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
My thesis from May 2010 that covers the design of a low distortion oscillator with a frequency range of 3 Hz to 300 kHz. This first-generation design has several weak points (for example the chosen compensation of the amplitude control loop results in slow settling) and parasitic oscillation of the integrators prevented proper operation over the full frequency range. Nonetheless the text can be a useful starting point for your own design.
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
Low Distortion Oscillator Performance
This document shows the performance measurements I have carried out on my second-generation low distortion oscillator finished in June 2017.
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