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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

  • One way this problem can be solved, can be seen by looking at Ethernet cable. There, the signals are close to each other, but the coupling is reduced heavily by having each signal wire and its return twisted together (there are balanced signals there, but the point still remains), allowing 100m lengths to be supported.

    Similarly, on PCBs, it's a good reason why the ground plane is so necessary to reduce the loop area to a minimum. You'll see an improvement if you can replace that ground trace with a ground plane, although you may find the close parallel lines will still pick up some energy from each other, since the ground plane isn't so close compared to the distance between the lines.

    Other techniques are used too, e.g. try not to run wires closely like this on the PCB, and maybe even have a ground trace between the signals, if they are really sensitive, to reduce electric field impact too.

    Sometimes, parallel lines are actually deliberate; that's how some RF couplers work (at higher frequencies), one use-case being that it's handy for sampling a signal without extracting too much energy from it.

    EDIT: One more point; although the SA is a very suitable instrument for this, the H-field probe can be connected to the 'scope too, if the SA was not available, since these signals are likely still of sufficient power for resolving by the 'scope's FFT. Ideally needs 50-ohm termination at the 'scope end though (e.g. T-piece and a BNC 50-ohm load). 

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  • One way this problem can be solved, can be seen by looking at Ethernet cable. There, the signals are close to each other, but the coupling is reduced heavily by having each signal wire and its return twisted together (there are balanced signals there, but the point still remains), allowing 100m lengths to be supported.

    Similarly, on PCBs, it's a good reason why the ground plane is so necessary to reduce the loop area to a minimum. You'll see an improvement if you can replace that ground trace with a ground plane, although you may find the close parallel lines will still pick up some energy from each other, since the ground plane isn't so close compared to the distance between the lines.

    Other techniques are used too, e.g. try not to run wires closely like this on the PCB, and maybe even have a ground trace between the signals, if they are really sensitive, to reduce electric field impact too.

    Sometimes, parallel lines are actually deliberate; that's how some RF couplers work (at higher frequencies), one use-case being that it's handy for sampling a signal without extracting too much energy from it.

    EDIT: One more point; although the SA is a very suitable instrument for this, the H-field probe can be connected to the 'scope too, if the SA was not available, since these signals are likely still of sufficient power for resolving by the 'scope's FFT. Ideally needs 50-ohm termination at the 'scope end though (e.g. T-piece and a BNC 50-ohm load). 

Children
  • in reply to shabaz

    Differential pairs are definitely something that came to my mind as I was thinking about signal coupling.
    What is news to me, is this phenomena being used on purpose on electronics designs (like the RF couplers you mention).

    I definitely know that the H-field probe can be connected to the scope! I've done it in the past, but I always needed to add a small amplifier board in order to get noticeable and clear signals. That's what I found on "real" boards at least, On this PCB that is kinda designed to maximize coupling maybe I could have just connected the probe alone.
    But that's a good point, the Spectrum Analyzer is not quite necessary (although I imagine that if the frequency content is in the Ghz range I would need an extremely pricey scope, so something like a tiny SA would be a better alternative)