HP 54710A User's Reference Manual page 332

FFT Menu
FFT Scaling
Magnify
The magnify softkey allows you to zoom in on a portion of the FFT record.
Use off to view the entire FFT spectrum, and use on to view a portion of the
spectrum.
The magnify softkey affects which two keys are displayed below it. When
magnify is off, the Span and Resolution softkeys are displayed. When
Magnify is on, the Magnify span and Center freq softkeys are displayed. The
Magnify span and Center freq softkeys use software expansion to rescale the
FFT.
Span
The Span softkey sets the maximum frequency of the FFT function. Span is
the sample frequency divided by two (Nyquist frequency). Changing the
span setting also changes the time base settings. You may notice that the
current sample rate is displayed below the span frequency selection. In the
equivalent-time mode, the effective sample rate is displayed. It is equal to
one divided by the time between points, and this number can be much larger
than the oscilloscope's maximum sample rate.
You can only change the span for channel operands in the real-time
acquisition mode.
Because the keypad does not have keys for entering kHz or MHz, use the
enter exponent key (Eex). For example, to set the span frequency to 125
kHz, press the following keys: 1, 2, 5, Eex, 3, Enter.
Resolution
The Resolution softkey sets the frequency resolution of the FFT record
(number of points). Resolution is the sample rate divided by the record
length. Because resolution is a function of record length, changing the
resolution changes the record length. You may notice that the number of
points used for the FFT computation is displayed below the resolution
frequency selection.
21–5