Receiving FM broadcast images on every frequency

I’m trying to use the LimeSDR-USB around 25-30 MHz to reverse engineer a remote control I have, but all I’m getting is the FM broadcast stations from 88-108 MHz. It’s drowning out my signal really badly – I can use those frequencies to listen to FM broadcast radio very clearly. I am getting the same images at different frequencies throughout the entire range from 15 MHz to 82 MHz, and one station around 115 MHz (protected air band). This entire spectrum below 115 MHz is in fact unusable.

I am using a whip antenna connected to RX0 through LNA_L. Using SDR# with 2MSPS. Same results with CubicSDR. Although I notice that below 30 MHz, changing the center frequency has no effect at all and does not change the tuning. For a device that is supposed to tune all the way down to 100 kHz, it’s not really working out.

Digging deeper, it seems even 95.7 MHz is copied onto 96.3 MHz and follows properly if I change the tuning, whereas some images will move in the opposite direction of the tuning. With a cheap RTL-SDR, I do not have those images within the FM broadcast band itself (surprisingly getting better quality with RTL-SDR than LimeSDR), but I do get the images around 30 MHz, so… digital aliasing problems?

Changing the sampling rate does seem to change the location of the individual images, but I really shouldn’t have that many images floating around like that.

Well, it seems unplugging the LimeSDR and plugging it back in fixed the image issues in the FM broadcast band. Strange it even occurred…

Correction: With 2MSPS I have the aliasing, with 2.56MSPS I do not. Still have all the images in the lower part of the band and unable to tune below 30 MHz.


You need to use a Lowpass Filter that can pass the HF band (2.0 to 30 MHz) to eliminate the images of the FM band - the input to the LimeSDR and the Lime Mini is wide enough that it cannot take those images out and they show up in the HF band. If you use a commercially available or homebrew Lowpass Filter then those images will be gone and you’ll see a clear HF band that you can tune.

Hope this helps - 73 de Marty, KN0CK

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You turn on NCO to tune below 30 MHz - check that out in LimeSuite.

73 de Marty, KN0CK


Using SDRAngel will disclose A LOT of what you’re seeing with how to set sample rate and apply NCO to tune further down. I always found that setting the sample rate to 6,000,000, and then having a ratio of 32 and 16 made it possible to tune down to the AM Broadcast band and demodulate AM there. But you can play with it however it makes sense to you.

Are you developing an app for the Lime? Just curious - that would be a good thing to see as SDRAngel is what I use (but it’s not for everybody) and SDRConsole only works for VHF to SHF (not sure why Simon refuses to work on HF for 'Console since there appears to be a ton of Hams waiting for it). But SDRAngel is elegant in its own way and I use it A LOT to tune HF and generally make my Lime work in the HF band for all modes (AM, FM, SSB, CW).

73 de Marty, KN0CK