The constant-input stability of the generalized immitance converter (GIC) digital filter structures proposed by A. Antoniou and M.G. Rezk (1978) is investigated. This stability is essential for these structures since they are used in cascade form. It is shown that only one of the five sections previously proposed can be stabilized against constant-input limit cycles. It is also shown that each one of those five sections can be made to realize multiple transfer functions without any additional multiplier. A universal filter based on the GIC concept is proposed which realizes simultaneously all the five standard second-order transfer functions and which is also free from both zero- and constant-input limit cycles. The sensitivity and noise properties of the proposed universal filter are compared with those of other known universal structures for a specific example.
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