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PHY performance metrics for JESD204B transmitter

Posted: 12 Feb 2013 ?? ?Print Version ?Bookmark and Share

Keywords:JESD204? JESD204B? common mode voltage? differential peak-to-peak voltage? impedance?

With the increased adoption of the JESD204 interface in data converters, it has become imperative 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 revisions of the standard, JESD204 in 2006 and JESD204A in 2008, specified data rates of 3.125Gbit/s. The latest revision, JESD204B released in 2011, lists three speed grades with the maximum data rate of 12.5Gbit/s. These three speed grades are governed by three different electrical interface specifications formulated by the Optical Internetworking Forum (OIF). For data rates up to 3.125Gbit/s, OIF-Sx5-01.0, details the electrical interface specifications while the CEI-6G-SR and CEI-11G-SR detail the specifications for data rates up to 6.375Gbit/s and 12.5Gbit/s, respectively. The high speed data rates require more attention be given to the design and performance of the high speed CML drivers, receivers, and interconnect network which 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 focus on three key performance metrics that are typically used to evaluate the quality of the transmitted signal: the eye diagram, the bathtub plot, and the histogram plot. These measurements are made from the perspective of the receiver as that is where the signal must be properly decoded.

The eye diagram overlays multiple acquisitions of the output data transitions to create a plot that can give many indications of the link quality. This plot can be used to observe many characteristics of the JESD204B physical interface such as impedance discontinuities and improper terminations. This is just one way that the physical layer can be evaluated. The bathtub plot and the histogram plot are two other important performance metrics that are used to evaluate the quality of the JESD204B link. The bathtub plot gives a visual representation of the bit error rate (BER) for a given eye width opening given measured in terms of the unit interval (UI). The unit interval is the specified time given in the physical layer specifications for JESD204B that gives the amount of time between data transitions.

The third measurement is the histogram plot which gives the distribution of the measured UI variation. The measurement is also an indication of the amount of jitter present in the measured signal. This plot along with the eye diagram and the bathtub plot can be used to gauge the overall performance of the physical layer of the JESD204B interface. A JESD204B transmitter with an output data rate of 5.0Gbit/s is presented. The performance for a transmitter of this data rate is detailed by the OIF CEI-6G-SR specification.

The eye diagram
Figure 1 shows an eye diagram for a JESD204B transmitter with a 5.0Gbit/s data rate. The ideal waveform is overlaid on a measured waveform. Ideally the transitions would be almost instantaneous with no overshoot or undershoot and without any ringing. In addition, the cross points, which determine the UI, would be without jitter.

Figure 1: 5.0Gbit/s eye diagram.

As can be seen from figure 1, in a real system ideal waveforms are not possible to achieve due to non-ideal transmission media which has loss and terminations that are not 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 the transmitter and receiver and a good transmission media with no large impedance discontinuities. It does exhibit an amount of jitter, but not in excess of the specifications for the JESD204 interface. The eye diagram does not exhibit any overshoot, but does have a slight amount of undershoot on the rising edges due to the slowing of the signal as it passes through the transmission media. This is to be expected however after passing through the connector and the 20cm of differential transmission lines. The mean UI looks to match the expected UI of approximately 200ps with the signal having 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.

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