dBFS is defined in AES Standard AES17-1998, IEC 61606, and ITU-T Recs.
P.381 and P.382, such that the RMS value of a full-scale sine wave is
designated 0 dBFS.
A full scale sine wave on HackRF has the range -127 to 127.
We calculate the full scale ratio relative to a signal in which both I
and Q are held at 127. This represents a signal with sqrt(2) times
the amplitude, and thus twice the power, of a full scale sine wave.
Therefore to obtain dBfs by the above definition, we add 3dB.
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