New LimeSDR Mini - loopback test failed

Projects LimeSDR
2

Something doesn’t add up here. Most people note that their boards “appear to work,” yet LimeQuickTest is intermittently reporting failures. These failures are usually on the same channel/frequency combo, e.g. CH0 (SXR=1000.0MHz, SXT=1005.0MHz) on the LimeSDR Mini, or CH1 (SXR=1800.0MHz, SXT=1805.0MHz) on the LimeSDR.

It doesn’t make sense that one particular transmitter or LNA channel at one particular frequency would be more prone to damage across multiple boards. If this is a manufacturing failure, I would expect other channel combinations to fail more often, but this doesn’t seem to be happening.

If there is actual damage, I would expect the tests to consistently fail. Intermittent failure is something I would expect from a cold solder joint, not a damaged part of an integrated circuit.

There are claims that the loopback tests in LimeQuickTest are supposed to be done on a cold board.
This makes no sense to me. A loopback is a switch, not an amplifier or a filter with temperature sensitive analog components. How could you possibly have a loopback test that is designed to only work on a cold board? Either you have a loopback that works at all reasonable temperatures, or you don’t have a loopback. The LMS7002 datasheet also mentions a loopback amplifier, but if this is the temperature sensitive component, you’d see the output changing by a few dB at most, not -14 dBFS at 5 MHz to -45 dBFS at a completely different frequency.

I’m starting to suspect that there may be a flaw in the way the loopback test is performed by the LimeQuickTest utility. Perhaps the delay between consecutive tests run by LimeQuickTest is marginally too short. If the board isn’t finished configuring itself by the time the next test is run, it’s conceivable that it could return garbage more frequently for one particular test than for any of the others.

Here are other threads documenting similar loopback test failures on the LimeSDR Mini:

Here’s are some LimeSDR (non-mini) loopback test failures that look similar:

There are many more, but you get the point.