Ethernet standards. TDP has generally not been implemented in manufacturing test processes due to test time and complexity. Instead, results have been correlated to the simpler mask margin and hit ratio tests.
Two factors have led the adoption of TDECQ in manufacturing test for PAM4 transmitters instead of the continued use of eye mask testing. First, the transmitter test requires the use of an FFE (Feed Forward Equalizer) to open the eye. Second, TDECQ is relatively simple and fast to measure. Along with TDECQ, transmitters are tested with Outer Optical Modulation Amplitude (Outer OMA), Outer Extinction Ratio (Outer ER), and Transition Time. Fortunately, these measurements are made on the input data of the TDECQ equalizer and are orders of magnitude faster to compute than TDECQ.
This paper discusses the issues that can affect measurement time and throughput and outlines some best practices for automating the process. An overview of the TDECQ test process and measurement setup is presented below followed by detailed instructions for automating the measurement process. An analysis of test time and computer specifications are detailed, along with recommendations. Finally, improvements to test time are shown with the use of FlexEye sessions.
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