I have a strange issue with LimeSDR (classic not mini). I’ve completed the tests and calibration successfully, but I couldn’t find any fm stations (antenna connected to RX1_W - CubicSDR software on Windows 10) until I started looking in 200MHz range. There I found all the stations I was looking for in the 88 to 104MHz range. For example a station that should be on 94.4MHZ is at 195.10, another station that should be on 93MHz is on 205MHz etc.
This is very strange especially that I can reliably tune to 446.08MHZ and I can see a transmission from my PMR radio that is transmitting exactly at that frequency.
In my attempts to troubleshoot this I closed CubicSDR and opened LimeSuite. I read the config from the board and I tried to find differences between the settings that allow me to receive the signal of my PMR radio on 446.08MHZ and the wrong FM signal at 205MHz as well as the example.ini from the wiki. Other than different frequency settings I found that the example has the “bypass SX LDO” box ticked.
Can anyone suggest how to troubleshoot this further? Is CubicSDR settign some registers of the board to wrong values? If it does why can I receive signals on 446MHZ correctly and not the lower frequencies?
I also noticed that the “Active VCO” setting is different between 446 and 205 MHZ. Perhaps this is a clue of some sort?
Also, if I load the example.ini file and then I change the frequency to 446.08MHz I can receive the signal sent by the PMR radio. If I then lower the frequency to 94.40MHz I can definitely see some signals in FFT viewer, but they are much lower amplitude than the FM stations when I manage to find them in the 200MHZ range with CubicSDR.
Those are called images, it is a common occurance in all radios without filters as mixing products up convert the lower frequency signals.
Use an FM trap filter and the FM stations will go away at the higher frequencies.
In fact use filters for every band you want to use and you’ll get much better results.
Thanks for the reply, but those are not copies of stations on the higher frequencies. Those stations are on those high frequencies instead of the lower ones.If they were let’s say on both 94.4 and 195MHZ I would say they are images, but if a station that is supposed to be on 94.4MHz is on 195MHz and not on 94.4MHz I wuold say this is some tuning issue.
I misunderstood, now that is interesting.
Do you have any offsets or NCO’s set that might appear to shift the signal upwards like that?
I’ll load up CubicSDR myself and see if I can replicate it.
I haven’y got any offset set. Initially when I thought this was just a straight frequency shift I applied an offset of -100.7MHz but this didn’t make the stations as they should be. It appears that in addition to the frequencies being offset they are also further apart from each other.
I’ve also done more troubleshooting and now I think this is purely a CubicSDR issue as in LimeSuite during my testing I had a tick next to “input of LNAW” in shorting switches on the RFE tab. So essentially LNAW antenna input was shorted to the ground. Once I unticked it I now can receive those FM signals as I should in LimeSuite, but in CubicSDR the problem of them being shifted remains. (Please note I never saw the frequency shift in LimeSuite - only that I couldn’t receive them at all.)
Also during the troubleshooting I tried to use manual gain setup in CubicSDR and I noticed it is practically impossible to set the TIA gain. As the moment I change it… it changes back to 0 by itself. Very strange. I have no other programs running that could change that setting on the board.
I’m sure now that this is a CubicSDR issue because once I managed to get GQRX working it works fine. BTW, GQRX doesn’t work if someone selects Lime from the list of devices. One has to choose other and then enter soapy=0, driver=lime in the driver details field, but this is beside the point.
I can reply to myself here in hope it helps someone else. If you;re having such issues do two things. Both equally important. First get some extra cooling to the limeSDR (in my case USB). If you can’t plug it into usb2. Somehow it heats up less in usb2 port.
But adding 2 radiators and a fan is a must. You definitely need a radiator for the RF chip. Those small 10x10mm RaspberryPi radiators made of copper work fine. Then you need a bigger radiator for the FPGA. Also, just a radiator for the FPGA is not good enough.
You need a fan as well. While you’re at it you may as well plug it into power permanently (next to the power input jack, you’ll have 5V from USB there as well) because the board quickly heats up even at idle. So there is no point trying to run it only when hot.
The second thing is updating gateware.
If you do those two things and you manage to keep it below 50C consistently it will actually work pretty well. Not fail every second calibration and so on.
The power budget is much less with USB 2.0 and I’d be surprised if you don’t run into other issues due to this, such as the stream dropping out. You may have been lucky, using a single channel and lower sample rates, but USB 3.0 is really recommended with a LimeSDR USB.
