LimeSDR mini with tx_samples_from_file confusion

I am trying to use a limeSDR mini, via UHD/Soapy on OSX, and using tx_samples_from_file:
./tx_samples_from_file --file sample.bin --type short --rate 2500000 --freq 1575420000 --gain 20 --repeat
It starts up, but then I get this:
[INFO] [UHD] Mac OS; Clang version 9.1.0 (clang-902.0.39.2); Boost_106700; UHD_3.12.0.0-0-unknown
[INFO] [UHDSoapyDevice] Make connection: 'LimeSDR Mini [USB 3.0] 1D3AC988444E8A'
[INFO] [UHDSoapyDevice] Reference clock 40.00 MHz
[INFO] [UHDSoapyDevice] Device name: LimeSDR-Mini
[INFO] [UHDSoapyDevice] Reference: 4e+07 MHz
[INFO] [UHDSoapyDevice] LMS7002M calibration values caching Disable
Using Device: Single USRP:
Device: FT601
Mboard 0: LimeSDR-Mini
RX Channel: 0
RX Dboard: 0
RX Subdev: SoapyRF
TX Channel: 0
TX Dboard: 0
TX Subdev: SoapyRF
Setting TX Rate: 2.500000 Msps...
Actual TX Rate: 2.500000 Msps...
Setting TX Freq: 1575.420000 MHz...
Actual TX Freq: 1575.420000 MHz...
Setting TX Gain: 20.000000 dB...
Actual TX Gain: 20.000000 dB...
Checking TX: LO Locked: ...
Press Ctrl + C to stop streaming...
[FATAL] [UHDSoapyDevice] Tx Calibration: MCU error 5 (Loopback signal weak: not connected/insufficient gain?)

Can any LimeSDR Mini gurus help? (Or maybe it is a UHD/Soapy Guru?)
Cheers Hugh

If possible I’d suggest trying with tools that use Lime Suite native API, rather than going through UHD and SoapySDR. E.g. see: