Then other thing. SSA specs start 9Khz. Using SSA trace B, I have done some kind of "level correction" curve. Yes this is not now rocket science... only just for fun and make SDS FFT bit questionable for this purpose. Look these "harmonics" in SDS image specially after 7th . What generate these all peaks. It is sure they do not come from generator - least not even close this level.
This requires further research. It also requires different methods and tools.
We must remember that the DSO FFT is created from data gathered from the ADC, be it 8, 10 or 12 bit core. Whereas the SA is from a swept type measurement. Recall most SA that use the swept mode also feature a log amp which has a large DR, more so than conventional linear amps, and possible input scaling after the passive LPF or BPF in the signal processing chain.
Also, the FFT is not benefiting from the dynamic scaling like we see with the FRA/Bode modes of these DSOs, it just the preamp and ADC at a fixed scale factor, and thus limited by such since the "log" dB scale is just a computation from the ADC raw data and not "preprocessed" before ADC conversion like the Log Amps & Scaling in a conventional SA.
I'm not surprised the FFT shows additional "artifacts" & various different harmonic levels over the conventional SA with it's swept capability.
Agree, the THD may not be an attractive added feature for signals with low THD, likely limited to something on the order of the core ADC and input amplifier linearity, so maybe 40~50dB, and thus not very attractive for any low level distortion characterization.
Like the use of the
Trace B correction display to revel the error when the SA is operated below the lower frequency spec limit
Edit: Should also add that these signals in question here originate from a 14 or 16 bit core DAC, whereas the DSO has a 8, 10 or 12 bit core ADC. In signal processing terms this is bass-ackwards (old engineering term to highlight a backwards condition
), the measurement should have a higher effective resolution/precision than the signal of interest if one expects the results to be meaningfull regarding the "quality" of the original signal.
Best