  In this paper, we discuss a new technique for measuring transceiver noise,
skew, and for the detection of uncompensated transceiver impairments. The
method consists on the introduction of frequency domain notch or notches at the
transmitter, which allows to estimate of the Signal-to-Noise and Distortion
Ratio (SNDR) at different stages over the transmission chain. The proposed
technique requires the detection of the signal spectrum using either: a modem
receiver, an Optical Spectrum Analyzer (OSA), an oscilloscope, or an Electrical
Spectrum Analyzer (ESA), depending on the stage where the spectrum is measured
and requiring minimal processing of the received signal.
  Our analysis is focused on Ciena's WaveLogic Ai commercial transceiver and a
95 Gbaud DAC. We demonstrate that symmetrically placed frequency domain notches
can be used to mitiguate the influence of crosstalk on the SNDR estimates.
  Finally, we show how a single notch spectrum can be used to detect and
compensate for impairments, and we perform skew estimation as an example.
