Spurs at +2MHz and -1.523 MHz (LIMESDR-USB)

Dear Friends,
We are working with RX1_L, we connected a 50 ohm termination at the input to determine if there are any on board noise sources.
We disabled the Transmitter.
At 60, 70 and 80 MHz input frequency, if the LNA gain is below 30 dB, there are no spurs.
At 40,50 60 db gain settings, we observe spurs at +2 MHz and -1.523 MHz.
The 2 MHz one is as strong as 13 dB above the noise floor (at 40,50,60 dB gain).
We guess that the 2 MHz Spur is a harmonic of the DC-DC Power Supply working at 1 MHz.
However we have no idea what the -1.523 MHz spur is (or where it is originated from).

Thanks a Lot in advance.