Distortion is another concern with high frequency signals and can be specified as Spurious-free Dynamic Range (SFDR) or Total Harmonic Distortion (THD). SFDR measures the difference in amplitude between the signal and the largest spurs. At low amplitudes, SFDR is dominated by noise spurs. At higher amplitudes, SFDR is dominated by harmonics. THD sums all the distortion and all harmonics. The 2nd & 3rd harmonics usually contribute the most to THD because they are the largest.