The SDG2042X doesn't have the problem with 10MHz and 1Hz signals but it does have a phase accumulation problem when using modulation. What Shahriar is showing is a different limitation but it is well within the specs of the Tektronix AFG31000 series where it comes to the frequency accuracy. AFAIK the Siglent generators don't have a frequency accuracy specification so it is a guess whether these are better or worse.
I've heard of the phase accumulation bug but never looked at it, but in the context of this discussion I just was curious. It turns out there's something a bit weird going on with the SDG2042X and I'll have to think about this. If you set an output of 10MHz, that output will drift
relative to it's own clock (!) but as soon as you press the MOD key, even with 0% AM selected, the clock and output are in sync. So if you have a 10Hz pulse and 10MHz sine output, those will be coherent with each other (at first at least) but will drift relative to the clock,
even with an external clock input. Then if you select MOD on the channel with 10MHz, that channel will sync with the clock but the 10Hz pulse will now be adrift.
I suppose this belongs in that other SDG2042X thread but I can't find it right now. Can anyone else duplicate this?