IEEE Standards for 400G optical Ethernet links have been in development for many months, and the specifications are finally stabilizing as of mid-2017. The result will be the first optical standards to employ PAM4 modulation. This requires a new set of measurements, with the TDECQ (Transmitter and Dispersion Eye Closure Quaternary) measurement getting the most attention. Increasingly more cutting edge companies are ready to evaluate their 400G products and need the TDECQ capability now.
TDECQ is a new and significantly easier method of calculating the penalty for transmitters that have unequal sub-eyes. This software calculation requires only an oscilloscope and are achieved by a direct measurement of the transmitter eye diagram.
It is simpler, faster and less expensive than older TDP (transmitter dispersion penalty) measurements, which would need a reference transmitter or an optical enabled BERT.
In its simplest definition, the TDECQ measurement creates two vertical histograms measured on an eye diagram like Figure 1 below. The histograms are centered at 0.45 and 0.55 unit intervals and each spans all modulation levels of the PAM4 eye diagram.
The amount of noise captured in the histogram is compared to an ideal receiver, and the dB difference in noise levels represents the power penalty for the transmitter under test.
One common mistake is to confuse the TDECQ result in dB with BER measurements.
We must keep in mind that TDECQ is a measurement of transmitter eye opening quality relative to an ideal transmitter and not a bit error rate.
Keysight engaged early in the development of a TDECQ solution with the IEEE Standards Association, sharing many of our hardware evaluations with the 400G committee. As a result, our TDECQ solution is easy to set up and creates fast measurements for use in both R&D and manufacturing environments. Our competition has also developed TDECQ capability, but their solution is separated from the scope and runs much slower.
The Keysight solution was designed for easy integration into a manufacturing system and can be quickly updated at the customer site if any changes are made to the IEEE 400G Standard.
Due to its low noise and fast sampling, the primary Keysight TDECQ solution is the low-cost N1092 DCA-M sampling scope module. It has predefined TDECQ reference receivers for both 26 GBaud and 53 GBaud transmitters. In addition, the Keysight 86105D-281 and 86116C-025 plug-in modules can also be used with the 86100 mainframes with the TDECQ option for the same result.
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