Frequency

Eyes wide open for signal integrity

31st August 2023
Kristian McCann
0

Eye diagrams give a quick overview of data signal jitter and noise while also revealing the robustness of data transmissions. Numerous fast bus interface standards, such as USB and PCI Express (PCIe), now define eye masks for signal integrity testing, clearly illustrating their importance. 

This article originally appeared in the July'23 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

Reliable analysis requires superimposing large numbers of data bits to form an eye diagram. However, a bit-based time reference is needed to extract the individual bits from a data stream. The time reference can be in the form of a second signal (clock or data strobe) as seen in parallel DDR interfaces. Serial signals have the clock signal embedded in the data stream. The clock signal must therefore first be recovered using clock data recovery (CDR). Then the oscilloscope can use it as a reference for displaying the eye diagram. This step usually involves software based post processing of the data signal and demands a certain amount of computation time. The final eye diagram reveals signal quality by the height and width of the eye opening, time and amplitude errors in the form of jitter and noise, or predistortion effects.

Hardware clock data recovery

Hardware clock data recovery (HCDR) developed by Rohde & Schwarz is now available for the first time in the R&SRTP high-performance oscilloscopes. Direct integration into the trigger hardware eliminates the need for software computation and saves lots of time. This is an entirely new approach to eye diagram analysis for serial signals. Users can choose any input channel as HW CDR source for data rates up to 16 Gbit/s and can set a tracking bandwidth between 1/500 and 1/3000 of the nominal bit rate. HW CDR runs continuously once it successfully locks in on the DUT signal. Timestamps are stored in acquisition memory in parallel with the waveforms and are available for mathematical analysis and eye diagram calculations.

Real-time eye diagrams

Triggering with HW CDR enables eye diagram analysis in near real time. The oscilloscope simply acquires short signal sequences and displays them immediately in the diagram. When the persistence is set to infinite, the eye diagram will form quickly. It can reach a maximum rate here of over 400 000 unit intervals (nominal bit width) per second. This means sporadic errors, such as cross-talk between adjacent components, are detected quickly and reliably. Mask tests and histograms enable further analysis without significantly slowing the acquisition rate. This also includes long-term observations, where mask test errors can stop measurements to track down error sources. The high HW CDR acquisition rate ensures that fast and sporadic signals are not missed, even with long observation times.

Standard-specific eye diagram analysis

RTP HW CDR also enables eye diagram analysis for long contiguous bitstreams, as required, for example, in compliance with the test specifications for the USB or PCIe standard. Along with the actual waveforms, it stores HW CDR edge times and makes them available for fast eye diagram calculation. The eye diagram function also makes other settings for computation and visualisation possible. For example, users can display an eye diagram for only those bits preceded by a bit change or by a user-defined bit sequence.

From quick start to detailed analysis

Users can carry out further detailed analyses once the eye diagram is displayed. Along with simple cursor measurements, the oscilloscope has a broad range of automated eye diagram measurements. Eye mask testing is another important function. Signals passing through a mask are registered as violations. The oscilloscope provides predefined masks for many standards. But users can also quickly create other masks, either directly on the screen or using specific settings in the mask dialogue. The eye stripe function marks a violation of the eye mask as a red stripe at the corresponding time point in the original waveform. This makes it easier to identify the causes of mask violations.

Figure 1. The eye stripe function makes debugging much easier. Mask violations are marked in red in the eye diagram and in the time signal of the waveform (blue arrows). All violations are listed in the results table and the currently displayed signal section is marked in blue.

When zoom coupling is enabled, the waveform zoom function focuses on the violation selected in the current result table. The bit transitions at the left and right of the eye diagram show signal jitter while the signal amplitudes in the centre of the eye diagram show signal noise. The histogram function makes it quick and easy to see the distribution of jitter and noise.

Users can also break down the individual components even further using the R&SRTP-K134 jitter and noise decomposition option.

Summary

The R&SRTP high-performance oscilloscope offers a broad range of comprehensive features for analysing the signal integrity of electrical signals. Integrated HW CDR triggering is available for the first time thanks to the R&SRTP-K137 advanced eye analysis option, which supports data rates up to 16 Gbit/s. This allows users to record quickly short bit sequences for fast eye diagrams and to analyse long bit sequences in compliance with test standards.

Figure 2. The RTP164B oscilloscope

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