Measurement Computing eZ-PostView rev.2.0 - Manual (Page 105)

eZ-Analyst 969795 Edit Menu 4-27
Blackman-
Harris:
This window function was designed to provide the minimum side lobe level of
any three-term window. Compared with the very similar Hanning window, it
has a slightly wider main lobe but much better dynamic range. This window
has the smallest 60 dB bandwidth of any window listed. The Blackman-Harris
window may be preferred over the Hanning for measurements requiring
better dynamic range.
Exponential:
An exponential weighting window is equal to 1.0 at the beginning of the block
and decays exponentially to a smaller value at the end of the block.
Exponential is used only with transient data that is captured with pre-trigger
to assure that the initial values in all data channels are very close to zero.
Exponential can be used with all transient excitation methods in order to force
the signals to decay close to zero, (See Response Decay Percent,) even if the
block length is not sufficient to capture all of the naturally occurring response.
If the data decays naturally to a low amplitude within the block, so that
leakage is not significant, exponential windowing can improve the signal-to-
noise ratio by giving reduced weight to the very low-amplitude data at the
end of the block.
Decay % :
Decay Percent is used when an Exponential Window is applied to the
Response Channel. It is the weighting factor present at the last block value.
For example, a decay of 5% means that the weighting factor at the beginning
of block is 100% and that the weighting factor of the last block is 5%. In
other words, the weighting factor dropped (decayed) to 5% through the
length of block.
Edit Menu > Configuration > FFT Setup Tab: FFT Window Functions / Reference Channels
The FFT Reference Window is applied to the output of a transducer to avoid collecting
extraneous signals caused by an excitation device, such as an impulse hammer. Possible
selections are: Response, Rectangular, and Cosine Taper.
When Rectangular is selected, specify the Start(%) and End(%) of the rectangle as a
percentage of the Frame Period.
When Cosine Taper is selected, specify the Start(%), End(%), and Edge(%) the Cosine
Taper as a percentage of the Frame Period.
FFT Window Functions, Reference Channels Pull-Down List
Start (%): The percent of the Frame Period at which the Rectangular and
Cosine Taper Windowing functions start.
Stop (%): The percent of the Frame Period at which the Rectangular and
Cosine Taper Windowing functions stop.
Edge (%): This is the percent of the Frame Period during which the Cosine Taper
Windowing function tapers up and down.