Spider DSA User’s Manual
189
Test Parameters
The Test Configuration window, Analysis Parameters tab is used to configure
tests.
Figure 133. Analysis Parameters Tab of Test Configuration Window
Overlap Ratio sets the proportion of the samples in a time block that are
overlapped (redundant with samples in a prior block) when calculating the FFT of
(un-triggered) continuous signals. Higher overlap ratios result in faster variance
reduction per unit time producing smoother data but they increase the processing
requirements. The Overlap Ratio options are: no overlap, 25%, 50%, 75%, 87.5%,
95% and As High As Possible. For most applications employing a symmetrically
tapered window function (such as Hanning), an overlap of 50% proves optimal.
Block Size/ Lines are the number of samples in each time blocks and the
number of (un-aliased) spectral lines in each resulting spectrum. Increasing the
block size increases the resolution of the frequency transform and allows lower
frequencies to be detected but it 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 its anti-aliasing filter. In general, this ratio is
about 0.46, meaning that 1024 samples in the waveform will produce about 0.46
* 1024 = 471 lines in the spectrum.
Sampling Rate (together with block size) determines the resolution and the
span of all time and frequency data blocks. Increasing the sample rate increases
time resolution (makes dT smaller), and decreases the time span (Block T)
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