Time Base Settings For Fft's - HBM Genesis GEN5i User Manual

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The above selection is made in the Perception software in the Settings Sheet
► Mainframe ► Internal Clock Base and is therefore mainframe-wide, i.e. the
same for all recorders.
A binary clock base is a useful time base settings when doing FFT's (frequency
domain analysis).
9.4.2

Time base settings for FFT's

When doing FFT's there are two topics that affect the acquisition:
1 It makes life easier when the final FFT yields spectral lines with a distance
∆ f that is a "nice" value. Otherwise stated: the FFT bin size should
preferably be a nice value. Sometimes this is also called the "frequency
resolution". The bin size is determined by the actual acquisition length or
sweep length: bin size = 1 / T in which T is the total recording time. E.g. a
one-second sweep will result in a 1 Hz bin size, a 0.5 second sweep results
in a 2 Hz bin size.
2 Preferably the acquisition length is equal to a power of two. Fundamentally
most FFT algorithms work on data sets with a length of 2^N.
The binary clock base of the internal time base in combination with the division
factors allow for a broad range of values that meet both requirements. In the
table below various sample rates are given as well as the corresponding
division factor (divisor). The table shows the bin sizes that result from these
sample rates in combination with various sweep lengths.
Example: from the table you can read that a sample rate of 40.960 kHz and a
sweep length of 8192 samples result in a 5 Hz bin size, i.e. the spectral lines
are 5 Hz from each other.
"Nice" values are considered to be "minor" values that easily fit in "major" values
for (grid) display purposes.
In the table below the values are in the colored cells and basically comprise the
range 1.25, 2.5, 5, 10, 20.
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