A patented instrument has been developed for measuring SNR, SINAD, and THD for DACs and ADCs. For DACs, a synthesized source bucks the sine-wave source being measured. For ADCs, the sine-wave source is the input to the ADC under test. The quality of the THD measurement is limited by the harmonics generated by the source (less than −120dB). A technique has been developed to further reduce harmonics from the source. The mathematics that is used to reduce the harmonics is described. A technique for measuring a source's THD independent of the internal source purity is demonstrated. Experimental results demonstrate that spurs can be reduced to −140dB relative to the fundamental. The technique is also applicable to non-harmonic spurs. A simulation technique using equiripple polynomials was developed to evaluate the algorithms.