Page 91 - DSP Integrated Circuits

76 Chapter 3 Digital Signal Processing

Figure 3.13 Attenuation for the third-order Cauer lowpass filter

3.10.3 Phase Function
The phase function of the frequency response is defined

Figure 3.14 shows the phase response for a third-order digital lowpass filter.
Notice the discontinuities in the phase response at the frequencies of the trans-
mission zeros, the first at about 96° and the second at 180°, and note that the
phase response is almost linear up to about 45°. The jump in the phase response
just below 80° is an artifact due to the periodicity of the phase response.
Linear-phase response implies that the impulse response must be either sym-
metric or antisymmetric. Exact linear-phase response is only possible to realize
using FIR filters since an IIR filter cannot have perfect linear-phase response.
However, the phase response of an IIR filter can be made arbitrarily close to a lin-
ear-phase response within a finite frequency band.
Linear-phase response, which will be further discussed in the next section, is
particularly important in applications where the variation in signal delay through
the filter is important.