Hi, I tested my QA403 with an external unbalanced 1 kHz oscillator which has around -140 dB THD.
Attached the FFT plot. Question, what I see is this the limit of the QA403 ADC ?
A direct QA403 loop unbalanced gave a worse plot.
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Is this a Victor generator?
Yes, it is the Viktor Generator.
I have one too. I think my results are even a little bit worse than yours, but the harmonic pattern is similar. Perhaps the slightly higher distortion is due to using a step up unit to get 36 volt from a single 9 volt battery, but I never tried with 4 batteries.
can you show a link or a picture of the device (Viktor Generator)?
Greetings from Germany
This link shows the oscillator with pictures. It is not my page.
I use an external conventional mains transformer power supply. Needs to be 1 m apart from the oscillator since otherwise there will be mains magnetic hum induced. First I as you used a step up converter with li-ion battery. But this was giving switching residuals in the spectrum above 50 kHz. This was bad since I use still older analog distortion and level measuring instruments which go far beyound 20 kHz.
The spreading of the peak is spectral leakage of the Hann window - you’d need a much higher dynamic range window to lose those skirts. The inbuilt inbuilt windows can’t do it, but you can define your own. See https://holometer.fnal.gov/GH_FFT.pdf for some very high dynamic range flat-top windows.
-126.92dB THD is likely to be mainly the ADC performance.
It might be worth trying 12dB full scale and see if you can do better, a generous amount of ADC headroom can improve ADC performance.
Thank you for the information. For me the FFT plot is sufficient since my applications are not in this low dB region. It was just only to know the performance of my QA403 ADC. And it shows that an analog generator can outperform the DAC.
