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Ultra-low THD generator, audio+ band.
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MasterT:

--- Quote from: blueskull on May 26, 2019, 10:51:50 pm ---What kind of flexibility do you need? If you only need a few frequencies, a consumer audio DAC plus banks of selective LPF or BPF could be a good idea.

This is a typical trick used in many audio testing setups to allow measuring THD below the capability of the hardware.

--- End quote ---
Objective is to make a tool, THD meter in this context to get kind of the chart below.
I start to play with audio DAC since it's relatively cheap, but my intention is a measurements with high precision in general, not audio related. For evaluation purposes of the OPA, capacitors, different topology of the schematics.
For example, I start to dig into internal stm32h743 16-bits ADC, got high noise+ THD level but I'm not completely sure that buffer OPA to drive ADC is behaving as I expect. Nothing is linear at 3.6 MSPS, having cranking  20-47 pF caps at the output, so I need a tool that would confirm THD level loaded-interfaced  OPA is the same as non-loaded in real-time over different sampling rates, frequencies etc. THD distortion that I observed is not related to a  wrong part number (OPA), not capable to drive specific ADC. That ADC itself generates a distortion.


MasterT:
It's an another example (Figure 34. Distortion Test Circuit, from opa1612 DS), how DO NOT to measure THD.  OPA with 10 OHm at the input has different characteristics compare to the one with 10k. OPA with MUX, switching not very fast, a few kHz is not the same one that driven by pure sine wave from the ideal source. 
It's all about interfacing. There may be two parts, that have very good performance in the lab test environment,  but all beauty disappear  at the moment as you try to connect one to another.  So, I 'd like to film this moment on the LCD screen of the THD meter, when one line somewhere at -100dB jumps up to -60 because some poorly educated tech from Digilent has no idea about X5 and COG caps. They put cheap ceramics to their board
https://reference.digilentinc.com/reference/pmod/pmodi2s2/reference-manual deeply disappointing me with -40dBc THD-2.
 
floobydust:
Rather than re-invent the wheel, look at Bob Cordell's High Performance THD analyzer from Audio 1981. It's all analog with lots of parts and uses spot frequencies.
"... measurement floor below 0.001 percent out to 20 kHz."

It has also been updated and discussed a lot on the DIY Audio forums.

Then he has the "Distortion Magnifier" that can use a PC sound card and is as good as that and the op-amps used, I think -140dB is the noise floor. You may find the project details on the Internet.
texaspyro:
A rather nice analog low distortion oscillator:

http://w140.com/tekwiki/wiki/SG505

Companion audio analyzer:

http://w140.com/tekwiki/wiki/AA501
amirm:

--- Quote from: MasterT on May 26, 2019, 01:25:32 pm ---For my new project I need 0.0001% THD sine generator 20 : 20000 Hz (better 20 : 100 000 Hz). I already find out, that audio codec available on a market can't perform below 0.001% in the best case scenario, most of them hardly passing 0.005%.
What options do I have? DDS using 20-bits DAC? In that case where to get DAC with INL 1 LSB at 20-bits resolution?

--- End quote ---
I have measured a number of DACs that come pretty close to your spec.  See: https://www.audiosciencereview.com/forum/index.php?threads/master-sinad-distortion-comparison-graph-for-dacs.4814/

SINAD of 115 dB translates to 0.00018% so pretty close to your requirements. 

If you can tolerate 0.0003%, the Khadas Tone Board will do the job at less than $99.  Has USB input so very easy to drive.  You can see my review here: https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-wesiontek-khadas-tone-board-dac.4823/

Amazon, Digikey, etc. carry it.

My Audio Precision analyzer goes beyond your specs but will cost you $28,000.  :)
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