Spider DSA User’s Manual
274
Phase measurement at higher orders will exhibit similar behavior, although they
are more difficult to comprehend intuitively.
It must be noted that Complex Order Tracks order tracks with phase, are not
regular complex signals as frequency response or cross spectrum. They are really
auto spectra with assigned phase. These synthesized signals can certainly be
viewed as a complex signal using tools including Bode Plot, polar and orbit
diagram. However, the user must keep in mind that the magnitude and phase of a
complex order track are calculated separately.
Bode Plot
The term Bode Plot is borrowed from the field of control theory, referring to a plot
of magnitude and phase angle between the input and output verses frequency of a
control system. Many in the rotating machine vibration industry have adopted
this term to describe the steady-state vibration response amplitude and phase
angle versus rotational speed (RPM). It turns out that the Bode Plot is the best
way to describe order tracks with phase. You typically use Bode plots for
transient analysis in both Run-up and coast-down conditions. A Bode plot can
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