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Let's talk about Crosstalk !

battlecoder

I've been a member for a long time, but this is my first time posting.

I guess it's a bit too late for a presentation, but I'm a software engineer with a fascination for electronics; It's been my hobby for more than 20 years, and I keep learning new things almost every day.

I recently became interested in frequency domain analysis. I know it has countless applications in the Ham/radio world, but I was thinking about more general-purpose use cases; like identifying cross-talk or interference on a PCB design.

I know that's a huge topic and there's some well known general advice and practices to reduce this, like ground planes and decoupling capacitors, and filters... but I wanted to check how bad it could be, if absolutely no sane advice or practice was followed.

So I came up with this ugly PCB as my ultimate test-bed:

image

This is honestly not far from a PCB I would have designed when I was starting in electronics. I liked having nothing but the tracks on the board.

For a first test, I injected two signals from a signal generator on the left side of the PCB, both very stable and clean. One of them a 7 Mhz sine wave, and the other one a 10 Mhz square wave. I picked two relatively close frequencies to see how easy (or hard) was going to be telling them apart later.

Both traces start very far from each other on the left side of the PCB on big pads, and then they get closer together near the center, just to take on their own separate paths again as their approach the right side.

I'm monitoring the "output pad" on the right side for both waveforms on an oscilloscope, and while I was initially watching both on the screen of the scope, the one that I think looked more interesting was the output of the "sine wave" trace:

image

That's one messed up sine wave. For the record; without the square wave signal connected (or with the signal turned off), that wobbly sine-wave becomes a very clean, proper sinusoidal wave, so this was clearly the influence of the other signal.

I'm sure it's possible to use the FFT on the scope to dissect the frequency content of that signal,but I wanted to explore none-invasive methods. Something that I could use for a quick inspection and that didn't require probing around test points or component pins. Something that could potentially identify higher-frequency content outside of the range of my scope. And for that  I went for a cheap "near field" probe, connected to a TinySA Ultra Spectrum Analyzer. You can actually see the near field "probe" hovering over the tracks in the first picture.

This is a capture from the TinySA:

image

I was honestly amazed at how well this worked. The probe captured perfectly the two main components of the coupled signal (both a 7 and a 10 Mhz component) even without touching the PCB. Because I zoomed tightly on the frequency range of both signals there's no harmonics from the square wave in sight.

The results were pretty exciting to me, but this left me curious about a couple of things. And I thought this forum was the perfect place to ask::

How often is this kind of debugging needed in real life? How frequently is this kind of analysis required? (other than when performing EMI emissions testing). Is there an area in electronics where cross-talk issues are more of a problem than in others? (for example is this problem worse in high-frequency applications?)

If any of you have an anecdote or story where cross-talk has been an issue, a mystery, or a lesson well learned, I'd love to hear it!

Parents

  • Interesting experiment.

    I was the ISV engineer for a large integrated avionics project and while visiting the prime contractor, they thought they were fighting a cross talk issue on their bus.

    I soon proved to them that they really had a timing issue, not a cross talk issue. 

    Unfortunately, they had already spent six weeks trying to find the problem in the lab at a time when they were spending $100,000 a week.

  • in reply to DAB

    Six weeks? Yikes. Hunting a problem for that long must have already created a lot of pressure. Add how expensive it was to run the show, it must have been extremely awful to be in the debugging team.

    Out of curiosity, how did you determine it was a timing issue and not cross-talk?

    Thanks for sharing that, btw.

  • in reply to battlecoder

    The Prime contractor was under a lot of pressure, two years late 100% overbudget.

    I had built a computer model of the architecture and was able to do detailed timing analysis of the software.

    My model showed that they had several poorly implemented signal processing modules that blew their double buffers.

    Once they explained what they were seeing in the lab I was quickly able to show them which modules were the problem and why.

    They were so impressed they later hired me to analyze other projects for them after my ISV contract ran out.

  • in reply to DAB

    That's a hell of a story. Having a model was a genius idea. And I guess it goes to show in a way the power of simulation and computing modeling. That led me to search on Google for EMI/EMC and cross-talk simulation software for PCBs, and unsurprisingly, there are some alternatives, some of them even baked in into commercial PCB design software.

  • in reply to battlecoder

    When I proposed building the model, the government was not sure what I would do with it, but since we had about two years before the contractor would have software to evaluate they gave the go ahead so I had something to do.

    I soon showed them how useful the model was and how it could predict it the final implementation would work. They took this data to the funding organization and they funded both the extension of the project plus my work.

    I used the same model ten years later on another project that was even more expensive. My model helped prove to congress that the government was taking necessary risk reduction actions to justify continuation of the project.

  • in reply to DAB

    That's incredibly cool. Thanks for the follow up to the story!

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