Now that I have an Output Attenuator prototype I wanted to look at the low-level performance of the A/D to make sure it has enough spurious-free dynamic range at low level to accurately assess the notch filter output.
The following FFT has the attenuated oscillator output compared to the notch filter output sampling the unattenuated +22 dBu output of the oscillator.
The FFT is 1M point with a 50 sample average.
The blue trace is the left channel of the Focusrite 2i2 connected to the output attenuator's -20 dB position. (About -40 dB actual.)
The spurious-free dynamic range (SFDR) of the A/D at -20 dBu level is about -115 dBc. (H3).
The red trace is the notch filter output.
There is 40 dB of post-filter gain; the reference level is about +22 dBu.
Thus, the notch filter depth is about 100 dB.
The resulting level is about -38 dBu at the converter input.
The harmonics in the red trace are from either the oscillator itself or the oscillator and notch filter.
The largest spur, H3, is only about 57 dB below the 1 kHz notched carrier.
ULDO is running very near its clipping point which is around +26 and the higher-order harmonics in its output (or the notch filter) can be seen.
Despite this its THD is an astonishing, almost too-good-to-be-true level of around -151 dBc.
With THD typically running -140 to -146 dBc I suspect that there is some distortion cancellation occurring.
But, to be honest, I've never measured the oscillator at such high level at the 2.26KΩ load impedance of the attenuator with a 1M point 50 sample average FFT.
I was content to say distortion was in the noise floor - now I'm looking for needles in haystacks.
Without regard to the oscillator's THD (-140 or -151?) it appears that the A/D, at low level, has plenty of SFDR to measure the notch filter preamp output.
A more fair comparison would be to check the A/D at a lower level comparable to the -40 dBu notch level but I think it would be in the noise.
The combination of ULDO, Nacho and the 2i2 has the dynamic range to accurately measure THD in the range of -100 to -120 dBc to perhaps -130 dBc.
With a DUT having distortion at -120 dBc ULDO-Nacho has a measurement range that is at least 20 dB better.
ULDO-Nacho FFT
Blue Is Output At -20dB_Pad Feeding Focusrite 2i2 Left Input
Red Is Notch Out With 40 dB Gain at A +22.24 dBu Ref Level Feeding A/D Right Input.