TABLE OF CONTENTS For systems, which cater for wide-band signals, it is important to ensure same time differences between signal spectrum components before and after filtering. This requirement is met if phase response of the filter is linear without constant factor, thus having a constant group delay function, ?( ?):
A flat group delay function is all that is required to assure the phase response linearity for a polyphase lowpass filter. This is due to the phase response equal zero for DC and continues across the whole baseband. The phase corrector can be designed in two ways, either straight by designing the corrector phase response shape to be opposite to the one of the filter or by making the group delay of the filter flat by designing its coefficients accordingly. The first method can be applied if a typical IIR corrector is designed. Then the shape of its phase can be specified. The latter method uses the correction filter or a set of them, phase responses of which are only approximately opposite to the shape of the phase response of the original filter. It can be noticed that phase response of the allpass subfilter with negative coefficient can be used to correct the phase of the polyphase lowpass filter for frequencies below ?=0.25. Certainly the quality of correction is better for smaller frequency bands. A cascade of different order allpass sections can be used for better correction. Phase correction is normally required only...
