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For the LEDs, take a look at the HDL and schematic to see how the LEDs are connected. The over/underflow LEDs are normally "on" (no over/underflows). When they turn off, an over/underflow has occurred (bad) and you may need to reduce sample rates, especially if operating in USB 2.0 or with a slow host processor. Unfortunately, there was a mistake in the the bladeRF x40 and x115 layout in which LED1 and LED2 are in the wrong order .. so just something to be aware of when looking at the LEDs. (This could be fixed in the HDL, but it hasn't officially been done, I assume, because it would cause mismatch/confusion between the schematic and HDL).