Column: JESD204 standard
of the signal as it passes through the transmission media – which is to be expected. The mean UI looks to match the expected UI of approximately 200ps, with the signal showing a 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 in Figure 2 is
measured with the same transmission media as Figure 1, except that the termination impedance is incorrect. The effects can be seen in the increased jitter present 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 receiver’s BER, potentially resulting in the loss of the JESD204B link at the receiver if the eye closes beyond what the receiver can tolerate. The eye diagram in Figure 3 presents
another case of non-ideal transmission of data, in this case an impedance discontinuity at a point midway between transmitter and receiver (here for an oscilloscope). 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 data’s rising and falling edges are 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.
The bathtub plot The bathtub plot is a measurement of the BER. It is generated by moving the sampling point across the eye diagram and measuring the resultant BER at each point. As Figure 4 shows, the closer the
Figure 1: 5.0Gbps eye diagram
Figure 2: 5.0Gbps eye diagram with improper termination
Figure 3: 5.0Gbps eye diagram with with impedance discontinuity
Figure 4: Above, 5.0Gbps eye diagram, and (right) bathtub plot
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