Order selection for lowpass IIR filters

Selection of the appropriate orders of the numerators and denominators for specified lowpass infinite impulse response (IIR) digital filter design criteria can be difficult. REcent work by H. L. Gray, et. al. suggests a general method by which orders may be selected prior to filter design. In this note, infinite impulse response lowpass digital filters are designed from finite impulse response (FIR) lowpass equivalents, wherein the Gray technique is used to select the order before design. On determination of the order, the IIR denominator is determined from the autocorrelation of the FIR and the numerator is obtained using the minimum phase Hilbert transform relationship.