The postman just brought me one of these "toys" as well and, unfortunately, I've got to chime in with the same findings: The Bode Plot function is pretty much useless as it is right now!
I found the same "wigglyness" of the traces while at least a monotony should be shown in case of a simple low pass. Maybe they designed too many "wiggles" in the traces on the instrument's PCB...
Whatsoever, there are several points that are possibly worse (but easier to get right):
- The Bode Plot function won't start (and it also won't display any message) if the signal exceeds the range of the "Output" channel (didn't test this for the "input" channel). So it may be necessary to manually adjust the sensitivity of the channels correctly before starting the Bode Plot.
- The preset number of points relates to a decade, not to the whole measurement interval (this may be a convention and I'm not aware of it).
- The setting for the measurement frequency spacing ("Line"/"Log") only relates to a decade, within the next decade, the intervals may change even if the spacing is chosen "Line".
- The on-screen display of the plot is abysmal: Even though the markings suggest a Logarithmic frequency scale, the values are placed linearly within each decade which results in kinks in the diagrams, regardless of Lin/Log setting, see RigolDS7(log) and RigolDS8 (lin)
! I'm not sure if the situation is the same for Amplitude values since the scale of the Y-axis apparently cannot be adjusted to a range that allows a better resolution. Same for the frequency axis, only automatic scaling is available.
So right now, for displaying the bode plots, you've got to take the route via the CSV output. If you think, in its current configuration, the DHO900 is a good tool for high resolution Bode Plots, think again... I'm actually considering to return the instrument since this function was the one that I'ld needed the most for a project I'm currently working on. I'm pretty sure Rigol will get it right eventually, but how long that may take is written between the stars... I guess for the time being, I'm better off with my ADALM2000 and the "Scopy / NWA" tool.