A broadband pseudo-cascade active control system

Because conventional feedback control systems have a gain-bandwidth limitation, a given level of noise reduction can only be achieved over a limited bandwidth. For a conventional control system, this limitation can be alleviated by using multiple stages in cascade, each stage dedicated to a portion of the desired operating band. However, for active noise cancellation there is a problem because only one entry point to the system is available and a direct cascade is not physically possible. The pseudocascade configuration is introduced for alleviating this problem. An analysis of a single-stage broadband active control system characterizes the response by the cancellation bandwidth and the out-of-band overshoot. These results are then used to synthesize a multistage cascade with offset reference frequencies. A numerical example of a sixth-order three-stage design shows that for a 4 dB overshoot constraint, the 15 dB cancellation bandwidth is increased by a factor of 2 compared with the single-stage design. A further increase of cancellation bandwidth can be achieved with additional stages