I know the earlier Siglent model SDG1000 had trouble doing this but they may just have improved the way they do it (e.g. not try and use sines at lower frequencies).
I realise that you've taken it apart so and have done a lot of analysis, so do you know how they generate square waves? They can't use the arb approach as the jitter
is a lot better than they get for arbitrary waves.
My comment was some amount loosy. I mean, they do not use comparator as they have done in SDG1000. This is sure.
9.999999ms period square and peak to peak around 1.5ns jitter where sdev is 66ps. Peak-peak 0.15ppm (0.000015%) and jitter rms <100ps. (0.0067ppm, period jitter sdev (rms) 0.00000067% )
It do not need any other proofment that this can not do from this period sinewave with simple comparator. If someone can, I want see this world 8'ths miracle.
But, also how they generate now Square, I do not know enough.
It is also possible that with different time period ranges there is some changes how it produce sqr.
Due to many other busy things I have not find time for look deeper.
Of course I'm interest to do it later somehow.
Also there is one undefined IC (only this chip markings removed) near FPGA and final DAC.
And becouse they have so much advertised this Siglent's new EasyPulse technology this chip is perhaps related somehow to this?
But more than how they do it internally in circuits, I'm interest now how it works for user. This is now priority. Accuracy and other things related to signal quality and also some amount of UI.
It is nice they have added push switch to this round control knob and now can jog between displayed signal parameter settings without need select with separate key..