Hi I am building an USB sound card circuit for raspberry pi. It involves a button, LED and Sound card IC PCM2912A (TI).

I am mounting raspberry pi over the 2 sided pcb on top (top has full digital ground). All analog components are in bottom but the wires to and from mic/speaker are in open. Bottom have both digital and analog ground which are connected at only single point (as power supply is one). I am also using PAM8403 to amplify the speaker output. (see below)

Basically i am building 2 way audio video system enclosed in a 5 face 'metal' box. 6th face is ABS plastic. Wifi connected to router is working perfectly inside box.

My problem is that when i start communication I hear a noise (most likely EMI noise). I have tested power supply to amplifier and it does have little spikes but thing is when I keep system inside but take just raspberry pi outside (wifi out) the noise is significantly reduced and when i keep pi out and keep wifi parallel to body, noise is zero percent. So the inference i have drawn is that noise is not from power supply but from RF waves.

I have tried to shield top of pcb, connectors for mic and speaker and camera circuit with aluminium shield inside electric tape but noise is still there. Also i have found out that if i keep my direct hand over antenna for raspberry pi noise is reduced to 0 but if i shield the inductors of antenna with foil (yes wifi still works) noise is slightly reduced. I have tried shielding various parts but not able to find perfect solution that works not sometimes but always.

Question: What may be the cause for noise? How touching wifi antenna makes noise zero? How to reduce the noise effectively?

Additional info: There is also NRF24L01+ in system but it is in power down mode most of the time and is only transmitting for less than a second. Also I am not grounding the shield at the moment.

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  • \$\begingroup\$ are you hearing the onset, and then the end, of RF packet energy that gets rectified in the transistor junctions? \$\endgroup\$ Nov 2, 2019 at 12:20
  • \$\begingroup\$ I am hearing the noise like a automobile being started. Not that high pitched but very irritating. \$\endgroup\$
    – Akshit
    Nov 2, 2019 at 12:31


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