Electronic – arduino – Square Wave pulses generating EMI

arduinoaudioemcserial

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

Best Answer

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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