Transform IIR lowpass filter to IIR bandstop filter


[Num,Den,AllpassNum,AllpassDen] = iirlp2bs(B,A,Wo,Wt)


[Num,Den,AllpassNum,AllpassDen] = iirlp2bs(B,A,Wo,Wt) returns the numerator and denominator vectors, Num and Den of the bandstop digital filter. AllpassNum and AllpassDen are the vectors of numerator and denominator coefficients of the allpass mapping filter. The prototype lowpass filter is given with a numerator specified by B and a denominator specified by A.

This transformation effectively places one feature of an original filter, located at frequency -Wo, at the required target frequency location, Wt1, and the second feature, originally at +Wo, at the new location, Wt2. Choice of the feature subject to the lowpass to bandstop transformation is not restricted only to the cutoff frequency of an original lowpass filter. In general it is possible to select any feature; e.g., the stopband edge, the DC, the deep minimum in the stopband, or other ones. It is assumed that Wt2 is greater than Wt1. Frequencies must be normalized to be between 0 and 1, with 1 corresponding to half the sample rate.

This transformation implements the "Nyquist Mobility," which means that the DC feature stays at DC, but the Nyquist feature moves to a location dependent on the selection of Wo and Wts.

Relative positions of other features of an original filter change in the target filter. This means that it is possible to select two features of an original filter, F1 and F2, with F1 preceding F2. After the transformation feature F2 will precede F1 in the target filter. However, the distance between F1 and F2 will not be the same before and after the transformation.

For more details on the lowpass to bandstop frequency transformation, see Digital Frequency Transformations.


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Design a prototype real IIR lowpass elliptic filter with a gain of about –3 dB at 0.5π rad/sample.

[b,a] = ellip(3,0.1,30,0.409);

Create a bandstop filter by placing the cutoff frequencies of the prototype filter at 0.25π and 0.75π.

[num,den] = iirlp2bs(b,a,0.5,[0.25 0.75]);

Compare the magnitude responses of the filters using FVTool.

fvt = fvtool(b,a,num,den);



Numerator of the prototype lowpass filter


Denominator of the prototype lowpass filter


Frequency value to be transformed from the prototype filter


Desired frequency locations in the transformed target filter


Numerator of the target filter


Denominator of the target filter


Numerator of the mapping filter


Denominator of the mapping filter

Frequencies must be normalized to be between 0 and 1, with 1 corresponding to half the sample rate.


[1] Constantinides, A.G., “Spectral transformations for digital filters,” IEEE® Proceedings, vol. 117, no. 8, pp. 1585-1590, August 1970.

[2] Nowrouzian, B. and A.G. Constantinides, “Prototype reference transfer function parameters in the discrete-time frequency transformations,” Proceedings 33rd Midwest Symposium on Circuits and Systems, Calgary, Canada, vol. 2, pp. 1078-1082, August 1990.

[3] Nowrouzian, B. and L.T. Bruton, “Closed-form solutions for discrete-time elliptic transfer functions,” Proceedings of the 35th Midwest Symposium on Circuits and Systems, vol. 2, pp. 784-787, 1992.

[4] Constantinides, A.G., “Design of bandpass digital filters,” IEEE Proceedings, vol. 1, pp. 1129-1231, June 1969.

Introduced in R2011a