Butterworth Low-pass Filter Design Tool Class

This Butterworth low-pass filter design tool can be used to design a Butterworth filter in continuous and discrete-time from the given specifications of the filter performance. The Butterworth filter is a class implementation. The object is created by a default constructor without any argument.

The filter can be prepared in three ways. If the filter specifications are known such as the passband, stopband frequencies (Wp and Ws) together with the passband and stopband ripple magnitudes (Ap and As), one can call the filter's buttord method with these arguments to obtain the recommended filter order (N) and cut-off frequency (Wc [rad/s]). An example call is demonstrated below;

ButterworthFilter bf();

Wp = 2.0; // passband frequency [rad/sec]
Ws = 3.0; // stopband frequency [rad/sec]
Ap = 6.0; // passband ripple mag or loss [dB]
As = 20.0; // stop band rippe attenuation [dB]

// Computing filter coefficients from the specs
bf.Buttord(Wp, Ws, Ap, As);

// Get the computed order and Cut-off frequency
Order_Cutoff NWc = bf.getOrderCutOff();]

cout << " The computed order is ;" << NWc.N << endl;
cout << " The computed cut-off frequency is ;" << NWc.Wc << endl;

The filter order and cut-off frequency can be obtained in a struct using bf.getOrderCutoff() method. These specs can be printed on the screen by calling PrintFilterSpecs() method. If the user would like to define the order and cut-off frequency manually, the setter methods for these variables can be called to set the filter order (N) and the desired cut-off frequency (Wc [rad/sec]) for the filter.

Obtaining Filter Transfer Functions

The discrete transfer function of the filter requires the roots and gain of the continuous-time transfer function. Therefore, it is a must to call the first computeContinuousTimeTF() to create the continuous-time transfer of the method;

bf.computeContinuousTimeTF();

The computed continuous-time transfer function roots can be printed on the screen using the methods;

bf.PrintFilter_ContinuousTimeRoots();
bf.PrintContinuousTimeTF();

The resulting screen output for a 5th order filter is demonstrated below.

 Roots of Continous Time Filter Transfer Function Denominator are : 
-0.585518 + j 1.80204
-1.53291 + j 1.11372
-1.89478 + j 2.32043e-16
-1.53291 + j -1.11372
-0.585518 + j -1.80204


The Continuous-Time Transfer Function of the Filter is ;

         24.422 
--------------------------------------------------------
1.000 *s[5] + 6.132 *s[4] + 18.798 *s[3] + 35.619 *s[2] + 41.711 *s[1] + 24.422 

Discrete Time Transfer Function (Difference Equations)

The digital filter equivalent of the continuous-time definitions is produced by using the bilinear transformation. When creating the discrete-time function of the ButterworthFilter object, its Numerator (Bn) and Denominator (An ) coefficients are stored in a vector of filter order size N.

The discrete transfer function method is called using ;

bf.computeDiscreteTimeTF();
bf.PrintDiscreteTimeTF();

The results are printed on the screen like; The Discrete-Time Transfer Function of the Filter is ;

0.191 *z[-5] + 0.956 *z[-4] + 1.913 *z[-3] + 1.913 *z[-2] + 0.956 *z[-1] + 0.191
--------------------------------------------------------
1.000 *z[-5] + 1.885 *z[-4] + 1.888 *z[-3] + 1.014 *z[-2] + 0.298 *z[-1] + 0.037

and the associated difference coefficients An and Bn by withing a struct ;

DifferenceAnBn AnBn = bf.getAnBn();

The difference coefficients appear in the filtering equation in the form of.

An * Y_filtered = Bn * Y_unfiltered  

To filter a signal given in a vector form ;

Calling Filter by a specified cut-off and sampling frequencies [in Hz]

The Butterworth filter can also be created by defining the desired order (N), a cut-off frequency (fc in [Hz]) and a sampling frequency (fs in [Hz]). In this method, the cut-off frequency is pre warped with respect to the sampling frequency [1, 2] to match the continuous and digital filter frequencies.

The filter is prepared by the following calling options;

 // 3rd METHOD defining a sampling frequency together with the cut-off fc, fs
 bf.setOrder(2);
 bf.setCuttoffFrequency(10, 100);

At this step, we define a boolean variable whether to use the pre-warping option or not.

// Compute Continuous Time TF
bool use_sampling_frequency = true;
bf.computeContinuousTimeTF(use_sampling_frequency);
bf.PrintFilter_ContinuousTimeRoots();
bf.PrintContinuousTimeTF();

// Compute Discrete Time TF
bf.computeDiscreteTimeTF(use_sampling_frequency);
bf.PrintDiscreteTimeTF();

References:

1. Manolakis, Dimitris G., and Vinay K. Ingle. Applied digital signal processing: theory and practice. Cambridge University Press, 2011.

2. https://en.wikibooks.org/wiki/Digital_Signal_Processing/Bilinear_Transform