Second-order IIR notch filter
[num,den] = iirnotch(w0,bw)
[num,den] = iirnotch(w0,bw,ab)
[num,den] = iirnotch(w0,bw) returns the numerator
num, and the denominator coefficients,
den, of the digital notching filter with the notch located at
w0, and with the bandwidth at the -3 dB point set to
bw. To design the filter,
w0 must meet the
condition 0.0 <
w0 < 1.0, where 1.0 corresponds to π radians
per sample in the frequency range.
The quality factor (Q factor)
q for the filter is related to the filter
w0 is the notch frequency.
[num,den] = iirnotch(w0,bw,ab) returns the digital
notching filter whose bandwidth,
bw, is specified at a level of
ab decibels. Including the optional input argument
ab lets you specify the magnitude response bandwidth at a level
that is not the default -3 dB point, such as -6 dB or 0 dB. If not specified,
ab defaults to the -3 dB width (10*log10(1/2)).
Design and plot an IIR notch filter that removes a 60 Hz tone (f0) from a signal at 300 Hz (fs). For this example, set the Q factor for the filter to 35 and use it to specify the filter bandwidth:
wo = 60/(300/2); bw = wo/35; [b,a] = iirnotch(wo,bw); fvtool(b,a);
The notch filter has the desired bandwidth with the notch located at 60 Hz, or radians per sample. Compare this plot to the comb filter plot shown on the reference page for iircomb.
Usage notes and limitations:
All inputs must be constant. Expressions or variables are allowed if their values do not change.