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fdesign.decimator

Decimator filter specification object

Syntax

D = fdesign.decimator(M)
D = fdesign.decimator(M, RESPONSE)
D = fdesign.decimator(M, CICRESPONSE, D)
D = fdesign.decimator(M, RESPONSE, SPEC)
D = fdesign.decimator(...,SPEC,specvalue1,specvalue2,...)
D = fdesign.decimator(...,Fs)
D = fdesign.decimator(...,MAGUNITS)

Description

D = fdesign.decimator(M) constructs a decimator filter specification object D with the DecimationFactor property equal to the positive integer M and the Response property set to 'Nyquist'. The default values for the transition width and stopband attenuation in the Nyquist design are 0.1π radians/sample and 80 dB. If M is unspecified, M defaults to 2.

D = fdesign.decimator(M, RESPONSE) constructs a decimator specification object with the decimation factor M and the 'Response' property.

D = fdesign.decimator(M, CICRESPONSE, D) constructs a CIC or CIC compensator decimator specification object with the decimation factor, M, 'Response' property equal to 'CIC' or 'CICCOMP', and D equal to the differential delay. The differential delay, D, must precede any specification string.

Because you are designing multirate filters, the specification strings available are not the same as the specifications for designing single-rate filters. The decimation factor M is not included in the specification strings. Different filter responses support different specifications. The following table lists the supported response types and specification strings. The strings are not case sensitive.

Design String

Valid Specification Strings

'Arbitrary Magnitude'

See fdesign.arbmag for a description of the specification string entries.

  • 'N,F,A' (default string)

  • 'N,B,F,A'

'Arbitrary Magnitude and Phase'

See fdesign.arbmagnphase for a description of the specification string entries.

  • 'N,F,H' (default string)

  • 'N,B,F,H'

'Bandpass'

See fdesign.bandpass for a description of the specification string entries.

  • 'Fst1,Fp1,Fp2,Fst2,Ast1,Ap,Ast2' (default string)

  • 'N,Fc1,Fc2'

  • 'N,Fst1,Fp1,Fp2,Fst2'

'Bandstop'

See fdesign.bandstop for a description of the specification string entries.

  • 'N,Fc1,Fc2'

  • 'N,Fp1,Fst1,Fst2,Fp2'

  • 'Fp1,Fst1,Fst2,Fp2,Ap1,Ast,Ap2' (default string)

'CIC'

'Fp,Ast' — Only valid specification. Fp is the passband frequency and Ast is the stopband attenuation in decibels.

To specify a CIC decimator, include the differential delay after 'CIC' and before the filter specification string: 'Fp,Ast'. For example:
d = fdesign.decimator(2,'cic',4,'Fp,Ast',0.4,40);

'CIC Compensator'

See fdesign.ciccomp for a description of the specification string entries.

  • 'Fp,Fst,Ap,Ast' (default string)

  • 'N,Fc,Ap,Ast'

  • 'N,Fp,Ap,Ast'

  • 'N,Fp,Fst'

  • 'N,Fst,Ap,Ast'

To specify a CIC compensator decimator, include the differential delay after 'CICCOMP' and before the filter specification string. For example:
d = fdesign.decimator(2,'ciccomp',4);

'Differentiator'

'N' — filter order

'Gaussian'

'Nsym,BTNsym is the filter order in symbols and BT is the bandwidth-symbol time product.

The specification string must be preceded by an integer-valued SamplesPerSymbol.

'Halfband

See fdesign.halfband for a description of the specification string entries.

  • 'TW,Ast' (default string)

  • 'N,TW'

  • ‘N'

  • 'N,Ast'

If you use the quasi-linear IIR design method, iirlinphase, with a halfband specification, the interpolation factor must be 2.

'Highpass'

See fdesign.highpass for a description of the specification string entries.

  • 'Fst,Fp,Ast,Ap' (default string)

  • 'N,F3db'

  • 'N,Fc'

  • 'N,Fc,Ast,Ap'

  • 'N,Fp,Ast,Ap'

  • 'N,Fst,Ast,Ap'

  • 'N,Fst,Fp'

  • 'N,Fst,Ast,Ap'

  • 'N,Fst,Fp,Ast'

'Hilbert'

See fdesign.hilbert for a description of the specification string entries.

  • 'N,TW' (default string)

  • 'TW,Ap'

'Inverse-sinc Lowpass'

See fdesign.isinclp for a description of the specification string entries.

  • 'Fp,Fst,Ap,Ast' (default string)

  • 'N,Fc,Ap,Ast'

  • 'N,Fp,Fst'

  • 'N,Fst,Ap,Ast'

'Inverse-sinc Highpass'

See fdesign.isinchp for a description of the specification string entries.

  • 'Fst,Fp,Ast,Ap' (default string)

  • 'N,Fc,Ast,Ap'

  • 'N,Fst,Fp'

  • 'N,Fst,Ast,Ap'

'Lowpass'

See fdesign.lowpass for a description of the specification string entries.

  • 'Fp,Fst,Ap,Ast' (default string)

  • 'N,F3dB'

  • 'N,Fc'

  • 'N,Fc,Ap,Ast'

  • 'N,Fp,Ap,Ast'

  • 'N,Fp,Fst'

  • 'N,Fp,Fst,Ap'

  • 'N,Fp,Fst,Ast'

  • 'N,Fst,Ap,Ast'

'Nyquist'

See fdesign.nyquist for a description of the specification string entries. For all Nyquist specifications, you must specify the Lth band. This typically corresponds to the DecimationFactor.

  • 'TW,Ast' (default string)

  • 'N'

  • 'N,Ast'

  • 'N,Ast'

D = fdesign.decimator(M, RESPONSE, SPEC) constructs object D and sets the Specification property to SPEC for the response type, RESPONSE. Entries in the SPEC string represent various filter response features, such as the filter order, that govern the filter design. Valid entries for SPEC depend on the RESPONSE type.

Because you are designing multirate filters, the specification strings available are not the same as the specifications for designing single-rate filters with such response types as fdesign.lowpass. The strings are not case sensitive.

The decimation factor M is not in the specification strings.

D = fdesign.decimator(...,SPEC,specvalue1,specvalue2,...) constructs an object D and sets its specifications at construction time.

D = fdesign.decimator(...,Fs) provides the sampling frequency of the signal to be filtered. Fs must be specified as a scalar trailing the other numerical values provided. Fs is assumed to be in Hz as are all other frequency values provided.

D = fdesign.decimator(...,MAGUNITS) specifies the units for any magnitude specification you provide in the input arguments. MAGUNITS can be one of

  • 'linear' — specify the magnitude in linear units.

  • 'dB' — specify the magnitude in dB (decibels).

  • 'squared' — specify the magnitude in power units.

When you omit the MAGUNITS argument, fdesign assumes that all magnitudes are in decibels. Note that fdesign stores all magnitude specifications in decibels (converting to decibels when necessary) regardless of how you specify the magnitudes.

Examples

These examples show how to construct decimating filter specification objects. First, create a default specifications object without using input arguments except for the decimation factor m.

d = fdesign.decimator(2,'Nyquist',2,0.1,80) % Set tw=0.1, and ast=80.       

Now create an object by passing a specification type string 'fst1,fp1,fp2,fst2,ast1,ap,ast2' and a design — the resulting object uses default values for the filter specifications. You must provide the design input argument, bandpass in this example, when you include a specification.

d=fdesign.decimator(8,'bandpass',...
'fst1,fp1,fp2,fst2,ast1,ap,ast2');

Create another decimating filter specification object, passing the specification values to the object rather than accepting the default values for fp,fst,ap,ast.

d=fdesign.decimator(3,'lowpass',.45,0.55,.1,60);

Now pass the filter specifications that correspond to the specifications — n,fc,ap,ast.

d=fdesign.decimator(3,'ciccomp',1,2,'n,fc,ap,ast',...
20,0.45,.05,50);

Now design a decimator using the equiripple design method.

hm = equiripple(d);

Pass a new specification type for the filter, specifying the filter order. Note that the inputs must include the differential delay dd with the CIC input argument to design a CIC specification object.

m = 5;
dd = 2;
d = fdesign.decimator(m,'cic',dd,'fp,ast',0.55,55);

In this example, you specify a sampling frequency as the last input argument. Here is it 1000 Hz. Design an equiripple filter and plot the magnitude response:

d=fdesign.decimator(8,'bandpass','fst1,fp1,fp2,fst2,ast1,ap,ast2',...
100,150,250,300,50,.05,50,1000);
fvtool(design(d,'equiripple'))

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