Spider DSA User’s Manual
259
Figure 172. Test Parameters for Acoustic Analysis
FFT Analysis Parameters
Block Size/ Lines defines the size of the time blocks in terms of the number of
samples, which the FFT algorithm transforms to a number of spectral Lines in the
frequency domain. Increasing the block size increases the resolution of the
frequency transform and allows lower frequencies to be detected but this also
increases the calculation time and slows down response.
The ratio between Lines and Block Size is determined by the characteristics of
A/D converter and anti-aliasing filter. In general, the ratio is about 0.46 which
means that a time block of 1024 samples results in about 0.46 * 1024 = 471 Lines
in the spectrum.
Window 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. Detailed descriptions about window types and average
modes can be found in the DSA Basics document.
Overlap Ratio sets the proportion of the samples in the time blocks that are
overlapped when calculating the FFT. Higher overlap ratios result in faster
response time but increase processing requirements. The Overlap ratio options
are: no overlap, 25%, 50%, 75%, 87.5%, 95%, and As High as Possible.
Average Mode gives options such as: Exponential, linear and peak hold as the
methods used to average the signal spectrum.
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