Multistage implementation of parameter-invariant null filter and its application to discrimination of closely spaced sinusoids

The authors present a two-stage parameter-invariant null filter based on the second-order null filters in cascade connection with low-pass sections. It is shown that such a filter is equivalent to a third-order parameter-invariant null filter by its ability to suppress two sinusoids of unknown frequencies. This filter was used to design the discriminator of two closely spaced sinusoids, and the results obtained demonstrate the potential of the proposed structure.<>