I had read through the whitepaper I linked to and connected the bodnar pulser to the scope again, in this case the DHO804(100Mhz).
Is it really the 100MHz model? Not hacked? There are no measurements enabled, but the risetime seems to be in the order of 1.6ns. This risetime rather suggests that the actual bandwidth is twice as much.
The signal from the bodnar is 10Mhz, so it should be interpolated cleanly, regardless of whether linear interpolation or sinx/x is used.
No, that's the wrong conclusion. It is not the fundamental frequency of the square wave which matters here, but the "smoothness" of the edges and corners.
But it doesn't, even at 1.25GSa/s you can already "discover" something
The superimposed persistent traces from subsequent acquisitions line up quite well and are not "blurred" (ilke in 312MSa.png), which suggests that there is no interpolation/reconstruction issue here. Most likely, the small overshoot is either already present in the original signal or introduced by the analog frontent.
and in the worst case, the 312.5MSa/s, it already looks pretty blurred.
Which is indeed an indication for a reconstruction problem at this bandwidth / sample rate combination.
So I fed in a square wave signal from the SDG2122X with the same frequency and amplitude. The only difference is that the signal is very "slow" in terms of rise time - 9ns, which shouldn't be a problem for the interpolation, and lo and behold, it is.
IMO It is not. Again, the superimposed persistent traces are not "blurred" (ilke in 312MSa.png), which suggests that there is no interpolation/reconstruction issue.
(Why the "roof" looks so "wobbly", I'll have another look tomorrow).
Check the waveform also on a different scope. I could well imagine that the top is not perfectly flat. A square wave is not an easy task for an AWG either, which is based on Nyquist-Shannon reconstruction from samples.
Now I also understand the behavior of the DHO4204, thanks for the food for thought@rf-loop.
I still do not understand it. Your doubt was well justified when you asked why the rise time of the 2Gsa/s trace was shorter than the rise time in the 4Gsa/s trace. This suggests that the front-end was running at a higher bandwidth when capturing the 2GSa/s trace than when capturing the 4GSa/s trace. But this would not make sense.