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enter image description here Is it normal to see periodic peaks going from 48.75 - 90 MHz? Also i was checking on my SDR dongle once and the whole band was raised above background offset for like a whole 50 MHz bandwidth and then it dropped to background again? Does anybody know what that means?

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  • $\begingroup$ You'll need to experiment a bit. How do these change when you change gain? what appens when you only slightly change the physical LO frequency? are they sampling rate-invariant? $\endgroup$ – Marcus Müller Dec 12 '19 at 16:44
  • $\begingroup$ I have added some figures for RF gain and sample rate change $\endgroup$ – ChemEng Dec 12 '19 at 17:01
  • $\begingroup$ My guess is that there's something wrong with your setup, or there's some weird kind of interference going on. Can you see known strong signals, such as FM stations? $\endgroup$ – MBaz Dec 12 '19 at 23:24
  • $\begingroup$ Yes i get peaks at radiostations and i can listen to the radio using the device. I was just looking through the spectrum and found these periodic peaks going from about 45MHz to 90 MHz when the radio stations start $\endgroup$ – ChemEng Dec 12 '19 at 23:49
  • $\begingroup$ how far does the noise floor drop when you remove the SDR from the antenna/any coax? $\endgroup$ – Marcus Müller Dec 13 '19 at 10:08
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Periodic noise in an SDR spectrum is a common problem. One likely source is EMI or RFI from nearby electronics.

The sources of EMI and RFI can be from computers, USB cables, networks cables and routers, power cords, wall plug or internal power supplies for various electronic devices, wireless chargers, LED lighting controls and voltage converters, and solar panels, etc., even radiated from neighboring buildings. This noise might rise and fall as people or controllers in other locations turn on/off lights and other devices. The power and high edge rates generated by some of these electronic components can radiate a vast range of periodic harmonics into the RF spectrum.

Kill the circuit breaker to your house or building, or wait for a neighborhood wide power outage, and a vast amount of this periodic noise in the spectrum might disappear. Or, if you are lucky, you can turn off or unplug things nearby, one at a time, and discover the dominant culprit. If you are running the SDR from a laptop and have a portable antenna, try running the SDR well away from any buildings and utility lines (for instance: a nearby park or open field, etc.)

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  • $\begingroup$ If it is from EMI or RFI what can i expect to see as i go down the frequency spectrum. Should it taper off, stay constant, or end abruptly? $\endgroup$ – ChemEng Dec 13 '19 at 17:16
  • $\begingroup$ Depends on the EMI source(s), edge rates/shapes, distance losses, radiation patterns, antenna, feedline, so could be anything. But often tapers. Gradually. $\endgroup$ – hotpaw2 Dec 13 '19 at 20:04

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