-(Sensor, Transducer,)(Conditioning, Gain, Filter, Driver,)RecorderA/D Sampling & Digitizing, Oscilloscope, Data Acquisition/Display/Storage( )

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-FFTFFTFFT?

-AliasingAliasingNyquist Criterionf2fHP2.56100HzResolution Lines 800f=100Hz/800=1/8HzT=1/f=8sec100Hz2.56100=256HzSample8256=2048Samples

(a)Measurements taken at the same frequency as the variation (b)The result is a constant value(a)Measurements taken at a rate which is less than the Nyquist criterion. (a)measurements taken at 4/3 the rate of variation. (b)The result is an apparent signal that has a frequency less than the real rate of variation, 1/3 the true frequency in this case

Input signal periodic in time recordInput signal not periodic in time record

-WindowLeakage,FFTFFTFFT,,

WindowHanning windowUniform window(window)Flat top window

Window()Force and Response (Exponential) windowForce widthLight DampingExponential decayingtime constant

Window

-Self-windowing functionFFTLeakageImpulseShock (Impact) Response-FRFSine Burst-Fix vibration sourceNoise Burst-RandomChip Burst-Mechanical,Control system FRF

-AveragingRMS AverageA: inTriggernoisenoise

Averaging()Vector (Linear) Average

A: inTrigger(Key Phasor)noise() ()

Averaging()Exponential Average (RMS or Vector)Average numbernnRMS Average(Linear AverageTrigger)n next value=(1/n)(new value)+[(n-1)/n](current value)exponential averagePause n

Averaging()Peak Hold Averageaverage modePeak HoldPeak Hold Average