Electronics > Projects, Designs, and Technical Stuff
Ultra-low THD generator, audio+ band.
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MasterT:
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?
coppercone2:
this might give you an idea

http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20with%20THD%20below%20-140%20dB.pdf
MasterT:

--- Quote from: coppercone2 on May 26, 2019, 03:05:10 pm ---this might give you an idea

http://www.janascard.cz/PDF/An%20ultra%20low%20distortion%20oscillator%20with%20THD%20below%20-140%20dB.pdf

--- End quote ---
I've seen this circuits earlier, as many others Wein bridge based oscillators. Unfortunately, almost all of them single tone, and introducing frequency adjustment would ruin their outstanding parametrics.
mzzj:
NEED or "need"?

Audio precision APx555 gets you almost there or does even better than -120db depending on voltage level.
About 30 000 usd, that should give you also idea of involvement if you want to attempt to build your own.
MasterT:
I came up with idea that it could be done using common audio DAC and good quality ADC. Monitoring sine wave output  from the DAC in real time,   I can feed forward antiphase THD data to the digital stream I'm seeding.  Locking the  control loop would eliminate distortion down to about +10 dB above the noise floor of the ADC.  My initial test ( CS4354 & CS5361 ) shows that idea works, with some limitation, control algorithm is slow and charges in frequency or magnitude demands some timing to stabilize the THD level back.
 Next step is to get noise floor down to -130 or so, and audio cs5361 is not the right part. Testing analog device (AD7984) can't get linearity level specified in DS, Thinking, I will straighten up this bitch first running data against ads1232.
Something like in two stages, linearizing ad7984 - slowly with oversampling using 24-bits slow adc but with linearity 20+ bits, than straighten up DAC at the next level.
Overall cost less than $100, including stm32H743 nucleo board, EVAL-ad7984 ADC and any CS or WM audio dac.

 
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