Measurement Computing eZ-Analyst rev.14.1 - Manual (Page 133)

eZ-Analyst 959294 Glossary G-3
Fast Fourier Transform (FFT) Response Window
The FFT (Response) window is a time-domain, weighting window. A response window is
usually applied to data to reduce FFT leakage errors. FFT theory assumes that the signal
being analyzed is periodic in the data acquisition block. When this is not the case, energy
from a signal at a specified frequency can leak into nearby spectral bins causing spectral
amplitude inaccuracies. Applying a windowing function controls, but doesn’t completely
eliminate, the error by multiplying each data frame by a suitable time-domain weighting
window. This calculation reduces the amplitude/magnitude of the data near the ends of each
data frame prior to performing the FFT and forces the data to be nearly periodic in the
window, thus reducing leakage errors. Response window options include Flat-Top, Blackman-
Harris, and Hanning.
Flat-Top Window, also written as Flattop Window
An FFT window function that provides accurate measurement of peak-amplitude. Compared to
the very similar 4-term “Max Flat Top,” this window also has a very low peak amplitude error,
and its frequency resolution is somewhat better. Its side lobes are considerably higher. Its
effective noise bandwidth is still almost twice that of the Hanning window, therefore this
window is used mainly to measure accurate peak amplitudes of discrete spectral components
that are known to be separated by several spectral lines.
Free Run (Free Running)
A state in which a data acquisition device acquires the next block of data when it has finished
the previous block. Free Run is a continuous collection of data, with a constant display
update.
Frequency-Domain
A domain where amplitudes are graphed in relation to frequency. Also see Time-Domain.
Frequency Slice
A 2-dimensional plot of amplitude versus time. A frequency slice consists of the amplitude
values of a specific frequency of multiple records. The plot can be seen in a 2-dimensional
Frequency Slice Window; or optionally, can be selected for viewing within a 3D Waterfall plot.
FRF (Frequency Response Function)
A dual-channel function for the single-input, single-output (SISO) frequency response function
between two specified input channels. FRF is the averaged cross-spectrum divided by the
averaged autospectrum of the input (the second named channel).
FSV (Full Scale Volts)
Maximum FSV refers to the high-end limit for the input voltage. Minimum FSV is the low-end
limit for the input voltage.
g
Acceleration due to gravity. In relation to earth’s gravity,
1 g = 980 cm/sec 2 , or 32.2 ft/sec 2 .
Gain
The degree to which a signal is amplified (or attenuated) to allow greater accuracy and
resolution; can be expressed as ×n or ±dB.
Hanning
An FFT window function. The Hanning window is typically used to analyze continuous signals.
The Hanning function reduces leakage and provides a relatively good compromise between
frequency resolution and amplitude accuracy.
Hz
Hertz, one cycle per second. Also, see CPM.
Impedance
Force/Velocity. A transfer type that is the reciprocal function of Receptance. Impedance is
derived by taking the inverse of the magnitude, and multiplying the phase angle by “-1.”
Inertance
Acceleration/Force. The transfer type function is calculated by dividing the cross spectrum of
the channel pair by the auto spectrum of the reference (force) channel.