The influence which a narrowband adaptive feedforward controller, operating in the presence of unreferenced random noise, has upon the frequency response characteristics of the controlled system away from the controller referenced frequency band is investigated. It is found experimentally that the adaptation of the controller weights is responsible for increasing the resonant peaks of the response and introducing new peaks in some instances. This phenomenon is examined analytically, where it is shown that the unreferenced frequency components can propagate through the adaptive algorithm error signal input and exert an influence on the controller output signal spectrum. This phenomenon is found to be most troublesome when there is a response peak near the feedforward system reference frequency. The only remedy for the problem is a reduction in the adaptive algorithm convergence coefficient to a value far below that dictated purely by stability considerations.

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