On the design of very low distortion oscillators

A circuit is described when permits the realization, using only common ports, of an oscillator with a total spurious output of below -120 dB relative to the fundamental in the audio band or a tunable generator with output distortion rising to just above -100 dB at the ends of the audio band. Performance methods are described, and results are presented for a fixed 1 kHz oscillator output. At 1 kHz, the tunable oscillator exhibited only second harmonics. The second was 119 dB below fundamental. Distortions were 106 and 103 dB down, respectively. Second-harmonic distortion rose as the frequency fell or rose and passed -100 dB shortly before reaching 20 Hz and 20 kHz, respectively. The top end limit is believed to be the result of the falling opamp desensitivity, and the bottom end limit a result of the (relatively fixed) shape of the amplitude control loop filter