Campbell Scientific CR9000X Measurement and Control System - Manual (Page 277)

Section 7. Measurement Instructions
7-53
The CR9052 computes the Hamming window function from:
⎟
⎠
⎞
⎜
⎝
⎛
−
−
1
2
cos 46 . 0 54 . 0
N
k π
for
( )
1 0 − ≤ ≤ N k .
The CR9052 computes the Blackman window function from
⎟
⎠
⎞
⎜
⎝
⎛
−
+ ⎟
⎠
⎞
⎜
⎝
⎛
−
−
1
4
cos 08 . 0
1
2
cos 5 . 0 42 . 0
N
k
N
k π π
for
( )
1 0 − ≤ ≤ N k .
The Kaiser-Bessel window function is calculated using:
( )
β
β
0
2
2
1
2
1
0
I
1 I
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛ −
−
−
−
N
N k
for
( )
1 0 − ≤ ≤ N k
where
( )
0
I is the modified zero th order Bessel function and
where 5 ≥ β (integer) ≥ 16
Spectral Options
The CR9052DC supports the following spectral options. The first five spectral
options are the same as the CR9000X FFT instruction. RMS Amplitude and
Decibels are new for the FFTFilt instruction.
Real and Imaginary
The real and imaginary option returns the raw real (r) and imaginary (i)
components from the FFT. The FFT calculation produces FFTLen/2 +1 pairs
of real and imaginary components. ILow and IHigh, described below,
determine which of these pairs of values are loaded into the destination array
by FFTFilt.
Amplitude
The amplitude option returns the amplitude of each spectral component. The
FFT calculation produces FFTLen/2 +1 amplitude components. ILow and
IHigh ,described below, determine the number of values returned by FFTFilt.
The amplitude of a sinusoid represented by
( )
t A ω cos is A. The CR9052DC
computes the amplitude from:
N
i r 2 2 2 +
for all components except the dc and Nyquist components. The dc and Nyquist
components are computed from
N
i r 2 2 +
.
N is the length of the original time series (FFTLen). The units of the
amplitude spectrum are mV.