Fluke VIBXPERT II User Manual page 75

3-24
Options
Sample frequency (Timewaveform): The sample frequency for the
time signal defines the upper frequency.
Measurement time (Timewaveform): The length of the time window
depends on the sample frequency. The maximum measurement
time is 640 s.
Number of lines (Spectrum, Cepstrum): Number of lines displayed.
Together with the 'upper frequency', this parameter specifies the
resolution of the spectrum (∆f).
Window (Spectrum, Cepstrum): The measurement of periodic signals
generates time data sets with periodic gaps as a result of the
finite observation period. Thus, the frequency lines appear wider
in the corresponding spectrum. The window functions suppress
these 'side lobes'.

Window features:
Hanning: Standard for FFT. Frequency precise, but amplitude er-
rors (<15%); for the analysis of continuous processes with
Note
frequency resolution and lowest possible leakage effect.
Rectangular: Frequency errors, but amplitude is precise; suitable
for the analysis of individual pulses, particularly if the pulse is at
the beginning of the time window.
Kaiser: As with Hanning, but lower amplitude errors (<12%)
Flat-Top: Lower frequency accuracy than Hanning, amplitude more
precise. For precision amplitude analysis of continuous processes.
Hamming: Frequency errors as in Rectangular; amplitude errors
(18%) less than in Rectangular, but higher than in Hanning.
Bartlett: Next to Rectangular, this is the simplest window (triangle);
lower amplitude and frequency accuracy.
Blackman: As for Hanning, but with higher frequency accuracy;
amplitude errors as in Kaiser (12%); next to Kaiser, the second
best alternative for nearly all applications.
Average type/ no. of measurements (spectrum, Timewaveform,
Cepstrum)
Without: During the measurement each measurement is
displayed. The 'Number of averages" specifies how many
single measurements are carried out. The last measurement is
evaluated.
Linear: The individual spectra are added and divided by the
number of averages. This averaging type emphasizes recurring
stationary sections in the spectrum.
Peak-hold: Highest measurement value is displayed. This type of
averaging emphasizes peaks that occur stochastically.
Exponential: Individual measurements are exponentially
averaged (the last spectrum is given the highest weighting).
Time-synchronous: The individual measurements are averaged
on a RPM synchronous basis. Reference transducer required.
Overlap (spectrum, Cepstrum):
Measurement range: See section A1, P. 3-22.
Trigger type/ Level/ Start (Spectrum , Cepstrum, Timewaveform) P.
3-22.
See section A1, P. 3-21.
high