Calculate random RMS functions from PSD input

The Fast RMS PSD Fit process computes a Fitted PSD function from input Power Spectral Density (PSD) data entered as an AFU file.

The total number of coefficients for the computed Fitted PSD function is equal to (N*N)+2N+1 where N is the number of orders. When you select the High Pass Filter option, N is equal to the number of orders plus 1.

1. Make sure that the AFU file that contains your excitation function is loaded in the XY Function Navigator (right-click the associated AFU node and choose Load All).Tip: If the XY Function Navigator is not visible on the Resource Bar, select Resource Bar Options →Tabs→XY Function Navigator.

2. Choose Response Dynamics tab→Function Math group→Single Math .

3. In the Function group, select the AFU file that contains the PSD input function and click to add the AFU records to the operation.

4. In the Operation group, select Fast RMS PSD Fit.

5. In the Parameter group, in the Number of Points box, type the number of points to fit the new function through.The software adds an additional 1000 points to the calculation.

6. In the Order box, type the order of the desired polynomial.

7. (Optional) Select the Use High Pass Filter check box, and in the Cutoff Frequency Fraction box, type a value between zero and 1.

8. (Optional) Select the Add Side Lobes check box, and in the Slope (dB/Octave) box, type a slope value. Tip: Side lobes are used to attenuate the fitted function to zero outside your specified frequency range. The side lobe slope required will depend on your function values at the frequency limits. Setting the side lobe roll off to a slope of 6 dB/octave or greater is usually needed to smoothly reduce the fitted function to zero. Using a very large side lobe slope may be useful to fit some functions.The software will add additional points to the input function to fit the function close to zero at the starting and ending frequencies.

9. Select the Interpolation Method type. This feature controls the interpolation of approximate data points when generating response functions with additional spectral lines.

10. Click Update Fit. The RMS Error of the Fitted PSD function is computed, compared to the input PSD.

11. When the software prompts you, select an existing graphics window or a new window to plot both the input and new function on the same XY plot.

12. Inspect the fitted function and the RMS Error to determine how well the Fitted PSD function matches your input function.

13. Click OK or Apply to save the new function.

14. Click Plot Fitto view the new function.

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Calculate random RMS functions from PSD input, Simcenter 3D 2021.1 Series

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