This is the ultra low distortion oscillator PC board described in the Pro Audio design Forum. "ULDO," the ultra low distortion oscillator portion of the board, generates 1 kHz tone with THD levels typically -140 dB below the "carrier' (oscillator output) level. -140 dBc is a THD level of 0.00001% or 0.1 parts per million.

ULDO's output is balanced with a 49.9Ω per leg output impedance. The maximum balanced output level (unloaded) is approximately +26 dBu with ±15V supplies.

The output level is DC-controlled by an external potentiometer over an approximate 26 dB range. An optional attenuator, "ATTO," is available to provide output attenuation to -70 dBu.

"Nacho," the notch filter portion of the PC board, extends measurement dynamic range.  Filtering the 1 kHz fundamental and adding 40 dB gain allows conventional audio-grade A/D converters and FFT software to measure distortion below the unassisted noise floor of the converter. A spreadsheet, available for download assists converting FFT levels to total THD.

Construction and assembly information for ULDO-Nacho can be found here: Ultra Low Distortion Oscillator and Notch Filter Assembly Instructions And Test

A Mouser Project Manager parts list is also available.