The Bathtub Plot
In addition to the eye diagram, the bathtub plot also provides useful insight into the quality of the serial data transmission on a JESD204B data link. The bathtub plot is a measurement of the BER (bit error rate) as a function of the sampling point as it moves across the eye diagram in time. The bathtub plot is generated by moving the sampling point across the eye diagram and measuring the resultant BER at each point. As Figure 4 illustrates, the closer the sampling point is to the center of the eye, the BER decreases. 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 (10-12
in this case).
Figure 4. 5.0 Gbps Eye Diagram – Bathtub Plot Measurement
The bathtup plot also provides information on the jitter (Tj
) components present in the signal. As Figure 5 shows, when the measurement point is at or near the transition points, it is relatively flat and the main jitter component is deterministic jitter. As with the eye diagram measurements, the bathtub plots are from measurements on a JESD204B 5.0 Gbps transmitter measured at the receiver after passing through a connecter and approximately 20 cm of transmission line. As the measurement point moves closer to the middle of the eye opening, the primary jitter mechanism is random jitter. Random jitter is the result of a large number of processes that are typically small in magnitude. Typical sources would include thermal noise, variations in trace width, shot noise, etc. The PDF (probability density function) of random jitter usually follows a Gaussian distribution. On the other hand, deterministic jitter results from a small number of processes that may 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.
Figure 5. Bathtub Plot – Jitter Components
An expanded view of the bathtub plot discussed in Figure 4 is given in Figure 6 below. This represents an eye opening of approximately 0.6 UI (unit interval) the receiver for a 5.0 Gbps serial data transmission with a BER of 10-12
. It is important to note that the bathtub plot such as the one in Figure 6 is an extrapolated measurement. The oscilloscope used to capture the data takes a set of measurements and extrapolates the bathtub plot. If one were to use a BERT (bit error rate tester) and acquire enough measurements to build the bathtub plot, it could take hours or even days, even with the high speed operation of today's measurement equipment.
Figure 6. 5.0 Gbps Bathtub Plot
Just as in the eye diagram, an improper termination or impedance discontinuity in the system can be seen in the bathtub plot. In contrast to the bathtub plot in Figure 6, the bathtub plots in Figure 7 and Figure 8 exhibit much shallower slopes on each side.
Figure 7. 5.0 Gbps Bathtub Plot – Improper Termination
Figure 8. 5.0 Gbps Bathtub Plot – Impedance Discontinuity
The eye opening for a BER of 10-12
is only 0.5 UI in both cases which is more than 10% less than the 0.6 UI for the good condition. The improper termination and impedance discontinuity contribute a large amount of random jitter to the system. This is evidenced 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 as well. This is evidenced again by the decreasing slope near the edges of the bathtub plot.