This is the ultra low distortion oscillator PC board described in the Pro Audio design Forum. Includes the bare PC board, and THAT ICs with two THAT340, two THAT1246 and a THAT2180B. 

"ULDO," the ultra low distortion oscillator portion of "ULDO-Nacho," generates 1 kHz tone with THD levels typically -140 dB below the "carrier' (oscillator output) level. Expressed in the more familiar percent -140 dBc is a THD level of 0.00001% or 0.1 parts per million.

ULDO's oscillator output is balanced with a 49.9Ω per leg output impedance. The maximum balanced output level (unloaded) is approximately +27 dBu with ±15V supplies.

Oscillator output level is DC-controlled by an external potentiometer over an approximate 28 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. A balanced input passive Twin-T filter removes the 1 kHz fundamental. The notch filter is followed by a 40 dB gain instrumentation amp and common mode rejection stage allowing conventional audio-grade A/D converters and FFT software to measure distortion below the noise floor and residual distortion of the converter. A spreadsheet, available for download, assists converting FFT levels to total THD.

ULDO-Nacho has the following features:

• Balanced output. The balanced output can also be used to provide two opposite polarity unbalanced outputs.
• Low Distortion, typically -141 dBc at +20 dBu into 10KΩ; -138 dBc at +20 dBu into 600Ω. For low distortion performance at lower operating levels it's recommended that ULDO be used at approximately +20 dBu and attenuated with "ATTO" or an external pad.
• A balanced passive notch filter extends measurement A/D dynamic range. The balanced notch filter can also be switched or jumper-selected to provide two independent filters for simultaneous Left and Right measurements.
• A balanced instrumentation preamp provides 40 dB post-filter gain. The instrumentation amp can be switched or jumper-selected to provide two unbalanced preamps for Left and Right measurements.
• Common Mode Rejection after the preamp using cross-coupled THAT1246 allows balanced distortion measurement and provides a balanced filter output to feed an A/D converter.
• Output level is adjustable over a 28 dB range.
• DC-controlled output level can use multiturn (wirewound) panel pots for precise level control.

Mechanical and Power Specifications

• Board Size: 6.3" x 3.3" Mounting holes: M3 on a 6" x 3" pattern. Assembled height: Approximately 1" not including stand-offs.
• Power Requirements: ±15 to ±18V (max) approximately 40 mA with no load current. At the maximum output level of +25 dBu into 604Ω the average current is 56 mA.

Connections

• Inputs: Power, DC Level Control, Notch Filter Input.
• Outputs: Oscillator Output, Oscillator Monitor Output, Unbalanced Left and Right Notch Filter Outputs, Balanced Notch Filter Output.
• Notch filter configuration: Three jumpers to switch between two single-ended or a single balanced filter. A DPST switch may be used to switch modes.

Oscillator Specifications

• Frequency: 1 kHz ±2%.
• Maximum Output Level: Balanced +27.5 dBu 10KΩ load, +25 dBu 604Ω load (+25 dBu into 600Ω is 316 mW). Unbalanced +21.5 dBu 10KΩ load, +20.5 dBu 604Ω load.
• Output impedance: 49.9Ω unbalanced, 100Ω balanced.
• Distortion: At +20 dBu into a 10KΩ balanced load typically -141 dBc (0 dBc = +20 dBu). +20 dBu into 604Ω (+20 dBm/100 mW) typically -138 dBc. (-140 dBc is 0.00001%.)
• Output Noise: -90 dBu (typical) unweighted over a 20-20 kHz measurement bandwidth. Spot noise at frequencies greater than 1 kHz is typically -130 dBu. (FFT averaging is required to accurately measure ULDO's residual distortion.)
• DC level control: -0.4V to -12V for approximately -2 dBu to +26 dBu balanced output level. (Unbalanced 6 dB less.) Unbalanced Vout peak = 1.2 * Vcontrol.
• Outputs: Oscillator output has high-current ground return path. Oscillator monitor output is to feed notch filter or level meter.
• Oscillator start-up time: 20-30 seconds from power application to stabilized amplitude.
• Output level stability: Less than ±0.02 dB at +20 dBu.

Notch Filter and Preamp Specifications

• Notch Filter Frequency: 1 kHz adjustable to match oscillator frequency.
• Notch filter depth: Typically 60 dB. Depth is not critical to FFT measurement results as long as the measurement A/D is within its spurious free dynamic range.
• Notch frequency sensitivity to source impedance: Minimal. The source impedance appears as a common point impedance to the two "Wye" arms of the unbalanced Twin-T filters or the four Wye arms of the balanced input "double Twin-T."
• Notch filter input impedance: Frequency dependent. Single ended 100KΩ at DC, 53KΩ at 20 Hz, 12KΩ at 1 kHz, 3K8Ω at 20 kHz. Balanced values are twice the single-ended specifications.
• Notch filter balanced input common mode impedance: Frequency dependent. 50KΩ at DC, 30KΩ at 20 Hz, 27KΩ at 1 kHz, 23KΩ at 20 kHz.
• Post filter preamp gain: +40 dB balanced or unbalanced. Balanced common mode gain: 0 dB. Preamp inputs are AC-coupled with a 4.8 Hz cutoff.
• Notch filter and post preamp equivalent input noise: -114 dBu at the balanced output, 100Ω source impedance.
• Notch filter/post filter preamp common mode rejection ratio: >60 dB. (Balanced output.)
• Notch filter preamp output impedance: Left and right unbalanced outputs 49.9Ω. Balanced output 100Ω differenetial.

Note: Noise measurements are for a 20-20 kHz bandwidth. Maximum level specifications are for ±15V supplies.

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.