Time record, span and resolution bandwidth
The 89400 series analyzers eliminate a restriction inherent in previous FFT
analyzers by allowing you to choose span and resolution bandwidth independent of
one another. This implies a different method of handling the time record before the
FFT.
The 89400 series analyzers take a number of samples (time points) directly related
to span. These samples are acquired in a time length inversely related to the
resolution bandwidth. The FFT converts this time record to the frequency domain
and displays the result across the number of frequency points (display points)
selected by the user.
Measurement speed and time record length
The length of a time record is inversely proportional to the resolution bandwidth.
For smaller resolution bandwidths, an FFT analyzer needs a longer time record and
therefore takes longer to make a measurement. For larger resolution bandwidths,
an FFT analyzer needs a shorter time record and can therefore make a
measurement much faster. The different time length required will become
noticeable as you start making measurements. Refer to the earlier discussion:
“Enhancing the Measurement Speed.”
Fundamental Measurement Interactions
19 - 17
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