Campbell CR9000, Инструкция по эксплуатации

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Section 7. Measurement Instructions
7-40
is equal to power, an equivalent normalization to dB is
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
ref
P
P
10
log 0 1
where
P
is the value from the power spectrum, and
ref
P
is power reference
level. The square of the inverse of the multiplier parameter ( Mult
-2
) gives
ref
P
. The multiplier parameter of the FFTFilt performs two functions for the
dB spectrum option. The first function is to convert the raw signal
measurements from mV to the units in which the dB reference is specified, and
the second function gives the dB reference. For example, users may convert
signals from a microphone to sound pressure level (SPL) spectra in dB relative
to 20
µ
Pascals RMS, by setting Mult to:
RMS Pascals 10 20
6
−
×
k
where
k
is the microphone calibration in Pascals per mV. The FFT calculation
produces FFTLen/2 +1 deciBell components. ILow and IHigh, described
below, determine the number of values returned by FFTFilt . The dB spectrum
is unitless.
FFT Spectral Bins
The FFT calculation produces N/2 +1 spectral bins, where N is the number of
points in the original time series. These bins may contain a single value (i.e.,
amplitude) or a pair of values (i.e., Real and Imaginary). Each of these bins
represents a frequency range. Let
i
be the bin number, ranging from 0 for the
DC component to N/2 for the highest frequency range. The center frequency of
each range is:
( )
i
N
f
i f
SR
c
=
where
SR
f
is the sample rate of the time series processed by the FFT
(parameter FSampRate ), and
N
is the length of the FFT (parameter
FFTLen) .
( )
0
c
f
is the center frequency of the first spectral component
calculated by the FFT,
( )
1
c
f
is the center frequency of the second spectral
component, and so on.
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.