Column: JESD204 standard


Physical layer performance metrics for a JESD204B transmitter


By Jonathan Harris, Applications Engineer, Analog Devices


W


ith 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 can’t be solely 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 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.


PHY performance To evaluate the performance of the PHY for a JESD204B transmitter, several performance metrics 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 is used to observe


16 July/August 2021 www.electronicsworld.co.uk


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 opening width, 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. It 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 under- shoots and with no ringing. In addition, the cross points that determine the UI should be without jitter. As seen in Figure 1, ideal waveforms


are not possible to achieve in a real system 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 reaching the measurement point. This eye diagram indicates a reasonable impedance match between transmitter and receiver, and good transmission media with no large impedance discontinuities. It does show some jitter, but not in excess of the JESD204 interface specifications. The eye diagram doesn’t show any


overshoot, but a slight undershoot on the rising edges due to the slowing


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