MATLAB SIGNAL PROCESSING TOOLBOX 6 Bedienungsanleitung PDF herunterladen (Seite 58)

4 Filter Design with the FDATool GUI

4-20

Designing Filters Using Command Line Functions

You can specify and design filters at the command line using fdesign and design.

The use of fdesign and design provides a powerful and efficient way to specify and

implement digital filters. With fdesign and design, digital filter design is a two-

step process. In the first step, the user specifies the desired characteristics of the filter

and saves those specifications in a Filter Specification Object. In the second step, the

user implements the filter as a dfilt object, which can then be used to filter data. As an

example of this two-step process, we will design a lowpass filter for data sampled at 20

kHz. The desired passband frequency is 1 kHz with a stopband frequency of 1.2 kHz.

We will limit the passband ripple to 1 dB and require 60 dB of attenuation between the

passband and stopband frequencies.

To specify the filter use fdesign.lowpass with the parameters given above. You

can copy and paste the following code at the MATLAB command prompt.

d=fdesign.lowpass('Fp,Fst,Ap,Ast',1000,1200,1,60,20000);

To design the filter use design with the appropriate design methods. In this

example, we create FIR equiripple and IIR Butterworth designs and compare the

filters' magnitude responses.

Hd1=design(d,'equiripple'); %FIR equiripple design

Hd2=design(d,'butter'); %Butterworth design

Hd=[Hd1 Hd2];%filter object with both designs

%compare filters

fvtool(Hd,'legend','on'); axis([0 2 -70 10])

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MATLAB SIGNAL PROCESSING TOOLBOX 6 Bedienungsanleitung Seite 58