I have recently received my LimeSDR Mini. I am having issues with images of the FM radio band appearing where they shouldn’t. I have found two previous threads with similar issues (Receiving FM broadcast images on every frequency and GSM Images over FM Band on LNAL), but both recommend low-pass filters to alleviate the issue. As I am getting images both above and below where FM radio should be, I would need multiple band-pass filters, which would not be ideal. Are there any other methods of eliminating these?
- FM images appear at 25, 58, 125, 155 MHz (about 31 MHz apart). I can get clear audio from these if I apply a FM demodulator. LTE seems to appear sometimes as well.
- Changing sample rates does not affect the images in any way.
- Using a different antenna does not remove the images.
- Setting the analog low-pass filter to a lower value only eliminates the FM bands to the side. FM bands which appear close to the center frequency are still very clear. From the block diagram (https://wiki.myriadrf.org/LimeMicro:LMS7002M_Datasheet), I would expect the low-pass filter to be able to eliminate these images, but it seems the FM band is at the wrong frequencies from the output of the tuner?
- The images at 25 and 58 MHz seem to be reversed.
- When tuning 1 MHz higher, the image around 25 MHz shifts in the opposite direction by about 3 MHz.
- This issue does not occur with an RTL-SDR.
- LimeSuite v20.10.0-ga45e482d, built from git master
- sdrangel 6.15.0-11-g697c709f9, built from git master
- LimeSDR mini gateware version 1.30 (latest)
I am quite new to radio, and I apologize if I am unclear or mistaken. I have asked a friend more familiar with radio, and after a few hours, they were as confused as I am. I have flipped a few settings in LimeSuiteGUI (probably not the best idea considering I don’t know what I’m doing), but it didn’t change anything. If comedic relief is required, consider that I have written this post with the SDR tuned to 31 MHz, listening to NPR.
A screenshot of sdrangel: