GnuRadio reporting incorrect frequencies, LimeSDR sink

General Discussion
1

I have unexpected behaviour with the GNU Radio LimeSDR source.

My test flowgraph is a LimeSDR source block into a QT GUI Sink. LimeSuite version 23.11.0, gr-limesdr version 3.0.2.0. The test signal is from a signal generator that checks out OK with my Rigol spectrum analyzer. GNURadio version 3.10.9.2, Ubuntu 24.04.

For an input signal at 50.05 MHz, the GUI sink shows the signal at 50.0037 MHz. For an input at 50.2 MHz, the signal is shown as 50.0168 MHz.

For an input signal at 144.2 MHz, the GUI sink shows the signal at 144.362. A signal at 144.3 MHz is shown as 144.356 MHz.

A test with a LimeSuiteNG (version 0.3.0) source into the same QT GUI sink shows the correct signal frequencies. LimeSuiteGUI (version 23.11.0) using FFT Viewer shows correct frequencies.

A separate test shows the LimeSDR Sink performs correctly, i.e. it generates a signal at the expected frequency.

2

What was the center freuqency set at?

Given the fact LimeSuiteGUI is showing it correctly, the problem must be within ‘gr-limesdr’ plugin when setting the LO frequency, most likely a lack of floating point precision issue.

Can’t find this version in github repo, was it installed from source or somewhere else?

3

I’m confident that the centre frequency was set at 50 MHz (specified as 50000000 in the GR block) and 144 MHz respectively. I can’t repeat the test at the moment because I need to repair my Lime Mini. The USB connector has broken away. I expect to be able to repeat the test (or any other tests) in a week.

Note that the GR LimeSuiteNG Source works fine also.

That version 3.0.2.0 seems to have come from Launchpad for Ubuntu 24 (Noble), ie installed by apt-get install gr-limesdr. I’m reading the version number from the GR Companion console log:

device_handler:info:gr-limesdr version 3.0.2.0.

My application is a transceiver, ie a flowgraph with both a source and a sink. These must be both LimeSDR or both LimeSuiteNG (or a get a device busy error if I mix). I’d prefer to use LimeSuite source and sink, but get a very noisy signal out from the LimeSuiteNG sink. Under investigation, The signal is similar to that reported in Sine Wave Transmission: High Noise Issue with LimeSDR Mini 2.4 Compared to V1,1. I am confident I do not have mismatch in sample rates.