I'm using this Mic:
And transmitting the data with XBee Pro.
The hum is awful, louder than most audio in the room.
It's a square wave at about the data bit rate.
Unless I use a USB cable, then it sounds perfect.
Any suggestions on how to fix this?
I've tried a few ideas, and so far only 1 is a success (with limitations).
Well I would never buy anything from any one who said:-
* Based on a high sensible Microphone
So not a stupid or reckless one but a sensible one! I would much prefer a sensitive one.
Basically if it works fine on USB power then there is something lacking in your alternative power supply which you make no mention about so I can't make any suggestions about it.
The problem is not the power supply, in fact it's the opposite of what you guessed. I have tried a battery with the XBee method of sending data. The problem is the RF square wave being picked up by the Mic. I have tried a different circuit entirely before I purchased the Sensible Mic. I am using USB to transfer data instead of XBee. The power is not the main issue. Or the noise coming from USB power is much smaller than RF.
That makes it more clear. So RF pickup on the audio front end a classic problem for a radio ham. Officially it is the microphone that is at fault because it it acting as a radio receiver and it is not designed as one.
Things to try are screening, it looks like most of the electronics are open. If you can put the microphone electronics in a grounded metal enclosure it might stop the pickup.
Great idea! I already tried it. It was insignificant. Somehow it's coming thru the signal wire? I had more success by changing the distance between the 2 boards. If you get it just right, it's better. But unpredictable. My best solution so far is to record the audio to SD, then send it when you're not recording. But that changes the functionality entirely.
You could try using ferrites round the leads, either in the form of beads you thread the wires through or a toroide to wrap them round. Don't discard the case either as that will be part of the eventual solution.
Great minds think alike? I've tried it. But I have not tried the 2 solutions together yet. Does the case have to completely enclose the audio board without any small openings? Or just get between them.
The gaps should be small compared with a wavelength of the interfering signal. By small at least 10 times less. There should be a solid connection between the case and the ground.
You can also try things like small ceramic capacitors across the microphone, like < 30 pF preferably surface mount.