This example shows how to design complex bandpass filters. Complex bandpass filters are used in many applications from IF subsampling digital down converters to vestigial sideband modulation schemes for analog and digital television broadcast. One easy way to design a complex bandpass filter is to start with a lowpass prototype and apply a complex shift frequency transformation. In this example, we review several cases of lowpass prototypes from single-stage single-rate FIR filters to multistage multirate FIR filters to IIR filters.

**Single-Stage Single-Rate FIR Design**

In the case of a single-rate FIR design, we simply multiply each set of coefficients by (aka 'heterodyne with') a complex exponential. In the next example, we rotate the zeros of the lowpass Nyquist filter prototype by a normalized frequency of .6.

Hlp = design(fdesign.nyquist(8)); % Lowpass prototype N = length(Hlp.Numerator)-1; Fc = .6; % Desired frequency shift j = complex(0,1); Hbp = copy(Hlp); Hbp.Numerator = Hbp.Numerator.*exp(j*Fc*pi*(0:N)); hfvt = fvtool(Hlp,Hbp,'Color','white'); legend(hfvt,'Lowpass Prototype','Complex Bandpass','Location','NorthWest')

The same technique also applies to single-stage multirate filters.

**Multirate Multistage FIR Design**

In the case of multirate multistage FIR filters, we need to account for the different relative frequencies each filter operates on. In the case of a multistage **decimator**, the desired frequency shift applies only to the **first** stage. Subsequent stages must also scale the desired frequency shift by their respective cumulative decimation factor.

f = fdesign.decimator(16,'nyquist',16,'TW,Ast',.01,75); Hd = design(f,'multistage'); N1 = length(Hd.Stage(1).Numerator)-1; N2 = length(Hd.Stage(2).Numerator)-1; N3 = length(Hd.Stage(3).Numerator)-1; M12 = Hd.Stage(1).DecimationFactor; % Decimation factor between stage 1 & 2 M23 = Hd.Stage(2).DecimationFactor; % Decimation factor between stage 2 & 3 Fc = -.2; % Desired frequency shift Fc1 = Fc; % Frequency shift applied to the first stage Fc2 = Fc*M12; % Frequency shift applied to the second stage Fc3 = Fc*M12*M23; % Frequency shift applied to the third stage Hdbp = copy(Hd); Hdbp.Stage(1).Numerator = Hdbp.Stage(1).Numerator.*exp(j*Fc1*pi*(0:N1)); Hdbp.Stage(2).Numerator = Hdbp.Stage(2).Numerator.*exp(j*Fc2*pi*(0:N2)); Hdbp.Stage(3).Numerator = Hdbp.Stage(3).Numerator.*exp(j*Fc3*pi*(0:N3)); hfvt.Filters = [Hd,Hdbp]; legend(hfvt,'Lowpass Prototype','Complex Bandpass','Location','NorthWest')

Similarly, in the case of a multistage **interpolator**, the desired frequency shift applies only to the **last** stage. Previous stages must also scale the desired frequency shift by their respective cumulative interpolation factor.

f = fdesign.interpolator(16,'nyquist',16,'TW,Ast',.01,75); Hi = design(f,'multistage'); N1 = length(Hi.Stage(1).Numerator)-1; N2 = length(Hi.Stage(2).Numerator)-1; N3 = length(Hi.Stage(3).Numerator)-1; L12 = Hi.Stage(2).InterpolationFactor; % Interpolation factor % between stage 1 & 2 L23 = Hi.Stage(3).InterpolationFactor; % Interpolation factor % between stage 2 & 3 Fc = .4; % Desired frequency shift Fc3 = Fc; % Frequency shift applied to the third stage Fc2 = Fc*L23; % Frequency shift applied to the second stage Fc1 = Fc*L12*L23; % Frequency shift applied to the first stage Hibp = copy(Hi); Hibp.Stage(1).Numerator = Hibp.Stage(1).Numerator.*exp(j*Fc1*pi*(0:N1)); Hibp.Stage(2).Numerator = Hibp.Stage(2).Numerator.*exp(j*Fc2*pi*(0:N2)); Hibp.Stage(3).Numerator = Hibp.Stage(3).Numerator.*exp(j*Fc3*pi*(0:N3)); hfvt.Filters = [Hi,Hibp]; legend(hfvt,'Lowpass Prototype','Complex Bandpass','Location','NorthWest')

**Single-Rate IIR Design**

Finally in case of single-rate IIR designs, we can either use a complex shift frequency transformation or a lowpass to complex bandpass IIR transformation. In the latter case, the bandwidth of the bandpass filter may also be modified.

Fp = .2; Hiirlp = design(fdesign.lowpass(Fp,.25,.5,80),'ellip'); Fc = .6; % Desired frequency shift Hiircbp = ciirxform(Hiirlp, ... % Shift frequency transformation 'zpkshiftc', 0, Fc); % DC shifted to Fc Hiircbp2 = iirlp2bpc(Hiirlp, ... % Lowpass to complex bandpass transf. Fp, [Fc-Fp, Fc+Fp]); % Lowpass passband frequency mapped % to bandpass passband frequencies hfvt.Filters = [Hiirlp,Hiircbp,Hiircbp2]; legend(hfvt,'Lowpass Prototype','Complex Bandpass #1',... 'Complex Bandpass #2','Location','NorthWest')

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