2016-10-12, 18:20
(2016-10-12, 13:23)DrRobin Wrote: I did wonder if there are any good techniques to find noise? I had a look at some VLF active antennas, or perhaps make a receiver and feed it in to a sound input? Not sure if there is anything on the market?
You have a receiver, the Blue system.
The Blue has buffered outputs available on each channel, pre/post the LP filter (if fitted). Modify a phone headset to get that signal into a Andriod or iOS device. The mic in should be on the second ring. There are many Frequency Analyser apps out there(*), OK they are limited to the low 20's kHz but still useful for nulling out VLF transmitters or watching the overall noise level then by rotating the antennas see if it is coming from a particular direction. Ferrites or loops have very similar polar patterns with a well defined and deep null along one axis. Use the null (ie minimum noise) to get a bearing to within a few degrees.
Some USB PSU's are just noisy. If you have a regular waveform on any channel in the Signals display with the aerials disconnected it's a fair chance you have a noisy PSU. Swap it out with others you may have or build a PSU filter but it would be better to find a quiet PSU.
AM radio can be useful and remember that has ferrite rod aerial as well, so if you do have noise source with direction you can use the radio to get several bearings on it and work out where it is.
(*) I've used Advanced Sprectrum Analyzer Pro (Vuche Labs, Android) goes up to 24 kHz.