Campbell CR9000, Instruction Manual

Page: 158 / 218

Section 7. Measurement Instructions
7-44
(parameter FFTLen) , and
bin
S
is the number of bins to combine (parameter
SBin ).
The difference between the center frequencies of adjacent spectral components
after linear spectral rebinning is
bin
SR
S
N
f
, and bandwidth of each spectral
component (except the dc component) is also
bin
SR
S
N
f
. The bandwidth of the
dc component is
N
f
SR
.
As with the original FFT results, ILow and IHigh . determine which part of
the rebinned spectrum to return. To return the entire spectrum, set ILow to its
minimum value, 0, and IHigh to its maximum value. The maximum IHigh is:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
×
bin
S
N
2
floor
where the
( )
x floor
is the largest integer that is not greater than
x
. To limit
the lower end of the spectrum, users first select a minimum frequency of
interest,
low
f
, and then set ILow to
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
+
×
2
1
1
round
bin
SR
low
bin
S
f
f N
S
,
where
( )
x round
is
x
rounded to the nearest integer. To limit the upper end
of the spectrum, users select a maximum frequency of interest,
high
f
, and then
set IHigh to:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
+
×
2
1
1
round
bin
SR
high
bin
S
f
f N
S
.
The total number of spectral components returned by the FFTFilt instruction is
IHigh - ILow + 1.
Logarithmic Spectral ReBinning (1/n Octave Analyses)
Logarithmic spectral rebinning combines the spectral components from a
variable number of adjacent bins into a single component of the final spectrum.
The number of bins that are combined increases logarithmically with
frequency. FFTFilt is programmed to return a logarithmic spectrum by setting
Fref to a non-zero value and SBin between one and twelve. The parameter
SBin determines the number of bins per octave in the rebinned spectrum. An
octave is a factor of two increase in frequency.
The dc component is never part of the final logarithmic spectrum.
Let
i
be the bin number of the rebinned spectrum. The center frequency of
each spectral component with logarithmic spectral binning is
( )
bin
S
i
ref c
f i f 2
=
for
high low
i i i
≤ ≤
where
ref
f
is an arbitrary reference frequency selected by the user (parameter