Agilent Technologies 89410A Operator's Manual page 253

Fundamental Measurement Interactions
The time record and zero padding
At the minimum resolution bandwidth in zoom mode, the time length necessary to collect
the time samples is equal to the time record length required by the FFT.
At any other resolution bandwidth, the number of frequency points remains the same, but
the time required to collect the time samples is shorter than the time record required by
the FFT. Therefore, the time record is extended by the addition of zeros.
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