Spider DSA User’s Manual
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captured. Increasing the sample rate also increases the maximum frequency (Fa)
in a spectral block and decreases its resolution (increases dF).
Available sampling rates on Spider series are:
Average Number is the number of blocks that are ensemble averaged for the
signal spectrum. Increasing the number of averages will reduce the variance of the
signal spectrum.
Average Mode options include: Linear, Exponential, and Peak Hold. Linear
averaging treats every block equally. The blocks are simply added together (at
each frequency) and the result divided by the Average Number. Exponential
averaging is a moving or evolving average that favors the most recently measured
block. Old data slowly loses its importance (time exponentially), so that the
average is dominated by the current instantaneous spectrum.
Window Type lets the user choose the window to be applied during FFT
operation. Windowing functions can help reduce leakage and increase the
precision of the frequency measurement. In general, select None for triggered
transients, Hanning for general continuous signals and Flat Top when studying
tonal data (such as a rotating machine) and needing extreme accuracy of spectral
peaks. Detailed descriptions about window types and average modes can be found
in the DSA Basics document.
FFT Average On/Off allows the user to turn on/off the FFT average.
LPF Cutoff Frequency allows the user to specify the cutoff frequency of the
low-pass filter.
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