HFU HF Underground

Technical Topics => Equipment => Topic started by: ChrisSmolinski on May 07, 2019, 1334 UTC

Title: More adventures in battling the RFI Demons
Post by: ChrisSmolinski on May 07, 2019, 1334 UTC
Another RFI source squashed.  The other day I repaired my 10/11 meter dipole antenna (actually the coax feedline to it had a corroded connector), and noticed a huge amount of RFI. After spending some time checking that antenna again for any faults, and cross checking against other antennas, which showed less RFI (likely because they are not quite as sensitive on that band)

I started to check around the house for the source. First with a portable radio which was nearly useless, then with my AFE822x SDR and laptop. The signal was definitely in the AC wiring (and probably radiating from it). At first I thought it was the phone or radio in the kitchen as the signal got stronger there, but it was likely due to being on a close AC circuit.

Then in the basement I found it to be stronger in my workshop. I finally identified it as the 12V power supply for a router.  Here are the before and after waterfalls.

I'll repair the power supply with a sledge hammer :)

(https://i.imgur.com/XYvdGBV.jpg)

(https://i.imgur.com/QEwCm29.jpg)
Title: Re: More adventures in battling the RFI Demons
Post by: ThaDood on May 07, 2019, 1748 UTC
Do you have old power strips with MOV's in them, (Metal Oxide Varistors.)? When they get old, or have taken some big spikes from lightning surges, they can also cause that noise.
Title: Re: More adventures in battling the RFI Demons
Post by: Σ on May 07, 2019, 1759 UTC
You still have some serious RFI around 27.025 MHz.   ;)
Title: Re: More adventures in battling the RFI Demons
Post by: Josh on May 07, 2019, 1941 UTC
Ahh the benefits of the unfiltered switching power supply!
Title: Re: More adventures in battling the RFI Demons
Post by: Ray Lalleu on May 08, 2019, 0937 UTC
On the waterfall, 27025 looks like an AM signal (standard CB mode).

The noise bands are around 26970 and 27040 kHz. So the switching frequency of the PSU is about 70 kHz. Other harmonics are likely to appear on any band from LW to low VHF, but that's depend upon the resonant frequencies of the wirings around the PSU.

Having a lot of old sets in my attic, thinking to save the integrated power supplies before disposing that stuff... The other way is looking to garage sale (attic emptying sales). I even bought a pair of old computer speaker because the PSU was a hefty heavy linear black box.

On the other way, I was fearing about the PSU of a new Lenovo laptop PC, but happily it is really quiet. So far I cannot find any noise from it.