• ULDO-Nacho Ultra Low Distortion Oscillator and Notch Filter, Bare PC Board

This is the ultra low distortion oscillator PC board described in the Pro Audio design Forum.

"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.

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ULDO-Nacho Ultra Low Distortion Oscillator and Notch Filter, Bare PC Board

  • $69.95


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