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.