Campbell Scientific CR9000X Measurement and Control System - Manual (Page 280)

Section 7. Measurement Instructions
7-56
The difference between the center frequencies of adjacent spectral bins is
N
f
SR
, and bandwidth of each bin is also
N
f
SR
.
The results described above are returned by the FFTFilt Instruction when Fref
is set to zero, SBin is either zero or one, ILow is 0, and IHigh equals N/2.
ILow and IHigh refer to the bin numbers of the first and last bins to load into
the destination array. For example, if the number of points in the original time
series, N=1024 then the resulting FFT would have 1024/2 +1 = 513 bins
numbered from 0 to 512. To get the entire FFT, ILow would be set to 0 and
IHigh would be set to 512.
ILow and IHigh can be used to return only a part of the spectrum. For
example, If only the higher frequencies were of interest, say bin 200 to bin
512, ILow could be set to 200 and IHigh to 512.
In terms of frequency:
To limit the lower end of the spectrum, select a minimum frequency of interest,
low
f , and then set Ilow to: ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
low
SR
f
f
N
round ,
where
( )
x round is x rounded to the nearest integer.
To limit the upper end of the spectrum, select a maximum frequency of
interest,
high
f , and then set IHigh to: ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
high
SR
f
f
N
round .
Not saving the higher frequency bins is particularly useful if you are used to
using some of the rules of thumb on over sampling that evolved to avoid
aliasing higher frequencies present because of the prolonged rolloff of analog
filters. For example, suppose you are interested in frequencies up to 1 kHz. To
get a 5 times oversample, FSampRate of 5 kHz is used with FiltOpt sampling
ratio = 5 and N=1024. The bin containing the 1 kHz information will be
Round((1024/5000)x1000) = 205. Bins containing spectra beyond the filter
stop frequency of 5000/3.37 = 1484 Hz will be drastically attenuated (
≥ 90
dB). The bin containing the stop frequency is: I = Round ((1024/5000)x1484)
= 304). Set IHigh to bin 205 and only spectra up to 1 kHz will be returned. Set
IHigh to 304 get the spectra through the filter roll off but discard the 208 bins
containing spectra beyond the stop frequency.
The total number of spectral components (spectral pairs for real and
imaginary, or amplitude and phase, spectral options) loaded into the
destination array by FFTFilt is IHigh - ILow + 1. Note that the bin numbers
ILow and IHigh are not the same as the array index numbers of the destination
array. For example, with a single (1Rep) 1024 point Amplitude FFT, if all the
bins were returned (ILow=0, and I High=512) into the destination:
FFTResult(1), FFTResult(1) would equal the amplitude for bin 0,
FFTResult(2) = bin(1), … FFTResult(513) = bin(512). If ILow were set equal
200 and IHigh equal 512, then FFTResult(1) = bin(200), FFTResult(2) =
Bin(201), … FFTResult(313) = bin(512).