Averaged Fft Spectrum - National Instruments NI Spectral Measurements Toolkit User Manual

Averaged FFT Spectrum

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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.
The following equations describe the three averaging methods applied to a
complex FFT spectrum.
Table 2. FFT Averaging Methods and Equations
Averaging Method
Vector averaging
RMS averaging
Peak-hold averaging
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.
Y = <X>
k
Y
=
k
Y = max(X
k
27 NI Spectral Measurements Toolkit User Guide
Equation
k
( <X conj X ( )> )
k
, Y )
k k – 1