In any case, the claim that R&S made that I expanded on was that Mpts should not be used as a figure of merit on modern FFT implementations. Using it to describe a limitation is like looking at a racecar and saying "pff, it's a four-wheel car." The categorization misses the point even if three wheels is a dumb idea and some applications demand six. My real objection was that every time I hear the drivers talking about acceleration and braking distance I shortly thereafter hear "pff, four wheels" and that's a too.
I get your point, however what you ignore is that on a DSO, the memory depth in FFT mode is actually relevant because, together with the sample rate, it determines the frequency resolution we get.
For FFTs, resolution bandwidth is equal to the sample rate divided by the number of points in the FFT. That means to get a more narrow RBW the scope must acquire and process more samples.
I'm away from home for the next two weeks otherwise I'd have posted some demo screenshots.
It's also possible that I'm simultaneously missing your point.
I guess so. You're arguing from a mathematical POV which is not wrong, and I am well aware that you can perform a DFT in different ways, some of where memory requirements don't play a large role as FFT as the way it's done on a scope, but since scopes don't work that way this isn't very relevant to the topic at hand.
It shouldn't come as a surprise that wideband SA architectures have similar implementations in both radio telescopes and MDO oscilloscopes: 8 bit N GS/s ADCs going into FPGA/ASICs implementing a "FFT+" with extra attention paid to spurs, linearity, and phase noise.
And still, neither is any better as with FFT on a decent scope (and phase noise of the "SA" in my MDO3k was even slightly worse than my 3GHz scope at similar settings).
I'm far from an oscilloscope expert, but I spent enough time playing with the very type of SA implementation that Tek is using -- in matlab, simulink, paper, and FPGAs -- that when I hear "it's just an 8-bit 1MPt FFT" I get a highly unfortunate case of someone-is-wrong-on-the-internet disease
You said you spent enough time playing with the very type of SA implementation that Tek is using, but how do you know what principle Tek uses in the MDO3000 SA?
You continue to claim that the MDO3k's "SA" mode works differently than scope's FFT, but so far you have not supported that claim with any facts or evidence.
So please, prove me wrong. Show me how Tek has managed to cram two completely different FFT implementations into the MDO3k which still perform exactly the same (BW aside, as this is limited by the inputs)?