the noise is coming from a cell phone which we use to transmit. No noise when its not transmitting. I located the actual chip on the board, an op amp, that injects the noise into the signal. Since it is from the cell phone GSM TDMA accessing (4.6 ms packets, looked it up on scope, 217 hz and its harmonics), I guess it is electromagnetic radiation. When I change to WLAN the noise completely goes away.
I am working with audible frequencies, the noise does not affect the other signals as we have a lot capacitance between power and ground at all parts of the board. I've already completely shielded the power source ( a battery several feet away ) using shielded power cables, and even wrapping the whole board and power cabling in aluminum foil which is grounded. all no luck.
There is no 217 hz and its harmonics on the power and ground from the opamp that creates the noise, so i am completely dumbfounded. I even added 220 uF capacitance (more than enough for 217 hz and above) and it doesnt affect it. I've even added capacitance to all the capacitors in the circuit surrounding the op amp, and none help (some just cut the sound completely off). The trace is a couple inches in length which COULD be a problem and I could try unsoldering the chip that it goes to but it would be a ton of work, or I could cut the trace with an exacto knife. However, even shielding it with aluminum foil and grounding it completely does not fix it!
Since everything else I've measured doesn't pick up the 217 hz it is not the oscilliscope cabling.
I tried disconnecting the microphone such that basically ground was going into the op amp. The noise very well could be coming from the line attached to the opamp output but I don't know how I can deal with it. There was still noise with none on ground/power.
I will try out some better than aluminum foil insulation and shielding methods between them. We place these devices within several inches from the cell phone in order to have it small in the woods.
This happens across over 50 boards with the same layout so it can't be the opamp. it only happens when the gsm phone is transmitting.I am going to try increasing the resistance in the feedback loop but I really see no 217 hz in the input of the opamp at all. so is it possible that the output trace which is several inches in length and whatnot could be picking up signal? What are some better ways to shield it?
Does anyone have any ideas?