It has been quite a while since our last PAM4 tutorial.
The 400G standards have matured since and on the electrical and optical side as expected pulse amplitude modulation (PAM4) is a predominant modulation format.
The transition from non-return-to-zero (NRZ) to PAM4 means many substantial changes. It comes alongside with new quality tests, test pattern and quality parameters.
In this part of the tutorial series we’ll introduce you to the differences of NRZ and PAM4 optical test parameters. New PAM4 measurements used in multiple IEEE 802.3bs/cd clauses include outer optical modulation amplitude and outer extinction ratio. For PAM4 the eye mask is no longer measured. A new key quality measure is being introduced: transmitter dispersion eye closure (TDECQ). Q stands for 'quatenary'.
While in NRZ the corresponding transmitter dispersion penalty (TDP) is determined by directly measuring the bit error rate (BER), for TDECQ the symbol error rate (SER) is measured indirectly.
This makes TDECQ cost-effective and relatively fast to measure in comparison to TDP.
See in detail how TDECQ is obtained to make statements about the performance of your transmitter and learn about the respective IEEE 802.3bs/cd standard requirements.