Measurement Computing eZ-PostView rev.2.0 - Manual (Page 142)

G-4, Glossary 957397 eZ-Analyst
Isolation
The arrangement or operation of a circuit so that signals from another circuit or device do not
affect the isolated circuit. Isolation usually refers to a separation of the direct link between
the signal source and the analog-to-digital converter (ADC). Isolation is necessary when
measuring high common-mode voltage.
Leakage
Leakage refers to an error in the FFT process due to the use of finite time blocks that do not
match at the blocks’ ends. At lower amplitudes, leakage results in a smearing of frequency
lines. FFT window functions, such as Hanning, can be used to minimize the error that is
introduced by leakage.
Linearization
Some transducers produce a voltage in linear proportion to the condition measured. Other
transducers (e.g., thermocouples) have a nonlinear response. To convert nonlinear signals
into accurate readings requires software to calibrate several points in the range used and then
interpolate values between these points.
Mobility
Velocity/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.
Nyquist Plot
A Nyquist plot is one method used to simultaneously display real and imaginary data. The real
numbered data is plotted on the X-axis and the imaginary numbered data is plotted on the Y-
axis. Line segments are used to join consecutive points. From the perspective of basic
vibration theory, a Nyquist plot of a mobility function should trace out a circle (counter-
clockwise) as frequency is increased through an isolated structural resonance.
Octave
The interval between two frequencies, where one frequency is twice the value of the other.
For example, frequencies of 500 Hz and 1 kHz are one octave apart; and 1 kHz and 2kHz are
one octave apart. Many sounds are broad band, having components that are continuously
distributed over a range of frequencies. The spectrum of such a sound can be approximated
in terms of a series of octave band or one-third octave band pressure levels. A band is
designated by its center frequency, f0, which is the geometric mean of the upper and lower
frequencies of the band. (See ANSI/ASC S1.6-1984.)
PSD Function
Power Spectral Function. A single-channel display function. It is the Fourier Transform of the
Auto-correlation function. This normalization should be used with continuous random signals.
Receptance, also referred to as Compliance
Displacement/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.
Response Decay Percent
See Decay %.
Sample (reading)
The signal value that is observed on a channel at an instant in time. When triggered, the
analog-to-digital converter [of the data acquisition device] reads the channel and converts the
sampled value into a digital representation.
Scan
A series of measurements across a pre-selected sequence of channels.
Spectrum Function
A dual-channel display function. Displays averaged linear spectrum computed as the square
root of the averaged autospectrum. This function is calibrated in peak engineering units (EU).
Time-Domain
A domain where amplitudes are graphed in relation to time. Also see Frequency-Domain.
Time Function:
A single-channel display function. Displays a time domain waveform of filtered, sampled data
scaled in either Volts or Engineering Units (EUs).