Something interesting about the SDS800x HD is that the Time Base is in the time domain is separate from the Frequency Domain Time Base (FFT). This is only seen in the top models of class "A" manufacturers. Siglent has "arrived there"...
This is very interesting for the analysis of low amplitude RF signals.
I have verified, through articles and images, that the SDS1204X the two time bases are linked. To "see" the FFT, you need to multiply the time base in time domais by 1000, to be able to see the well-defined FTT ranges. In the SDS800X HD, it is separate. Including numerically marking the FFT peaks, and being able to edit which ones should appear in the "Peak Table" is exceptional.
If anyone can, please clarify whether other models od Siglent above the SDS800X HD have this possibility of having separate time bases.
The only "separate timebase" which comes into my mind is the timbase of a zoom window, because a zoom window has its own time/div. Can you explain where else you see "separate time bases"? The main time base (and the derived sample rate and record length) applies to both, time and frequency domain. You cannot set these parameters separately for time and frequency domain.
Note that the time/div value is irrelevant for the FFT anyway. What matters for the FFT are
sample rate and
number of points. However, with automatic memory management, you cannot set time/div, sample rate and record length independently, but they are linked. IOW, if you want to set a sample rate and record length suitable for a "nice" FFT, then you can only do that by adjusting time/div.