Going to get a bit preachy here and suggest that just about everyone in this thread spend some *significant* time studying basic digital signal processing theory in general and Fourier transforms in particular. It's a shame to see people criticize an incredibly awesome piece of gear because it doesn't support a "25Mpt FFT" or some other such ridiculousness.

Good idea.

Most importantly, the inter-dependencies between FFT sample size, sample rate, BW and frequency resolution, for example

*df = F*_{s}/N (df = frequency resolution; f

_{s} = sample frequency, N = number of acquired points)

and

*f*_{max}<f_{s}/2 (f

_{max} = upper BW limit)

In short, the frequency resolution ('RBW') is dependent on the acquisition time, which again depends on the size of the memory and the sample rate. Lower sample rates allow longer acquisition times but also come with reduced BW due to Nyquist-Shannon laws.

For FFT to capture the full analog BW (6GHz) the sample rate must be at least 12GSa/s, a step that (if I remember right) doesn't exist on the DSOX6004A, with the next available sample rate step being the max of 20GSa/s.

With a 1M FFT sample size, the scope can capture 50us, which gives it a frequency resolution ('RBW') of 20kHz. To go lower, i.e. to 10kHz, the sample rate must reduce (10GSa/s which is the next lower step), which reduces the usable BW to <5GHz, <2.5GHz (5kHz), <1.25GHz (2.5kHz) and so on.

With 25M FFT size, the full sample rate of 20GSa/s could be maintained down to an RBW of 800Hz.

It should now be clear why most high bandwidth scopes use long memory for FFT and why having 25Mpts or more for FFT isn't as ridiculous as you seem to think (modern high BW scopes can typically use >100M for FFT). Of course Keysight knows this, too, because all their high bandwidth Infiniium scopes can use long memory for FFT, and have been for ages (even my old DSO8064A with 4GSa/s and 1.2GHz real BW uses up to 32Mpts for FFT). The reason the DSOX6004 doesn't lies in the limitations of its MegaZoom ASIC and it's (comparably) slow platform.

It doesn't make the DSOX6004 a bad scope. It just means that looking at the frequency spectrum is important then I'd look for an alternative, i.e. a cheap SA.