Frequency Response Plot Calculation Error

[John Bretz]

Javier Fuentealba and other good folks at Cessna found that the
magnitude and coherence of the Frequency Response Plot went to .667 of
the expected value when changing from a rectangular window to a
Hanning window. The cause has been found, and Nyquist and Octave Band
plots are also affected.

Please accept our apologies for any inconvenience this error has
caused. The fix will be in the next release. Thank you again Javier
for bringing this to our attention

Affected Versions: 5.3.3 thru 6.5

John

[John Bretz]

There are a couple of easy workarounds possible:

1. Use rectangular windows.
2. Scale the excitation or response parameter with the ENBW of the
window type using derived parameters.

Below is a list of approximate scaling factors for the windows -

Hanning, 1.5
Hamming, 1.362826
Blackman, 1.726757
Flat Top, 3.769080
Kaiser-Bessel Alpha=2.0, 1.496342
Kaiser-Bessel Alpha=2.5, 1.651920
Kaiser-Bessel Alpha=3.0, 1.795235
Kaiser-Bessel Alpha=3.5, 1.928448

Please remember to "unscale" your parameters once you start using 6.5.

John