Is there anything I can do to change the record length in FFT mode to be smaller? Here's why:
I often use the FFT function (love the new improvements) with relatively low frequency spans < 100kHz (think audio frequencies), and then the fixed record size quickly becomes annoying.
For example, looking at a 24kHz span with a 10Hz resolution bandwidth, the resulting window size is only 2400 points, which at the proper Nyquist rate of 48kHz (which the scope chooses) takes 100ms to capture. (The scope actually chooses a different width depending on the chosen windowing function, e.g. 89.04ms for a rectangular window, but I suspect the effective RBW depends on that function.)
However, the fixed record size of 131kSamples (2^17) is much larger than that, taking around 2.7
seconds to fill up at 48kHz. If I choose a lower span or a better RBW, it can get into tens of seconds per update.
In the screenshot, you can see that the window is pretty small in comparison to the record. The good thing is that I can stop the acquisition and move the window around those 2.7s to look at the spectrum at different times, or change the RBW without recapturing, but most of the time I'd rather have the quickest update rate possible, especially for fast-changing signals.
Interestingly, it seems that the scope would do the right thing in Roll mode, but then it does not display the FFT at all anymore (I suspect because there is no actual trigger).