I just found out the hard way that a too hot card (chip) may cause;
[ERROR] Rx calibration: MCU error 5 (Loopback signal weak: not connected/insufficient gain?)
This applies to SoapySDR writeSettings CALIBRATE_RX as well as in LimeSuiteGUI (the later may fail the calibration due to incorrect parameters as well, but this can easily be corrected with the reset/default buttons). LimeSuiteGUI may during the overheated condition produce other random errors as well.
Since the overheated card runs just fine once up and running, this error is quite hard to locate. Everything runs fine until you restart - and then the calibration fails. You perform some random pokes, and voilà - it starts working! Not because of your random pokes, but due to the fact the chips was given time to cool down… Hence, the next time you get this failure, same random pokes will accomplish nothing …
So keep cool and do not over-temp your LimeSDR. In my case I had missed to reconnect a fan after some hardware work, but as I understand this applies even to other hardware variants than the PCIe LimeSDR I’m using here.
Mounting the card vertically seems to be enough for passive convection cooling, but mounting the card horizontally with the chips facing down and it will overheat if not adding some explicit cooling.
A posting like this would have saved me a day’s work, hence this post.