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National Instruments Corporation
27
NI Spectral Measurements Toolkit User Guide
emphasis on the most recent data. The
averages sofar
parameter stops
incrementing at
N
while the averaging continues.
Linear weighting includes the following modes:
•
One-shot linear averaging—Average one time for the specified
duration of
N
measurements. When the duration is over, the averaging
stops.
•
Auto–restart linear averaging—Automatically repeat the one-shot
linear averaging after every
N
measurements.
•
Moving average—Average the most recent
N
measurements.
•
Continuous—Average all measurements taken with equal weight.
Averaged FFT Spectrum
The following equations describe the three averaging methods applied to a
complex FFT spectrum.
The RMS and peak-hold averaging methods produce real spectra, and
vector averaging produces a complex spectrum.
All the averaging operations in the Spectral Measurements Toolkit operate
on a complex FFT input spectrum. Create an averaged FFT spectrum using
the SMT Averaged FFT Spectrum VI, located on the
Spectrum Averaging
palette.
Table 2.
FFT Averaging Methods and Equations
Averaging Method
Equation
Vector averaging
Y
k
= <
X
>
k
RMS averaging
Peak-hold averaging
Y
k
= max(
X
k
, Y
k
– 1
)
Y
k
<
X
conj
X
( )
>
k
(
)
=
