HP 54710A User's Reference Manual page 333

FFT Menu
FFT Scaling
When the resolution is changed, the record length is set to a value that is a
power of 2. The FFT operates on the data points that are displayed on the
screen. If you have not changed the resolution or span, the number of data
points on the screen may not be a power of two. When the resolution is not a
power of 2, the FFT operates on the portion of the screen starting from the
left side that is a power of 2.
You can only change the resolution for channel operands in the real-time
acquisition mode.
When the Span or Resolution settings are changed, the time base scale is
automatically set so that the entire waveform record is displayed on the
screen. Also, the sample rate and record length modes are set to manual.
After you have finished with an FFT, it is a good practice to reset the sample
rate and the record length to automatic. That way the oscilloscope can
automatically pick the sample rate and record length to optimize the display
of signals on the display. You can either press the Autoscale key, or set the
record length and sample rate to automatic in the acquisition menu.
An FFT performed on a small number of points is computed faster than those
performed on a large number of points. An FFT performed on a large
number of points has better frequency resolution. The resolution affects how
accurately frequencies are measured and how well two frequencies that are
close together are resolved.
If the display is showing 2000 points and the sample rate is 1 MHz, the
resolution drops to the next lowest power of 2, which is 1024 points. The
FFT is then calculated from the first 1024 points on the display. If you want
the FFT to use more data points, increase the resolution (changing the
resolution from 977 Hz to 488 kHz increases the resolution) so that the
number of points increases to the next power of 2, which is 2048 in this
example.
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