basics of fft signal analysis
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Basics of FFT Signal Analysis
Crystal Instruments Webinar Series
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CI Webinar Schedule
Date Topic
August 18 Basic FFT setup
September 15 Routes for data measurement
October 20 Impact testingNovember 17 Sound Level Measurements
December 15 Order Tracking
See www.go-ci.com/webinars.html for up-to-date schedule
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Basics of FFT Signal Analysis
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FFT Line Response
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FFT Line Response
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Leakage
Frequency centered on FFT line Frequency between FFT lines
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Windowing Functions
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Force and Exponential Windows
Force window: excitation Exponential window: response
Pictures courtesy of University of Massachusetts, Lowell
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Analysis Parameters
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Resolution and Range
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Resolution and Range
Frequency Lines
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Resolution and Range
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Spectrum Type and Scaling
FFT returns complex-valuedamplitudes
Real part represents cosine components,
imaginary part represents sine components
(90 phase difference)
Can be converted to magnitude andphase
Squared magnitude represents signal power
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Spectrum Types
Spectrum Types:
Magnitude: Amplitude Spectrum
Squared magnitude: Power Spectrum
Squared magnitude per unit bandwidth: Power
Spectral Density
Squared magnitude block time length: Energy
Spectrum Squared magnitude block length per unit
bandwidth: Energy Spectral Density
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Spectrum Types
Periodic signals (discrete frequencies):
Amplitude or Power Spectrum
Broadband random signals: Power Spectral
Density
Transient Signals: Energy Spectral Density
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Amplitude Spectrum Scaling
From Wikipedia:
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Averaging
Combine multiple time blocks together to
form one spectral estimate
Random data: higher average number, better
estimate of random characteristics
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Averaging Types
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Averaging Methods
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Averaging
FFT
Average
(Linear/
Exponential/
Peak Hold
|| ||2
Average
(Linear/
Exponential/
Peak Hold
Time SignalData
Complex
FrequencyData
Sx
Magnitude-squared
Frequency Data
Sxx
Linear Spectrum
Average
Gx
Power Spectrum
Average
Gxx
In CoCo: Linear Spectrum:
Time Linear
Time Exponential
Power Spectrum: Linear
Exponential
Peak-Hold
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Overlap Processing
A proportion of each time frame can be reused in
subsequent frames
Faster updating
Better averaging in same time period, only up to 50%
Graphic from Understanding FFT Overlap Processing, Tektronix Corporation
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CoCo Operation
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CSAs
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Windows and Traces
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Signal Display
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Param. Menu
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Analysis Parameters
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Display Parameters
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dB, Mag, Log
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James Zhuge, President
Tim Hsiao
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Crystal Instruments Corporation4699 Old Ironsides Dr., Ste. 100
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