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Low Cost Eye Diagram Generation

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Weston:
I was recently watching a youtube video ( youtube.com/watch?v=u6lx1ntNoxE ) about a cable tester for HDMI / USB cables and I noticed in the teardown portion of the video that the front end looked really simple for something that can take eye diagrams up to 12.5 Gbps . The product documentation shows pretty decent eye diagram photos too: https://www.totalphase.com/products/advanced-cable-tester-v2/



I was pretty curious about how this works, especially given the manufacture bothered to sand the part number off the IC.

I thought about how it might work for a while and started digging through datasheets, and I am pretty sure I have found how it works: They are using the built in eye capture feature for a high speed serial re-timer IC. The IC also generates the test patterns, so no other high speed interfaces are required!

Based on the pinout and specifications, I am pretty sure they are using this quad channel retimer from TI: https://www.ti.com/product/DS125DF410

The chip is ~$30 in qty 1 on digikey and the eye capture works to the full data rate of 12.5Gbps. The eye capture mode provides the bit error rate for 64 offset values of time and for 64 bit offset values of voltage, which is a good, but not great eye diagram.

I thought this was pretty cool and worth sharing that you can capture eye diagrams that would otherwise require an absurdly expensive sampling scope with a rather low cost IC. In this application they are just testing a cable and controlling the TX and RX signals, but it seems you could use this IC to generate eye diagrams from arbitrary data sources. I wonder if you could even add a pre-amp to use it to probe data busses.

The biggest limitation is that the chip needs to be able to lock to the signal and recover the clock to generate the eye diagram and there is no provision for an external reference clock for the TX signal. I do wonder if there are other similar chips that have that feature though, the hardware for a low cost TDR or sampling scope is basically all there!

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