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🚀 The world's shortest HOW-TO - a parts-per-billion THD measurement

The whole chain, from "open the app" to "harmonics at −180 dBV", on one screen. Each step links to the full chapter if you want the detail. The green-play glyph marks a start button (generator, sweeps); the red-LED marks a record button (scope, FFT).

1 · Calibrate the chain (once)

  1. Start a sine and calibrate the DAC output voltage against a known level.
  2. Loop the DAC straight back into the ADC and calibrate the ADC full-scale.
  3. Measure the loopback frequency response and save it to a .frc file.
  4. Load that .frc in the Frequency-response view as a calibration and activate it - now the chain's own response is de-embedded from every measurement.

2 · Add the notch

  1. Plug the passive 1 kHz twin-T notch between DAC and ADC (why a notch).
  2. Tune the notch onto the tone with the tune-notch wizard (Tools -> Tune notch): set the target to your measurement frequency and trim the pots while the live sweep shows the notch's depth and how far its null sits from the target.
  3. Measure the notch's frequency response and save it to a separate file (you'll de-embed it too).

3 · Lock the tone

  1. Run a 1003 Hz sine - a few hertz off 1 kHz, to step clear of mains harmonics - with snap-to-FFT-bin on.
  2. Start recording the oscilloscope and the FFT analyser .
  3. Check the scope - the tone clearly above the noise, the trigger locked (use a little hysteresis so noise doesn't false-trigger).
  4. Set the FFT for your card's maximum sample rate - 2 M @ 384 kHz, 1 M @ 192 kHz, 512 k @ 96 kHz - with Get fundamental from generator on and Align generator = FLL.
  5. Watch ΔF settle to a tiny value (typically ±0.05 ppm) - that is the tone phase-locked and ready for deep coherent averaging.

4 · Pre-distort, then measure

  1. Open the DAC pre-distortion wizard - the wizard button .
  2. Enter the initial averages per round and a target THD, then Start - the loop drives the DAC's own harmonics into the noise.
  3. Save the result to a .dpd file and apply it to the generator.
  4. Switch the generator to "Sine compensated" - it now emits a tone cleaner than the DAC alone can make.
  5. Insert your DUT between the DAC and the notch.
  6. Reset the FFT statistics and start collecting .
  7. Wait for H₂...Hₙ to stabilise as the average deepens - and what you are reading is the device, not the source. 🎯

Live FFT: pre-distorted 1003 Hz tone behind the notch - THD H2..9 = 0.00000093 %

The payoff. A 1003 Hz tone behind the notch, pre-distorted, at ~730 coherent averages: THD H2...H9 reads 0.00000093 % - under one part per billion, the individual harmonics down at −160 to −185 dBV.

New here? The DAC pre-distortion chapter explains why every step above is the way it is - the notch, the manual fundamental, the deep averaging.