ction = new HighpassFilterButterworthSection Public HighpassFilterButterworthImplementation Protected HighpassFilterButterworthSection section Public class HighpassFilterButterworthImplementation compute the result as the cascade of the fir and iir filters return 0 * Fs * zeta * omegac) + (omegac * omegac)) / (-b0) 0 * Fs * zeta * omegac) + (omegac * omegac) iir section // normalize coefficients so that b0 = 1, // and higher-order coefficients are scaled and negated double b0 = ( 4. 0 * Fs * Math.Tan(Math.PI * cutoffFrequencyHz / Fs) ĭouble zeta = -Math.Cos(Math.PI * ( 2. compute the fixed filter coefficients double omegac = 2. ( double cutoffFrequencyHz, double k, double n, double Fs) Protected IIRFilterImplementation iirFilter = new IIRFilterImplementation( 2) Protected FIRFilterImplementation firFilter = new FIRFilterImplementation( 3) Public class LowpassFilterButterworthSection (cutoffFrequencyHz, i + 1, numSections * 2, Fs) įor ( int i = 0 i < i++) ction = new LowpassFilterButterworthSection ( double cutoffFrequencyHz, int numSections, double Fs) Public LowpassFilterButterworthImplementation Protected LowpassFilterButterworthSection section Public class LowpassFilterButterworthImplementation Then call the compute() method on the object repeatedly, passing in an input sample and receiving a filtered sample as the return value. To use the LowpassFilterButterworthImplementation, HighpassFilterButterworthImplementation, or BandpassFilterButterworthImplementation, construct the object with the desired cutoff frequency or frequencies, number of 2-pole sections, and the sample frequency (Fs).
#Bandpass filter designer code#
The code includes an implementation of an n th-order FIR filter for the zero (numerator) polynomials and an implementation of an n th-order IIR filter for the pole (denominator) polynomials. A bandpass filter is implemented as a cascade of a lowpass and a highpass filter. In the code, the lowpass and highpass filters are implemented according to the coefficient derivation from the design document. Then a substitution of variable s=1/s transforms the lowpass filter into a highpass filter, and the bilinear z-transform is used to derive the coefficients of the equivalent highpass digital filter. The design starts with a continuous-time lowpass Butterworth filter and uses the bilinear z-transform to derive the coefficients of the equivalent digital filter. BackgroundĪ Word document giving the filter design via bilinear z-transformation is included. This is a C# implementation of digital lowpass, highpass, and bandpass Butterworth filters of arbitrary order (n cascaded 2-pole sections).
#Bandpass filter designer download#
Download Butterworth_Filter_Design.zip - 17.5 KB.
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