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432 Chapter 8 IIR FILTER DESIGN
1
0.8913
Magnitude Response
1
Phase Response
|H|
π units
0
0.1778
0
0 0.2 0.3 1
frequency in π units
−1
0 0.2 0.3 1
frequency in π units
0
1
Magnitude in dB
15
Group Delay
decibels
15
Samples
10
5
0 0.2 0.3 1
frequency in π units
0
0 0.2 0.3 1
frequency in π units
FIGURE 8.23
design
Digital Chebyshev-II lowpass filter using impulse invariance
From the frequency response plots in Figure 8.23 we clearly observe the passband
as well as stopband degradation. Hence the impulse invariance design
technique has failed to produce a desired digital filter.
□
□ EXAMPLE 8.14 Design a lowpass digital filter using an elliptic prototype to satisfy
ω p =0.2π, R p =1dB
ω s =0.3π, A s =15dB
Solution
The elliptic filter is equiripple in both bands. Hence this situation is similar to
that of the Chebyshev-II filter, and we should not expect a good digital filter.
The MATLAB script follows:
>> % Digital Filter Specifications:
>> wp = 0.2*pi; % digital Passband freq in rad
>> ws = 0.3*pi; % digital Stopband freq in rad
>> Rp = 1; % Passband ripple in dB
>> As = 15; % Stopband attenuation in dB
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