I just figured id post this here, because it was months of work trying to figure this out and hope I can save someone else a headache.
So we have several repeaters that are Motorola MTR2000/3000 running ASL3, and one of the sites had a horrible noise problem, the repeater would sound fine on my bench with a dummy load, when it was at the site it would have a hum, almost like cricket sounds.
Tried two different repeaters, one was using a TechnobyGeorge ARA FOB (cheaper option), and the other was using a DMK URI(much more pricey), so at some point in the process we decided it had to be something to do with the site, we went crazy with shielding stuff checking all the feedline, etc, etc.
It turned out to be a ground loop between the PI and the USB interface.
A ADUM3160 usb isolator solved the problem, its a $10 US or less part.
Hope this post saves someone some extreme frustration!
It was also causing the CTCSS output from usbradio to not trigger yaesu radios.
i had an audio buzz and searching this Forum yielded an indirect mention of using a USB Isolator sometime in 2011
it probably would be good to mention this kind of problem solver a little more often!
not all of the ADUM3160 boards are alike, though. there is one presently available via Amazon where clearly the time was taken to route the input and output board traces away from each other, leaving a central isolation zone.
I think they may be talking about like professional audio recording interfaces and not sound cards as the speed and amperage is very limited. It looks like some of those that I have seen that as well.
Ive built a number of nodes and always struggled with noise when using a common 12 volt supply to feed the compute and the radio/interface. I tried everything as well and usually just powered the pc separate to eliminate the noise.
I ordered one of these to try it on a node with a common power supply that had noise. It eliminated it.
