Unformatted text preview: ) and response (output)) that are recorded or captured for each average (N c ). The number of capture blocks is also the number of cyclic averages that will be used to reduce the leakage error. Each group of contiguous capture blocks (N c ) is used as the time domain data contributing to one power spectral average that contributes to the estimate of the FRF measurements. Window Function  The window function refers to the digital signal processing, time domain window that is applied to the capture blocks. The application of the window function to the capture blocks is on the basis of the group of contiguous capture blocks not on each capture block individually. Average (Ensemble)  The average or ensemble refers to the total collection of contiguous time blocks that contribute to each power spectral average. The total time of each average is equal to the sum of the number of delay blocks (N d ) plus the number of capture blocks (N c ) times the observation period (T) which is the same for all delay and capture blocks. The number of averages (N avg ) refers to the number of these contiguous collections of time blocks and is, therefore, the same as the number of power spectral averages. The number of capture blocks can also be thought of as the number of cyclic averages (N c ). Cyclic signal averaging is often used with excitation characteristics in order to better match the time domain input and output signals to the requirements of the FFT prior to the application of the FFT. Cyclic signal averaging essentially digitally comb ﬁlters the time domain data to reduce the amount of information in the data that is not periodic with the observation period (T). This type of averaging reduces the effects of the leakage error. As long as the N c successive blocks of data are contiguous, the blocks of time domain data can be averaged together, with or without windows, to achieve the beneﬁt of leakage reduction [910]. Periodic  If the excitation signal is repeated for each delay and capture block, the signal is referred to as periodic. This classiﬁcation is consistent with the deﬁnition of a periodic function and includes typical examples of sinusoids and chirps as well as a random signal that is repeated on the basis of the observation period (T). The periodic classiﬁcation does not deﬁne whether
(543) +UCSDRLRJA CN20263663/664 Revision: June 12, 2001 + the same signal is repeated for each successive group of contiguous delay and capture blocks. Burst Length  Burst length is the percentage (0 to 100%) of the average or ensemble time that the excitation signal is present. Burst length is normally adjusted in order to achieve a signal that is a totally observed transient. The decay of the signal is a function of the system damping and the characteristics of the excitation hardware. Burst length can be deﬁned as the percentage of the total number of contiguous delay and capture blocks or of a percentage of just the capture blocks. For the pur...
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