| Filter Design Toolbox Reference Guide | |
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Arbitrary response magnitude and phase filter specification object
d = fdesign.arbmagnphase
d = fdesign.arbmagnphase(specification)
d = fdesign.arbmagnphase(specification,specvalue1,specvalue2,...)
d = fdesign.arbmagnphase(specvalue1,specvalue2,specvalue3)
d = fdesign.arbmagnphase(...,fs)
d = fdesign.arbmagnphase constructs an arbitrary magnitude filter specification object d.
d = fdesign.arbmagnphase(specification) initializes the Specification property for specifications object d to the string in specification. The input argument specification must be one of the strings shown in the following table. Specification strings are not case sensitive.
Specification String | Description of Resulting Filter |
|---|---|
n,f,h | Single band design (default). FIR and IIR (n is the order for both numerator and denominator). |
n,b,f,h | FIR multiband design where b defines the number of bands. |
nb,na,f,h | IIR single band design. |
The following table describes the arguments in the strings.
Argument | Description |
|---|---|
b | Number of bands in the multiband filter. |
f | Frequency vector. Frequency values specified in f indicate locations where you provide specific filter response amplitudes. When you provide f you must also provide h which contains the response values. |
h | Complex frequency response values. |
n | Filter order for FIR filters and the numerator and denominator orders for IIR filters (when not specified by nb and na). |
nb | Numerator order for IIR filters. |
na | Denominator order for IIR filter designs. |
By default, this method assumes that all frequency specifications are supplied in normalized frequency.
f and h are the input arguments you use to define the filter response desired. Each frequency value you specify in f must have a corresponding response value in h. This example creates a filter with two passbands (b = 4) and shows how f and h are related. This example is for illustration only. It is not an actual filter.
Define the frequency vector f as [0 0.1 0.2 0.4 0.5 0.6 0.9 1.0]
Define the response vector h as [0 0.5 0.5 0.1 0.1 0.8 0.8 0]
These specifications connectf and h as shown in the following table.
f (Normalized Frequency) | h (Response Desired at f) |
|---|---|
0 | 0 |
0.1 | 0.5 |
0.2 | 0.5 |
0.4 | 0.1 |
0.5 | 0.1 |
0.6 | 0.8 |
0.9 | 0.8 |
1.0 | 0.0 |
A response with two passbands—one roughly between 0.1 and 0.2 and the second between 0.6 and 0.9—results from the mapping between f and h. Plotting f and h yields the following figure that resembles a filter with two passbands.
The second example in Examples shows this plot in more detail with a complex filter response for h. In the example, h uses complex values for the response.
Different specification types often have different design methods available. Use designmethods(d) to get a list of design methods available for a given specification string and specifications object.
d = fdesign.arbmagnphase(specification,specvalue1,specvalue2,...) initializes the filter specification object with specvalue1, specvalue2, and so on. Use get(d,'description') for descriptions of the various specifications specvalue1, specvalue2, ...specn.
d = fdesign.arbmagnphase(specvalue1,specvalue2,specvalue3) uses the default specification string n,f,h, setting the filter order, filter frequency vector, and the complex frequency response vector to the values specvalue1, specvalue2, and specvalue3.
d = fdesign.arbmagnphase(...,fs) specifies the sampling frequency in Hz. All other frequency specifications are also assumed to be in Hz when you specify fs.
Use fdesign.arbmagnphase to model a complex analog filter:
d=fdesign.arbmagnphase('n,f,h',100); % N=100, f and h set to defaults.
design(d,'freqsamp');For a more complex example, design a bandpass filter with low group delay by specifying the desired delay and using f and h to define the filter bands.
n = 50; % Group delay of a linear phase filter would be 25.
gd = 12; % Set the desired group delay for the filter.
f1=linspace(0,.25,30); % Define the first stopband frequencies.
f2=linspace(.3,.56,40);% Define the passband frequencies.
f3=linspace(.62,1,30); % Define the second stopband frequencies.
h1 = zeros(size(f1)); % Specify the filter response at the freqs in f1.
h2 = exp(-j*pi*gd*f2); % Specify the filter response at the freqs in f2.
h3 = zeros(size(f3)); % Specify the response at the freqs in f3.
d=fdesign.arbmagnphase('n,b,f,h',50,3,f1,h1,f2,h2,f3,h3);
design(d,'equiripple') In the following figure, displaying the filter in FVTool shows both the magnitude response and the nearly linear phase.
fdesign, design, designmethods, setspecs
| | fdesign.arbmag | fdesign.bandpass | |
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