By Jonathan Harris, Applications Engineer, Analog Devices
With the increased adoption of the JESD204 interface in data converters, it has become necessary to devote more attention to the performance and optimisation of the digital interface; the focus must not be only on the data converter performance. The first two versions of the standard, JESD204 in 2006 and JESD204A in 2008, specified data rates of 3.125Gbps. The latest revision, JESD204B released in 2011, lists three speed grades with the maximum data rate of 12.5Gbps, governed by three different electrical interface specifications formulated by the Optical Internetworking Forum (OIF). For data rates to 3.125Gbps, OIF-Sx5-01.0 specifies the electrical interface, whereas CEI-6G-SR and CEI-11G-SR for data rates to 6.375Gbps and 12.5Gbps, respectively. These high-speed data rates require more attention be given to the design and performance of the high-speed CML drivers, receivers and interconnect network that make up the physical interface (PHY) of the JESD204B interface.
To evaluate the performance of the PHY for a JESD204B transmitter, there are several performance metrics that are evaluated. These include common-mode voltage, differential peak-to-peak voltage, differential impedance, differential output return loss, common-mode return loss, transmitter short-circuit current, eye diagram mask, and jitter. This article will cover the three key performance metrics typically used to evaluate the quality of the transmitted signal: the eye diagram, the bathtub plot and the histogram plot. The measurements are made from the receiver perspective since this is where the signal must be properly decoded.
The eye diagram overlays multiple acquisitions of the output data transitions to create a plot that gives indications of the link quality, and used to observe JESD204B PHY characteristics, such as impedance discontinuities and improper terminations.
The bathtub plot gives a visual representation of the bit error rate (BER) for a given eye width opening, measured in terms of the unit interval (UI), which is the specified time given in the PHY specifications for JESD204B for the length of time between data transitions.
The histogram plot gives the distribution of the measured UI variation. The measurement is also an indication of the amount of jitter present in the measured signal. Along with the eye diagram and bathtub plot, this measurement can be used to gauge the overall performance of the physical layer of the JESD204B interface.
The eye diagram
A JESD204B transmitter with an output data rate of 5.0Gbps is presented here, and its performance detailed by the OIF CEI-6G-SR specification; see its eye diagram in Figure 1.
The ideal waveform is overlaid on a measured waveform. Ideally, the transitions would be almost instantaneous with no over- or undershoots and with no ringing. In addition, the cross points that determine the UI should be without jitter.
As seen in Figure 1, in a real system, ideal waveforms are not possible to achieve due to non-ideal transmission media, and loss and terminations that can’t be 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 transmitter and receiver, and a good transmission media with no large impedance discontinuities. It does show an amount of jitter, but not in excess of the specifications for the JESD204 interface.
The eye diagram does not show any overshoot but a slight undershoot on the rising edges due to the slowing 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 having 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 presented in Figure 2 is measured with the same transmission media 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 receiver’s BER, with potentially resulting in the loss of the JESD204B link at the receiver if the eye closes beyond what the receiver can tolerate.
Figure 1: 5.0Gbps eye diagram
Figure 2: 5.0Gbps eye diagram, improper termination
Figure 3: 5.0Gbps eye diagram, impedance discontinuity
The eye diagram presented in Figure 3 presents another case of a nonideal 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). 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 as a function of the sampling point as it moves across the eye diagram in time. The plot 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 sampling point is to the center 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).
Figure 4: 5.0Gbps eye diagram, bathtub plot measurement
Figure 5: Bathtub plot, jitter components
The bathtub plot also provides information on the total 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 5.0Gbps 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 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.
An expanded view of the bathtub plot discussed in Figure 4 is given 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 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 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-speed operation of today’s measurement equipment.
Just as shown 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. The eye opening for a BER of 10-12 is only 0.5UI in both cases which is more than 10% less than the 0.6UI 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, evidenced by the decreasing slope near the edges of the bathtub plot.
Figure 6: 5.0Gbps bathtub plot
Figure 7: 5.0Gbps bathtub plot, improper termination
Figure 8: 5.0Gbps bathtub plot, impedance discontinuity
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. The histogram shows a mostly Gaussian type distribution with periods measured 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 in nature, which indicates there is at least a small amount of deterministic jitter also.
Figure 9: 5.0 Gbps histogram plot
Figure 10: 5.0 Gbps histogram plot, improper termination
Figure 11: 5.0Gbps histogram plot, impedance discontinuity
The histogram in Figure 11 indicates the results of having 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 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 to avoid impedance discontinuities in the transmission media. These have appreciable negative effects on the 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 to ensure a properly working system.