Campbell Scientific CR9000X Measurement and Control System - Manual (Page 282)

Section 7. Measurement Instructions
7-58
FFT. To be sure the spectrum output by the FFT includes the lowest frequency
of interest,
low
f , set N (FFTLen) so that:
low
SR f
N
f
≤
.
Frequency Resolution
Frequency resolution goes hand in hand with the minimum frequency. The
difference between the center frequencies of adjacent spectral bins is
N
f
SR
,
and bandwidth of each bin is also
N
f
SR
.
For a given sample rate,
SR
f , if better frequency resolution is required (i.e.,
more bins, each covering a narrower frequency range) increase the number of
points in the FFT, N. If less resolution is required (i.e., fewer bins each
covering a wider frequency range) decrease the number of points in the FFT or
(to keep the minimum frequency from slipping into the DC bin) combine bins
as described below.
Spectral ReBinning
An FFT spectrum can be “rebinned” into a spectrum containing fewer bins
where each of the new bins contains a component that covers the frequency
range of the bins that were combined. The dc component (bin 0 of the original
FFT) is not combined with other bins but may be returned with a linear
rebinned spectrum. The first bin to be combined is the first ac component. Bins
can be combined in two different ways:
1)Linearly with the resulting bins all having a fixed bandwidth equal to the
distance between center frequencies of adjacent bins (as in a the spectrum
created by the FFT).
2) Logarithmically with the bandwidth increasing with frequency.
The mathematical operations to combine bins depends on the spectrum type
(SpectOption). Amplitude, RMS amplitude, and dB spectra are combined by
summing the power in the adjacent bins and then converting this summed
power to the desired spectrum type (amplitude, RMS amplitude, or dB). Power
spectral density (PSD) functions are combined by averaging adjacent
frequency-normalized bins into to give the frequency-normalized result.
Combining Real and Imaginary, or Amplitude and Phase spectra is not
allowed.
Fref and SBin are constants that determine the type of spectral binning. ILow
and IHigh are constants that determine which part of the rebinned spectrum is
returned.