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Following on my unanswered question about bluetooth noise, I have now believe that I have learned a bit more about my problem.

I have built a Arduino controlled circuit to replace the pots and switches on an electric guitar. You can find full information at the webpage.

I communicate via Serial Port Protocol. A bluetooth module is connected via shielded cables to the circuit board.

The audio part of the circuit is fully opto-isolated from the command part.

At each data exchange, I can hear a faint click out of the amp.

After quite some searching, it seems that this click is a result of radiation from the 5v square waves passing along the rx and tx wires, despite the shielding.

I further shielded the cable with a copper mesh sheath, and this reduced the click intensity. As did wrapping the connectors in tin-foil.

I am wondering:

  • Why/how the shielded wire lets the EMI radiation out?
  • If changing to 3v square waves would further reduce the intensity of the click?
  • Why there are no clicks when using a USB cable to communicate instead of the rx/tx?
  • If there is any way of preventing this EMI from being radiated?

Here is my best attempt at showing the physical layout. Note that the cable to the BT module is now shielded.

enter image description here

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  • \$\begingroup\$ Have you tried changing the SPI clock frequency? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Sep 30, 2013 at 8:37
  • \$\begingroup\$ Is the shield of your shielded cable grounded at one end only? \$\endgroup\$
    – RedGrittyBrick
    Sep 30, 2013 at 8:41
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    \$\begingroup\$ I think there are quite a few hints and helpful suggestions in your original question so now I think you need to come clean with a diagram or schematic of what you have/want and also diagrams/schematics of what works when you use wires. It's hard for someone coming to these questions afresh to get a picture in their mind's eye what you are trying to do. You'll be helping yourself and following advice in the 1st question. \$\endgroup\$
    – Andy aka
    Sep 30, 2013 at 8:42
  • \$\begingroup\$ @Ignacio Vazquez-Abrams: No I have not... \$\endgroup\$
    – gratefulfrog
    Sep 30, 2013 at 10:03
  • \$\begingroup\$ @RedGrittyBrick: It is grounded at one end only, the external copper mesh is not grounded, since grounding increased the noise. \$\endgroup\$
    – gratefulfrog
    Sep 30, 2013 at 10:07

1 Answer 1

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Difficult to assess this without going round the circuit with a scope probe looking for spikes, including on the power supply; but there are further cheap tricks that can be applied to mitigate EMI even without understanding exactly where it's coming from:

  • small resistors in series with TX/RX; try 33 ohm to start with, up to maybe 100 ohm. Place near the side driving that signal. This mitigates "ringing" in the line.

  • decorate power supplies with decoupling capacitors to ground, 1-10 uf. Place near device using power.

  • small ferrite beads/chokes in line with power supplies

  • shield the RX/TX lines not just the pickup (it doesn't mention this in the diagram)

You mention a USB cable but I can't work out where it would have gone in the diagram? USB is both balanced and properly terminated, which gives it better EMI properties.

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  • \$\begingroup\$ I have tried resistors, but 1Kohm only which just disrupted the communication, but I will now try as you suggest. The power supplies are already decoupled. I tried ferrites on the power supplies with not results. I double shield the rx/tx lines and this has helped a lot, but there are parts on the circuit board that cannot be sheilded... I mention USB in relation to the Arduino Micro's usb connection. If I use the USB connection for the serial comms, there seems to be no clicking. Do you think running the serial comms at 3.3v instead of 5v would help? I'll report back on the resistors \$\endgroup\$
    – gratefulfrog
    Sep 30, 2013 at 15:18
  • \$\begingroup\$ I tried with 47 and 100ohm resistors, no change to the ticking on the rx/tx lines.... \$\endgroup\$
    – gratefulfrog
    Sep 30, 2013 at 15:40
  • \$\begingroup\$ What's the other sort of serial comms that isn't USB? \$\endgroup\$
    – pjc50
    Sep 30, 2013 at 15:49
  • \$\begingroup\$ Serial Port Protocol on the Arduino RX/TX pins, in my case those pins are wired to the Sparkfun BlueSMIRF board which is supporting the Roving RN42 radio. \$\endgroup\$
    – gratefulfrog
    Sep 30, 2013 at 15:58

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