Double SFDR
Spurious-free dynamic range in dB: the ratio of the largest marked peak to the second-largest marked peak.
Returns: Double - Spurious-free dynamic range in dB.
Remarks:
With A_((1)) >= A_((2)) the two largest mark amplitudes, SFDR = 20 log_(10)( A_((1))/A_((2)) ) [dB]. Domain: single-sided amplitude spectrum with Marks identifying the fundamental and any spurs. Returns 0 when fewer than two marks are present or on internal error. Independent of zero padding.