Hantek HDG2102B square wave not so square

[gnuarm]

While the overall shape of the magenta waveform may look like the second waveform qualitatively, if you do a quantitative measurement you will get a different result.  The ripple in the high and low levels is in the ball park of the seventh harmonic, not the third.  However, this is not the waveform of adding the seventh and first harmonics.  So the output is most likely the result of a slew rate limitation rather than a bandwidth issue.  They are not the same thing.

To me the magenta waveform looks basically like fundamental + 3rd harmonic, but with a lower amplitude for the 3rd harmonic. The presence of other (even smaller) components can hardly be judged by eye, but FFT were necessary. I rather do not see obvious signs of non-linear distortion due to a slew-rate limit.

It's an oscilloscope display.  It's easy enough to measure the period and frequency.  My measurement came up with about 6.something ns, so 160 MHz, far from the 3rd harmonic.  Not so far from the 7th.  BTW, the fundamental measures 24 MHz in that image, not 33 MHz matching the 30 ns that has been discussed.

[tkamiya]

So....  if the magenta trace is 25MHz with 9nS rise time, it makes sense that 30MHz at 10nS will be rounder than that.

Confirmed.  Thanks everyone!

[tautech]

So....  if the magenta trace is 25MHz with 9nS rise time, it makes sense that 30MHz at 10nS will be rounder than that.

No not round as the rising edge of a square wave should always be straight even when it is slow.

[tkamiya]

I think this is a language issue.  The magenta trace shows what I call a "rounding."  Even the straight part, there is a definite slope and slight inflection.  I got the answer I was looking for.  Thank you very much!

[gf]

While the overall shape of the magenta waveform may look like the second waveform qualitatively, if you do a quantitative measurement you will get a different result.  The ripple in the high and low levels is in the ball park of the seventh harmonic, not the third.  However, this is not the waveform of adding the seventh and first harmonics.  So the output is most likely the result of a slew rate limitation rather than a bandwidth issue.  They are not the same thing.

To me the magenta waveform looks basically like fundamental + 3rd harmonic, but with a lower amplitude for the 3rd harmonic. The presence of other (even smaller) components can hardly be judged by eye, but FFT were necessary. I rather do not see obvious signs of non-linear distortion due to a slew-rate limit.

It's an oscilloscope display.  It's easy enough to measure the period and frequency.  My measurement came up with about 6.something ns, so 160 MHz, far from the 3rd harmonic.  Not so far from the 7th.  BTW, the fundamental measures 24 MHz in that image, not 33 MHz matching the 30 ns that has been discussed.

Attached is a plot of

t = (0:1:99)/100;
x = sin(2*pi*t) + 1/6 * sin(2*pi*3*t);
plot(t,x);

I.e. fundamental + 3rd harmonic (no 5th or 7th).
Relative peak and valley positions on the timeline basically correspond to the magenta trace.

EDIT: And for comparison, the 2nd plot (plot-1+7.png) shows how fundamental + 7th harmonic looks like (-> sin(2*pi*t) + 1/6 * sin(2*pi*7*t))

[gf]

So....  if the magenta trace is 25MHz with 9nS rise time, it makes sense that 30MHz at 10nS will be rounder than that.

Basically yes. At least at/near the limit you rather need to compare frequencies in ratio to the sampling rate. I.e. you rather need to compare the 24 MHz @300MSa/s from the SDG to 20 MHz (not 30) @250MSa/s from the HDG.

The SDG has a clear advantage, though, in this regard. It has the higher sampling rate.
And besides that, its 4x interpolation/oversampling capability certainly relaxes the requirements for the analog reconstruction filter.
(Don't know if the oversampling is used for anything else - the docs don't seem to reveal that.)