## Documentation Center |

IIR comb filter specification object

`d=fdesign.comb` `d=fdesign.comb(combtype)``d=fdesign(combtype,specstring)`

`fdesign.comb` specifies a peaking or notching
comb filter. Comb filters amplify or attenuate a set of harmonically
related frequencies.

` d=fdesign.comb ` creates
a notching comb filter specification object and applies default values
for the filter order (

` d=fdesign.comb(combtype)` creates
a comb filter specification object of the specified type and applies
default values for the filter order and quality factor. The valid
entries for

Argument | Description |
---|---|

notch | creates a comb filter that attenuates a set of harmonically related frequencies. |

peak | creates a comb filter that amplifies a set of harmonically related frequencies. |

` d=fdesign(combtype,specstring)` creates
a comb filter specification object of type

`'N,Q'`(default)'

`'N,BW'``'L,BW,GWB,Nsh'`

The following table describes the arguments in * specstring*.

Argument | Description |
---|---|

BW | Bandwidth of the notch or peak. By default the bandwidth is
calculated at the point –3 dB down from the center frequency
of the peak or notch. For example, setting BW=0.01 specifies
that the –3 dB point will be +/- 0.005 (in normalized frequency)
from the center of the notch or peak. |

GWB | Gain at which the bandwidth is measured. This allows the user to specify the bandwidth of the notch or peak at a gain different from the –3 dB default. |

L | Upsampling factor for a shelving filter of order Nsh. L determines
the number of peaks or notches, which are equally spaced over the
normalized frequency interval [-1,1]. |

N | Filter order. Specifies a filter with N+1 numerator
and denominator coefficients.The filter will have N peaks
or notches equally spaced over the interval [–1,1] . |

Nsh | Shelving filter order. Nsh represents a
positive integer that determines the sharpness of the peaks or notches.
The greater the value of the shelving filter order, the steeper the
slope of the peak or notch. This results in a filter of order L*Nsh. |

Q | Peak or notch quality factor. Q represents
the ratio of the lowest center frequency peak or notch (not including
DC) to the bandwidth calculated at the –3 dB point. |

` d=fdesign(combtype,specstring,specvalue1,specvalue2,...)` creates
an IIR comb filter specification object of type

` d=fdesign.comb(...,Fs)` creates
an IIR comb filter specification object using the sampling frequency,

These examples demonstrate how to create IIR comb filter specification objects. First, create a default specification object.

d=fdesign.comb;

In the next example, create a notching filter of order 8 with a bandwidth of 0.02 (normalized frequency) referenced to the –3 dB point.

d = fdesign.comb('notch','N,BW',8,0.02); Hd = design(d); fvtool(Hd)

Next, create a peaking comb filter with 5 peaks and a quality factor of 25.

d = fdesign.comb('peak','N,Q',5,25); Hd = design(d); fvtool(Hd)

In the next example, create a notching filter to remove interference at 60 Hz and its harmonics. The following code creates a filter with 10 notches and a notch bandwidth of 5 Hz referenced to the -4dB level. The filter has a shelving filter order of 4 and a sampling frequency of 600 Hz. Because the notches are equidistantly spaced in the interval [-300, 300] Hz, they occur at multiples of 60 Hz.

d = fdesign.comb('notch','L,BW,GBW,Nsh',10,5,-4,4,600); Hd=design(d); fvtool(Hd)

Was this topic helpful?