The Eye Diagram
Figure 1 shows an eye diagram for a JESD204B transmitter with a 5.0 Gbps data rate. The ideal waveform is overlaid on a measured waveform. Ideally the transitions would be almost instantaneous with no overshoot or undershoot and without any ringing. In addition, the cross points, which determine the UI, would be without jitter.
Figure 1. 5.0 Gbps Eye Diagram
As can be seen from Figure 1, in a real system ideal waveforms are not possible to achieve due to non-ideal transmission media which has loss and terminations that are not matched exactly. The eye diagram shown is a measurement made at the receiver in a JESD204B system. The signal has passed through a connector and approximately 20cm of differential transmission lines before making it to the measurement point. This eye diagram indicates a reasonable impedance match between the transmitter and receiver and a good transmission media with no large impedance discontinuities. It does exhibit an amount of jitter, but not in excess of the specifications for the JESD204 interface. The eye diagram does not exhibit any overshoot, but does have a slight amount of undershoot on the rising edges due to the slowing of the signal as it passes through the transmission media. This is to be expected however after passing through the connector and the 20cm of differential transmission lines. The mean UI looks to match the expected UI of approximately 200ps with the signal having small amount of jitter. Overall, this eye diagram presents a good signal to the receiver which should have no trouble recovering the embedded data clock and properly decoding the data.
The eye diagram presented in Figure 2 is measured in with the same transmission media as used in the measurement for Figure 1 with the exception that the termination impedance is incorrect. The effects can be seen in the increased amount of jitter present in the signal at the crossing points as well as in the non-transition areas. The overall amplitude is compressed in many of the data acquisitions resulting in an eye diagram that is beginning to close. The degradation will cause an increase in the BER at the receiver and could possibly result in the loss of the JESD204B link at the receiver if the eye closes beyond what the receiver can tolerate.
Figure 2. 5.0 Gbps Eye Diagram – Improper Termination
The eye diagram presented in Figure 3 presents another case of a non-ideal transmission of data. In this case an impedance discontinuity is presented at a point midway between the transmitter and the receiver (in this case an oscilloscope).
Figure 3. 5.0 Gbps Eye Diagram – Impedance Discontinuity
As can be seen by the degraded performance in the plot, the eye opening is closing meaning that the area inside the transition points is getting smaller. The rising edges and falling edges of the data severely degraded due to the reflections of the impedance discontinuity on the transmission line. The impedance discontinuity also contributes to an increase in the amount of jitter seen at the data transition points. Once the eye closes beyond the limits of the receiver's capability to decode the data stream, the data link will be lost. In the case of Figure 3, it is likely that many receivers would be unable to decode the data stream.