Column: JESD204 standard
Just as in the eye diagram, an improper
termination or impedance discontinuity can be seen in the bathtub plot. In contrast to the plot in Figure 6, the bathtub plots in Figures 7 and 8 exhibit much shallower slopes on each side. The eye opening for a BER of 10-12
is
only 0.5UI in both cases, more than 10% less than the 0.6UI for the good condition. The improper termination and impedance discontinuity contribute a lot of random jitter to the system. This is shown by the decreasing slope on each side of the bathtub plot, along with the decreased eye opening at a BER of 10-12
. There is also a small increase
in the deterministic jitter, shown by the decreasing slope near the edges of the bathtub plot.
The histogram plot The histogram plot shows the distribution of the measured periods between transition points in the data transmission. Figure 9 shows a histogram for a
relatively good performing system at 5.0Gbps. It shows a mostly Gaussian distribution with periods between 185ps and 210ps. The expected period for a 5.0Gbps signal should be 200ps, which means the distribution is spread about -7.5% to +5% around its expected value. When an improper termination is
introduced (Figure 10), the distribution becomes wider – between 170ps and 220ps, increasing the percentage of variation from -15% to +10%, double that of Figure 9. These plots show that mostly random jitter is present in the signal, since they have a mostly Gaussian-like shape. However, the shape is not exactly Gaussian, which indicates there is also at least a small amount of deterministic jitter. The histogram in Figure 11 shows the
results of an impedance discontinuity along the transmission line. The shape of the distribution is not Gaussian-like at all, and has developed a small secondary hump. The mean value of the measured period is also skewed. Unlike the plots in Figures 9 and 10, the mean is no longer 200ps, but shifted to about 204ps. The
Figure 9: 5.0 Gbps histogram plot
Figure 10: 5.0 Gbps histogram plot with improper termination
Figure 11: 5.0Gbps histogram plot with impedance discontinuity
more bimodal distribution indicates that there is more deterministic jitter in the system, due to the impedance discontinuity present on the transmission lines. The range of values measured for the
period is again increased, although not as much as with improper termination. In this case, the range is from 175ps to 215ps, or approximately -12.5% to +7.5% of the expected period. The range isn’t as large, but again, the distribution is more bimodal in nature.
A properly working system It’s important to maintain good design practices to properly terminate a system and avoid impedance discontinuities in the transmission media. These have appreciable negative effects on data transmission and can result in a faulty data link between the JESD204B transmitter and receiver. Employing techniques like the ones discussed here to avoid these issues will help ensure a properly working system.
www.electronicsworld.co.uk July/August 2021 19
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