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| Siglent SDG1032X sine distortion at 1 kHz ? |
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| 2N3055:
--- Quote from: electr_peter on December 29, 2022, 08:20:49 am ---Simulations showed that ADC dither is not an issue (noise is high enough to hide quantization effect), but small non-linearities in AFE could cause FFT to show "comb". Assuming non-linear behavior is stable over time, it can be estimated by downloading raw time domain samples and comparing with "ideal" input signal. Not sure if at such low distortion level it is possible to do a digital non-linear correction after the fact or whether it is worth the effort, this requires more modeling. Of course, correction would be scope and input setting specific and would require very pure input signal. FFT view at different input levels and SDS HD scope settings could get small insight at which AFE part causes distortion, maybe there is a sweet spot somewhere. FFT shows something non-linear from AFE because noise is so low. IMO that's better than to drown (hide) distortion in high noise floor. --- End quote --- Simulations were not necessarily needed to gain that knowledge. We already knew there is enough noise for dithering. Nothing wrong with verifying. It is nice when theory and practice agree. Nonlinearities introduce harmonic and intermodulation distortions, phase delays and deform BW flatness,that is how it works, correct. Digital non-linear correction is possible in theory but not practical because of sheer volume of real-time data and needed models. As you go through ranges, various levels of attenuation and PGA gain is used in combination. Any of those combinations and different drive levels would create different distortions. There are many sweet spots in attenuation ranges, especially if you go from coarse to fine gain settings. I absolutely agree with last statement (this is what I said before): only reason why people see these spurs is because noise floor is good. |
| electr_peter:
--- Quote from: 2N3055 on December 29, 2022, 09:32:11 am ---Digital non-linear correction is possible in theory but not practical because of sheer volume of real-time data and needed models. As you go through ranges, various levels of attenuation and PGA gain is used in combination. Any of those combinations and different drive levels would create different distortions. --- End quote --- Checked possibility of digital non-linearity correction, IMO it's not worth the effort. 1) Correction requires non-trivial amount of math operations, proper calibration even more so. 2) Actual distortion is small, but it is non-linear and variable/unstable. Your could be chasing ghosts in AFE during calibration with questionable stability. 3) Adjustment effect is small and almost invisible in time domain for most cases, but it will have some impact on FFT at low signal levels. Too much effort/cost for small improvement in case of general purpose DSO, better to focus design effort on a more linear AFE instead. |
| electr_peter:
--- Quote from: 2N3055 on December 28, 2022, 04:30:44 pm ---So it's a limitation that oscilloscope has spurs 20 db below it's native dynamic range... Who knew it works that way... --- End quote --- In case of FFT spectrum view, number of points in FFT calculations has an effect on noise floor. For \$N = 12\$ bit ADC and \$L = 4096\$ point FFT, FFT noise floor is combination of signal to noise ratio (\$SNR\$) and FFT processing gain $$FFT\hspace{1ex}noise\hspace{1ex}floor = SNR + processing\hspace{1ex}gain = 107 dB$$ where $$SNR = 6.02 \cdot N + 1.76 = 74 dB$$ and $$processing\hspace{1ex}gain = 10 \cdot log_{10}{(L/2)} = 33 dB$$ With bigger number of data points for FFT noise floor is lower. This is related to SA where narrower \$RBW\$ results in lower noise floor. Alternative view is to think about relationship between power spectral density (\$V_{rms}/\sqrt{Hz}\$) and power spectrum (\$dB\$). Reference: Analog Devices, Taking the Mystery out of the Infamous Formula, "SNR = 6.02N + 1.76dB," and Why You Should Care |
| RoV:
Made a test on my SDG2042X (ok, 2122X ;) ). Used an E-MU 0202 connected to my PC, acquiring ~20 s at 48000 Hz sampling, 24 bit resolution. Generator at 1 kHz sine, 1 Vpp output. Acquisition gain set to have about 80% level on sin peaks. Processed with Welch method, using a Blackman window of length 4800 to minimize sidelobes: result in figure, with fundamental normalized at 0 dB. Highest harmonic is 2nd, at -96.3 dB. Then 3rd is at -105.5, 4th at -105.2, 5th at -105.9. |
| RoV:
Test repeated with a lower (~half) gain setting on the E-MU 0202, because I was suspicious about the 2nd harmonic. Now the 2nd is at -103.5 dB, the 3rd as before. Worst is the 10th at -101.9 dB. THD, computed up to the 10th harmonic, is -97.5 dBc, or 0.0013%. |
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