SOLVED: Identifying interference with oscilloscopes and software-defined radio

[djhuft]

25 kHz Source Found!

When I last posted, I had a slight suspicion in the back of my mind that the 25 kHz signal might also be VLF station. I now confirmed that.

The station is NML4, a US Navy transmitter broadcasting from Lamoure, ND. I verified the direction again by rotating my antenna and watching the amplitude. As I expected, it is coming from the northeast (or southwest, but I don't know of any other stations in that direction).

I calibrated the frequency of my SDR dongle and upconverter system against WWVB so that it was as accurate as I could make it. The oscillators in the SDR and upconverter are not perfect, so this was a worthwhile step. The resolution of the SDR is not great, but I measured the frequency of the interferer to be very close to 25.2 kHz, just what I expected.

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So, I suppose this mostly wraps up my investigation.. I ordered digital filters and will be installing them shortly.

The interference from NML4 is unfortunate. I suppose I will aim most of my antennas to avoid it. Adjusting one or more channel's filters to block 25 kHz and above is probably counterproductive.

It is interesting to note that another user encountered a very similar situation in 2014:

I recently got a RED station set up in the Pacific Northwest area of the US (Seattle, WA), and it turns out that the Jim Creek Naval Radio Station (callsign NLK) is broadcasting on 24.8 kHz at 1.2 MW just 50 miles to the north of me.  Needless to say it's coming across loud and clear on my station.  I carefully oriented my H-field antenna so that one channel was nulling-out the transmitter to the best of its ability, but the signal is still obviously there and the gain isn't even very high (8*2).  Is there any secret tricks for combatting this or should I just give up and sell the station to someone who can make better use of it?

None of the developers had a great solution, and just recommended doing nothing.