As line rates and channel attenuation increase, the receiver equalizers are more often enabled to overcome channel attenuation. This poses a challenge to system bring-up because the quality of the link cannot be determined by measuring the far-end eye opening at the receiver pins. At high line rates, the received eye measurement on the printed circuit board can appear to be completely closed even though the internal eye after the receiver equalizer is open.
Because the GTM receiver is ADC-based, the conventional eye scan used in the previous family of transceivers (such as GTH and GTY transceivers) is not possible. The GTM RX provides a non-destructive sampled eye diagram that can be used to measure and visualize the receiver signal margin after the equalizer, as shown in the following figure.
The sampled eye diagram is constructed by plotting one sample per symbol located at the CDR sampling point (after the equalizer). By plotting multiple samples, the image looks like the figure above. Sampled eye supports both NRZ and PAM4 modulation, with the only difference being that the NRZ eye consists of two amplitude margin levels, and PAM4 with four distinct amplitude margin levels. The Versal ACAP Transceivers Wizard allows access to the sampled eye data. See Versal ACAP Transceivers Wizard LogiCORE IP Product Guide (PG331).