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AGILENT 35670A
Supplemental Operator’s Guide
28
Window
[FFT]
Type: Force/Expo Width: 15 ms
Decay: 30 ms
Date: 10-02-96 Time: 09:02:00 PM
-9.766ms 115.112ms
X:488.281 us Y:-74.925 mV
X:488.281 us Y:-53.234 mV
-9.766ms 115.112ms
X:488.281 us Y:48.1237 mV
-9.766ms 115.112ms
A: CH1 Time
B: CH2 Time
X:488.281 us Y:67.1154 mV
80
mV
-20
mV
Real
300
mV
-200
mV
Real
C: CH1 Win Time
D: CH2 Win Time
250
mV
-250
mV
Real
70
mV
-30
mV
Real
-9.766ms 115.112ms
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