Column: JESD204 standard
sampling point is to the centre of the eye, the lower the BER. As the sampling point moves closer to the transition points of the eye diagram, the BER increases. The distance between the two slopes of the bathtub plot at a given BER gives the eye opening at the specified BER (in this case 10-12
).
The bathtub plot also provides information on the total jitter (Tj
Figure 5: Bathtub plot with jitter components ) Figure 6: 5.0Gbps bathtub plot
components present in the signal. As Figure 5 shows, when the measurement point is at or near the transition points, the plot is relatively flat and the main jitter component is deterministic jitter. As with the eye-diagram measurements, the bathtub plots are from a JESD204B 5.0Gbps transmitter measured at the receiver after passing through a connecter and approximately 20cm of transmission line. As the measurement point moves closer to the middle of the eye opening, the primary jitter mechanism is random jitter – a result of a large number of processes that are typically small in magnitude, such as thermal noise, trace width variations, shot noise, and so on. The probability density function (PDF) of random jitter usually follows a Gaussian distribution. On the other hand, deterministic jitter results from a small number of processes that have large magnitudes and may not be independent. The PDF of deterministic jitter is bounded and has a well-defined peak-to-peak value. It can have varying shapes and is typically not Gaussian. An expanded view of the bathtub plot
Figure 7: 5.0Gbps bathtub plot with improper termination
discussed in Figure 4 is shown in Figure 6. This represents an eye opening of approximately 0.6UI at the receiver for a 5.0Gbps serial data transmission with a BER of 10-12
. It is important to note that Figure 8: 5.0Gbps bathtub plot with impedance discontinuity
the bathtub plot, such as the one in Figure 6, is made of extrapolated measurements. Te oscilloscope used to capture the data takes a set of measurements and extrapolates it into the bathtub plot. If one were to use a bit error rate tester (BERT) and acquire enough measurements to build the bathtub plot, it could take hours or even days, even with the high speeds afforded by today’s equipment.
18 July/August 2021
www.electronicsworld.co.uk
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