Agilent Technologies Agilent 35670A manual Using Force/Response Windows, Fft

AGILENT 35670A

Supplemental Operator’s Guide

Window	Type: Force/Expo	Width: 15 ms
[FFT]		Decay: 30 ms
Date: 10-02-96 Time: 09:02:00 PM
A: CH1 Time	X:488.281 us	Y:67.1154 mV
80
mV
Real
-20
mV		115.112ms
-9.766ms
B: CH2 Time	X:488.281 us	Y:-74.925 mV
300
mV
Real
-200
mV		115.112ms
-9.766ms

C: CH1 Win Time	X:488.281 us	Y:48.1237 mV
70
mV
Real
-30
mV		115.112ms
-9.766ms

D: CH2 Win Time	X:488.281 us	Y:-53.234 mV
250
mV
Real
-250
mV		115.112ms
-9.766ms

Using Force/Response Windows

Hammer measurements for modal analysis typically require special windows, named Force/Response windows, to maximize measurement quality. The force window merely zeros out the trace after a fixed time interval, so that noise or bumps from the hammer after the desired impact do not get interpreted as excitation applied to the structure. The fixed interval must be long enough to capture the whole pulse, so that measurement of the impact does not get zeroed out. A typical value for the length of a force window is T/7, where T is the record duration in seconds. For a 1600 Hz span and resolution of 400 lines, the record length is 250 ms, so a “conservative” force window might have a duration of 36 ms. Experience may show that a much shorter duration may work better.

Response windows prevent leakage by artificially decaying the response so that by the end of the record the signal is less than 5% of the peak. The artificial decay is specified by a time constant, with shorter time constants providing higher decay rates. A time constant that is guaranteed to decay a signal to less than 5% is T/4. For a span of 1600 Hz and resolution of 400 lines, this translates into a response

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