Low Distortion Oscillators
To measure harmonic distortion, and other related measures such as THD+N, a sinusoidal oscillator is required. It should have very low distortion and noise, such that it does not limit the measurement resolution. Achieving this is a very demanding task, particularly if fast settling after a frequency change is also required. Below some findings from my studies of this topic.
Low Distortion Oscillator Design Master Thesis
Low Distortion Oscillator Comparison
The following file contains some comparative measurement for the Tektronix SG 505 and the Audio Precision System One, SYS-2722 and APx555: low_distortion_oscillator_comparison.pdf
Passive Notch Filter
A fixed frequency passive notch to measure distortion at very low levels.
Gerber files for this design can be downloaded here: passive_notch_filter_r1.zip