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I was trying to make a roaming device which helps me route my calls through the internet, to save on my bills while on international roaming.

My setup is like this:

GSM module SIM800l, Raspberry Pi 4, a USB audio adapter (Quantum-QHM623) and an Android app to get calls redirected.

Basically this setup routes all my calls from my simcard to my phone app via the internet.

The project is almost complete, but the issue I am have is that when the speaker of the SIM800l is connected to the mic of the USB audio adapter, I get a whole lot of background noise.

These are the trouble-shooting steps I followed which might help you assess the situation:

  1. I can hear the voice with a lot of interference only when I connect the speaker's +ve from the SIM800l to the +ve of the mic in the USB adapter's +ve side. If I connect -ve of the speaker out to the mic ground of the audio adapter, all I hear is noise, no voice.

The other audio line which is mic from the SIM800l to the speaker of the USB adapter works absolutely fine.

I tested the modules this way and they seem to work fine:

Audio USB adapter: to see if the audio adapter is working, I tested shorting out the mic and speaker and simulated a call from the Android app, and it worked fine (I was able to hear own voice after I said something, and the voice quality was good too).

SIM800l: AT commands work fine; I was able to send and receive messages, able to call receive calls and hang them up using minicom.

Audio lines of the SIM800l: I shorted the mic and speaker lines and made a voice call to my mobile number, and I was able to hear my voice fine, but the quality is ok.

Any help with this noise issue would be very much appreciated.


EDIT: I can work around this issue using a different power supply. I had previously used the same Raspberry Pi to power the SIM module, but now I have used a different power supply to power the SIM800l module, and it's working quite well.

Is there any way to use the same power supply and not get the noise?

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    \$\begingroup\$ It is pretty difficult to imagine the connection based on text. can you draw the connection? it is also okay if you do draw it on paper nicely and upload it. It really helps the other experts to help you with your question sooner. Warm welcome by the way :) \$\endgroup\$
    – User323693
    Commented Aug 31, 2019 at 18:06

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Just saw your edit, I'm gonna write a reply anyway.

It is probably due to your power supply and/or ground loops which inject noise into your circuit.

Make sure you are using an isolated supply and that the operating frequency is out of audible range (although it's not always easy to know as the control may change frequency upon the current drawn).

  • Avoid the cheap power supplies, take a good one.
  • Try to add a big cap 100/300uF and several small caps like 0.1uf 10nF at the power supply / input of your circuit.
  • Try making a LC filter by adding ferrite, inductor on the supply lines.
  • Make sure your circuit has all the necessary bypass cap (0.1uF) on each chip VDD pins.
  • Make sure you have a ground plane.
  • Make sure you don't have significant current going through the ground on your board, like having a module or another board connected on the "other side" of the board as the power input.

A picture of the board / pcb layout would help.

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