7Infinite Impulse Response Filters
Digital filters with an infinite impulse response (IIR), or recursive filters, have properties similar to those of analog filters, and consequently, their coefficients can be determined by similar techniques [1–3].
Before discussing the method for calculating the coefficients, it is useful to give some general expressions for the properties of these filters.
7.1 General Expressions for the Properties of IIR Filters
The general IIR filter is a system which, from the set of data x(n), produces the set y(n) such that:
The Z-transfer function for this system is written:
This is the quotient of two polynomials in Z, which are often of the same degree.
As the coefficients al and bk are real numbers, H(Z) is a complex number such that:
and the frequency response of the filter can be written with the same conventions as in the earlier chapters:
The modulus and the phase are expressed in terms of H(Z) by the following equations:
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