I made some tests with an SDS 1104X-E 6.1.37R9 (ext. to 200MHz) and an SDG 2042 X (ext. to 120 MHz).
Bode Plot II works very well within the audio range and more. For higher frequencies, impedance matched fixtures would work best, but this is not always possible and - anyway - better suited instruments would be needed for trustful measurements.
For practical purposes, I think that the supplied PP510 10x probes are fine up to a few tens MHz; better yet, low capacitance active probes would be ideal to minimize DUT loading.
First test: 10x probes on CH1 and CH2 directly clamped to the generator output.
This worked better than expected (to me), usable up to 65 MHz within +/-1 dB.
But, there's a problem.
CH1 and CH2 use the same ADC, and - as expected - there's no appreciabile skew.
The same setup with CH1 and CH3 (each on its own ADC) gives the following plots:
Here the phase begins to visibly change beyond 1+ MHz, due to a delay between CH1 and CH3. From the data table, we read -12.8 deg. @ 39.81 MHz, which corresponds to 12.8/360*1000/39.81 = 0.893 ns (CH1 is delayed with respect to CH3).
After setting up an 890 ps deskew [not sure about the sign] on CH3, this is the new run:
There is [almost] no change from previous plots, meaning that channel deskew is ignored by Bode Plot (or maybe I missed something).
Anyway, almost 1 full ns of skew between channels should never happen, so I attached the probes to the output of a 40 ps risetime L.Bodnar square wave generator to see what happens:
This confirms that CH1 is delayed with respect to CH3, but only by about 100 ps - not easy to evaluate.
Conclusion: Bode Plot II is a very handy and usable feature, but for a proficient use of the phase plot beyond a few MHz, it should be allowed to enter a delay parameter to each scope channel for manual compensation (automatic phase compensation at selected frequencies after a preliminary a test run would be great).
As a side note, the input attenuation (1x, 10x etc.) should be properly set
before entering the Bode Plot environment. I see that this setting is quite cumbersome:
1) select the desired channel
2) press button #4 "Probe nnX"
3) press button #1 "Probe nnX"
4) select attenuation using the Adjust knob
5) carefully press the knob without changing the selection
The most used attenuation factors are 1x and 10x by far, so it would be nice to alternate 1x and 10x by repeatedly pressing button #4. This way, selecting 1x or 10x would be much much faster.
That's all... Thanks to anyone who has had the patience to read this stuff