Maximum frequency is 120Mhz, according to the external connected generator I guess(SDG2000X for example).
SDG6000X got max 500Mhz, are there "natural" limitations which makes it not possible to get higher when the connected generator will "allow" this ?
Or can/will the maximum frequency in the menu increased someday ?
When Bode Plot was introduced in 2017, the 120 MHz limit has been defined because of the frequency range of the SDG2000X. The SDS6000X wasn't yet available back then.
Of course, when SDG6000X became available, a higher limit for the Bode Plot has been considered - and there have been even serious plans actually increasing it.
But at the same time we gathered experience with the existing implementation and it turned out that things aren't that straight forward. Keeping the phase equal between channels proved to be tricky, to say the least. It is hard enough to ensure equal phase at the scope inputs, when a coax length difference of only 4.5 mm already generates a phase shift of 1 degree at 120 MHz. But even when the users got their setup completely right, there were still inaccuracies in the Bode Plot itself. I do not know where they came from, because there were absolutely no visible errors in the time domain, but quite obviously some of the usual corrective measures weren't applicable for Bode Plot. We got phase deviations already within one ADC (channels 1+2 or 3+4) and all the more so when the reference channel was on one ADC and the measurement channel on the other (which might have been preferable because of even less crosstalk).
To cut a long story short, the error could be minimized, but not completely eliminated, see attached screenshot for the SDS2354X Plus.
SDS2354X+_FRA_Reference
Of course, I cannot rule out some residual phase shift in my test setup, and the 200 MHz bandwidth limit already starts to show an effect at 120 MHz, which might be slightly different for different channels, but the Bode Plot indicates that the error will hardly exceed about 1.5° at 120 MHz, because that is what I got for channel 2 when channel 1 was the reference. Channels 3 and 4 are worse, on the latter the phase error already exceeds 5°.
Under these conditions, we agreed that any higher frequency limit would take matters a bit too far.
Regarding the lower limit, this is another one I've requested a long time ago – and more than once.
The lower limit is 10 Hz because the errors introduced by AC-coupling can easily be handled down to this frequency. We don't want to use DC-coupling, because then we might have to take care of DC offsets (remember that Bode Plot utilizes vertical gain settings down to 500 µV/div), which might not even be constant during a single pass. This could cause enormous additional effort and headaches. Furthermore, it will be very slow, even without averaging. Yet averaging will be essential to get good results, especially at the very low frequencies, because 1/f noise will take its toll down there.
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