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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

  • Long ago I was developing a high-speed data collection system that contained both analog (Op-Amps and A/D converters) and digital (75Mhz DSP, large CPLD, multiple banks of RAM and a USB transceiver). I used large, flooded ground and power planes throughout the PCB.  In initial testing it became very clear that there was a lot of crosstalk in the design, with very clear signs of the digital signals being superimposed on the analog signals.  In analyzing the design, there were significant digit signal runs over large areas of the power and ground planes.  I started doing a lot of reading about grounding techniques and decided to minimize ground and power flooding, in exchange for a more purposeful routing, with a more star like routing.  That and forcing and high-speed signal to cross power/ground traces at a perpendicular path.   The effects were absolutely amazing.  I went from a huge crossover issue to a drastically reduced occurrence. up to the point that it was not visible in scope tracings or in the collected data.  

  • in reply to genebren

    Oh I love this! Thanks for sharing. This is exactly the kind of story I was hoping to find. Examples from the real world are extremely valuable.

    I wasn't expecting that over-doing ground/power planes could also be a problem. Making the signals cross perpendicularly later is not something I would have thought of. You mention reading more about grounding techniques. Do you remember if there was a specific source, book, magazine or something that contained the most insight about this?

    Thanks again

  • in reply to battlecoder

    This goes back a long way, but I vaguely remember having a book, that was a compilation of various articles from Analog Devices on the subject of PCB and circuit design techniques to reduce noise.  It was a great resource, but right at the moment I can't seem to locate it.  I did however find this link that seems to contain a lot of the same data. ( Microsoft Word - EDch 12 pc issues.doc (analog.com) )

    I will look a little deeper and see if I can find this (I can't imagine that I would have let go of this, although I have lost some things by loaning them out Angry).

  • in reply to genebren

    Thanks a lot! I wasn't expecting you to find it for me. I appreciate the effort. I'll be reading this for sure.

  • in reply to battlecoder

    I found the book that I was referring to, "Mixed-Signal Hardware and Housekeeping Techniques" by Walt Kester of Analog Devices.  This was a part of the handout for a seminar session that I attended on dealing with noise in analog/digital designs. The link I provided you with is also by Walt and is included in this book.

    I may have oversimplified my statement concerning ground planes.  A more correct statement would be that ground planes may need to be split into multiple sections (connected at a single point), where they serve digital and analog regions.  Also, the perpendicular comment applies most when traces of a different class of signals (i.e. Digital lines crossing an analog region) and not all traces in general.

    In the case that I was dealing with, there were several high-speed digital lines that crossed a solid ground plane and capacitively coupled noise into ground and was very noticeable in the low-level analog signals.

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  • in reply to battlecoder

    I found the book that I was referring to, "Mixed-Signal Hardware and Housekeeping Techniques" by Walt Kester of Analog Devices.  This was a part of the handout for a seminar session that I attended on dealing with noise in analog/digital designs. The link I provided you with is also by Walt and is included in this book.

    I may have oversimplified my statement concerning ground planes.  A more correct statement would be that ground planes may need to be split into multiple sections (connected at a single point), where they serve digital and analog regions.  Also, the perpendicular comment applies most when traces of a different class of signals (i.e. Digital lines crossing an analog region) and not all traces in general.

    In the case that I was dealing with, there were several high-speed digital lines that crossed a solid ground plane and capacitively coupled noise into ground and was very noticeable in the low-level analog signals.

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