LimeSDR curious transmitted noise problem

I use a LimeSDR-USB (not the mini) for various ham and general radio stuff. A long standing oddity is that sometimes on transmit it generates wideband noise up to half sample rate away from the TXLO. From starting it streaming about 20% of the time it does this, the rest of time its fine. Once doing it I can start and stop TX streaming and it does it all the time until I stop both TX and RX and start again. At all times the RX output is fine.
I can reproduce this effect on third party apps like SDRAngel as well as a C# test program using API calls that I have written to investigate this. Two pcs do the same. It seems to be something to do with the TX up-sampler, but I have now discovered that one or two registers accessed through the API seem to affect it.
Mainly 0x00A6, the later Limesuite dlls set this to 000F on Init but if I set this to 0001 the problem disappears. One or two other obscure registers seem to have an effect but none as clear and repeatable as A6.
I am beginning to think this is an odd hardware fault, although LimesuiteGUI and LimeQuicktest do not show any problems. It will happlily do both TX and RX calibration and there are no API fail errors. The board is externally powered and the power line is stable and noise free.
I hope I have attached a couple of spectrum analyser pictures showing good and bad start-ups. Has anyone else ever seen anything like this? Looking through the forum I can’t see anything that quite fits.
Mike Christieson


Tagging @Zack.

That looks like either clipping in a filter or data drop-outs to me. If you can, switch your analyzer to zero span with a resolution bandwidth of the sample rate of the TX, and see if you see drop-outs in the signal. Also try reducing the signal level you are feeding into the processing chain, and see if the noise goes away. If you see drop-outs in zero span, that suggests you are dropping data. If reducing the digital signal removes the fuzz, that suggests you are clipping in the digital processing chain.