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Broad signal

Any idea about this. Wasn't there before and is definitely not related to anything in my apartment




  • edited October 2018
    When I seem something like that on a waterfall the first words in my mind are "failing switching supply". Having said that, I've run across some OEM Chinese switching wall warts that were devoid of any semblance of RF filtering that radiated very well, gradually changing their RF characteristics as their low-quality filter capacitors gradually fail.

    Despite what you may think/hope, I always remember the following mantra: Most QRM begins at home! meaning that it's very easy to overlook something that isn't immediately obvious in one's own house, such as the power supply in the computer/monitor that one is using with the Kiwi. Even if one turns off all of the electrical power, many people overlook things that are battery-powered such as a back-up UPS and its loads, battery-backed security system and things like POE-powered cameras, etc.

    Often, one can hear a hum/buzz at twice the mains frequency (100/120 Hz) on such signals, rich in audio harmonics of these frequencies, verifying their source as being mains-powered and likely very local: This is typically done using AM - but some supplies' switcher frequencies are frequency-modulated by the mains and this may also be heard in FM.

    If there is a "sputtering" noise, that may also be a failing power supply - or even something being used to maintain a battery (automobile, for mobile phone, etc.) that is operating in some sort of "discontinuous" mode.

    I also see a mirror image of this same signal some 350-370kHz below, at the left edge of the screen, implying that the interfering device is a switching supply - and this high-ish frequency often implies a smaller point-of-load converter as most mains-powered wall warts operate in the 30-90 kHz range - but I have seen some devices that contain such supplies (TVs, DSL/cable modems, etc.) that are wont to radiate. The Kiwi itself has a POL switcher, but interference from it is usually very well quashed.

    Perhaps most insidious is that at even higher frequencies, these broad bands of QRM tend to overlap producing a continuous curtain of noise across large swaths of spectrum making it less apparent that the source may be a single signal: This is why it's usually easiest to "search and destroy" these things at the lowest-possible frequencies. Given the 350-370 kHz separation, it should be able to find one of these (not as) broad signals at this same interval as you go lower in frequency, each iteration getting proportionally narrow in bandwidth - finally ending up in the broadcast band at the (apparent) 2nd harmonic of 700-740 kHz where it can be located with a portable AM/Mediumwave receiver if you haven't the facility to tune directly to its fundamental.

    Good luck with the hunt!
  • Guess it was the distribution amplifier for cable TV within the house

  • Ah good to know, it's easy to forget those potential broadband RF amps in many locations.
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