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I have been asked to continue an unfinished project using Sigfox ATA8520 transceiver and Arduino DUE board. There are some LEDs and buttons on the board which I think are used for selecting different modes on the board. When I turn the board on, I see that the LEDs are driven through transistors that randomly turn on without touching the board. These LEDs do not glow quite dim but bright enough to see in a well lit room. What could be causing it and how can I fix it?

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  • \$\begingroup\$ Sounds to me like pull down resistors are not connected \$\endgroup\$ Aug 24 '20 at 3:57
  • \$\begingroup\$ SMPS are worst offenders then line noise on wires that may be suppressed with filters and RF cap to earth gnd. \$\endgroup\$ Aug 24 '20 at 4:08
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There could a few reasons why traces that carry high frequency often run into noise issues. Points below come from personal experience and what I learnt troubleshooting high frequency noise on boards.

  1. You need decoupling capacitors close the power supply preferably tantalum.
  2. Noise usually occurs because of parasitic capacitance or inductance building up in the circuit. Reasons for this could be length of traces that carry high frequency signals. These signals induce voltage onto neighboring traces thereby generating noise which for a beginner would be almost impossible to troubleshoot.
  3. When using high frequencies, linear power supplies must be used such as a battery pack or even bench power supplies that are linear.
  4. Use pull-down resistors (at base/gate of transistor/MOSFET) when switching semiconductors are used such as transistors, MOSFETS, etc.
  5. You need to be careful of how ground planes are connected. Usually it is recommended to run high frequency transceivers and digital logic of microcontrollers of separate power supplies or at least separate ground places to mitigate as much high frequency noise as possible.
  6. Proper shielding of high frequency components from low frequency components would be a useful technique to avoid issues you are facing.
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  • \$\begingroup\$ Thanks for your answer, I have been asked to design a new board and your recommendations will be very helpful. \$\endgroup\$
    – John Brown
    Aug 24 '20 at 5:20

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