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Is there any basic methods to calculate crosstalk between different layers in a multilayer PCB. I am not after anything exact just a way to grasp the magnitude of the issue.

The situation I am trying to solve is in a 4 layer space constrained PCB with a stackup of Signal-Ground-Power-Signal, on my bottom layer I have a number of differential +/- 10V Signals max freq 1KHz. On the power layer above these traces I have a power trace with a current varying between 0-200mA with a minimum rise time of approximately 0.1ms, the trace for the return current is directly adjacent.

Assuming there is no other way I can route my PCB how can I go about getting a feel for what kind of affect a step in my current can affect my signals.

Other data

Signal Output impedance <100 ohms

  • Signal Input impedance approx 10k ohms
  • Signal trace width 12mil, trace separation 12mil
  • Power trace width 40mil, separation 20mill
  • Copper Weight 1oz
  • Outer plane separation 0.2mm
  • Inner plane separation 1.2mm
  • Length of parallel run 75mm
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  • \$\begingroup\$ check out the PCB Toolkit by Saturn PCB Design Inc. \$\endgroup\$ – Techydude Mar 1 '16 at 6:38
  • \$\begingroup\$ Why have I never run across this tool before, it looks pretty cool, any recommendations how I can get it to work for my stackup which is effectively two signals above a ground plane. \$\endgroup\$ – Hugoagogo Mar 1 '16 at 6:50
  • \$\begingroup\$ So are you saying you've got a current-carrying pair on the 'power'-inner layer, and a signal diff-pair on the bottom layer, and they're both running parallel to each other for 75mm? But what's the lateral separation between them? \$\endgroup\$ – Techydude Mar 1 '16 at 7:46
  • \$\begingroup\$ On top of each other I know it's bad that's why I am trying to evaluate this vs running wires. \$\endgroup\$ – Hugoagogo Mar 1 '16 at 11:36
  • \$\begingroup\$ I recommend Hyperlynx SI, though it is very expensive software there is a trial version. mentor.com/pcb/hyperlynx/signal-integrity \$\endgroup\$ – DerStrom8 Mar 1 '16 at 13:22
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As @derstrom8 suggested, there are better tools for doing this, but tend to be expensive & aimed at pros. I vaguely remember there is a FOSS field-solver tool out there somewhere, but can't remember. But if I interpret your PCB stack-up figures correctly, you may be able to approximate your situation with the Saturn tool I mentioned:

Saturn PCB Toolkit

If you set it to 'stripline' mode where you're calculating crosstalk of 2 tracks sandwiched within 2 planes, and then set the distance of the bottom plane effectively to infinity (500mm) because that doesn't exist in this scenario, then set H1 to the gap between your 2 inner planes (where the lower plane is your power track on the inner-bottom, right?), and H2 to the gap between bottom-layer & bottom-plane (what you describe sounds like a typical 4-layer stack-up), I see -84dB, or 1.25mV.

Perhaps with that rough idea you can use your input-impedance & knowledge of the broader system to work out how significant this may or may not be.

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  • \$\begingroup\$ How can I relate this coupled voltage to an input impedance, what would the impedance of the coupling better. I assume the higher the input impedance the more effect it will have. \$\endgroup\$ – Hugoagogo Mar 1 '16 at 21:47
  • \$\begingroup\$ hang on - my answer isn't right at all! i mistook your +/-10V to be what the PSU is doing, but that's just your signal swing. So your PSU is switching 0-200mA, but what's its voltage? \$\endgroup\$ – Techydude Mar 1 '16 at 21:52
  • \$\begingroup\$ This particular traces is out of a current regulator, that will drive a signal somewhere between 0-200mA to a maximum of 15V. While not technically a power rail I though it should be separated out from my signal layers. \$\endgroup\$ – Hugoagogo Mar 1 '16 at 22:40
  • \$\begingroup\$ 15V - ok, so that's not much different to the 20V I put in based on your +/-10V anyway, so the result won't be hugely different, 0.94mV. \$\endgroup\$ – Techydude Mar 2 '16 at 2:04
  • \$\begingroup\$ So is crosstalk always a certain induced voltage, for some reason I thought of it as injecting a certain amount of power into the adjacent line, which then translates into certain voltages via the impedance. \$\endgroup\$ – Hugoagogo Mar 2 '16 at 2:06

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