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AGILENT 35670A
Supplemental Operator’s Guide
50
Section 18 : Measuring Frequency Response with the AGILENT 35670A
Frequency response functions characterize the behavior of systems to external
stimuli. For mechanical systems, the stimulus can be a transient force such as an
impact from a hammer, or a broadband stimulus, such as random excitation from an
electrodynamic shaker. The response is usually motion of the system, measured with
and accelerometer, velocity transducer, or displacement probe.
In either case the stimulus and the response measured with two channels of the
AGILENT 35670A. The stimulus is usually measured on Channel 1, and the
response is measured on Channel 2. The resulting frequency response is averaged to
reduce noise and variance, and the result is displayed in a variety of formats that will
be described in this section, including:
• Magnitude and phase
• Real and imaginary components (
• Nyquist Diagrams
The quality of the frequency response measurement is determined by how much of
the response is due to the stimulus instead of being due to noise or other undesirable,
unmeasured forces. Quality is described by coherence, which ranges from zero to
one, where zero means none of the response is due to the stimulus. A coherence of
one means that the response was entirely due to the stimulus
Measuring Frequency Response Using Impact Excitation
Refer to the section “Modal Testing Using a Hammer and Accelerometer” to
measure frequency response using an impact as a stimulus. An impact provides
broadband excitation, however the impact has a high peak to RMS ratio which tends
to reduce measurement quality.
Measuring Frequency Response Using Broadband Excitation
Excite a system using the AGILENT 35670A built-in source to provide broadband
excitation with low peak to RMS ratio and the best measurement quality. Use burst-
random noise to eliminate window distortion and reduce leakage. The following
instructions show how to set up a modal test with these example parameters:
electrodynamic shaker with amplifier
ICP force transducer with sensitivity of 8 mV/lb.
ICP accelerometer with sensitivity of 10.2 mV/G
wire sting to connect shaker to force transducer
burst random excitation, uniform window
frequency span of 800 Hz, 4 averages
results viewed as log magnitude and phase
Set up the shaker
• Attach force transducer to structure