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Cutoff frequency variable LPF by IIR 6th-order Butterworth filter for ST Nucleo F401RE.
Dependencies: UIT_IIR_Filter UIT_ACM1602NI UITDSP_ADDA mbed UIT_AQM1602
BilinearDesignLpfHpf/BilinearDesignLH.cpp
- Committer:
- MikamiUitOpen
- Date:
- 2014-10-23
- Revision:
- 0:a9412b9e85b7
- Child:
- 5:b291c4653eb9
File content as of revision 0:a9412b9e85b7:
//------------------------------------------------------------------------------ // Design of Butterworth LPF and HPF using bilinear transform // // 2014/06/29, Copyright (c) 2014 MIKAMI, Naoki //------------------------------------------------------------------------------ #include "BilinearDesignLH.hpp" namespace Mikami { // Execute design // input // fc: Cutoff frequency // output // c : Coefficients for cascade structure // g : Gain factor for cascade structure void BiliearDesign::Execute(float fc, Coefs c[], float& g) { Butterworth(); Bilinear(fc); ToCascade(); GetGain(); GetCoefs(c, g); } // Get poles for Butterworth characteristics void BiliearDesign::Butterworth() { float pi_2order = PI_/(2.0f*ORDER_); for (int j=0; j<ORDER_/2; j++) // Pole with imaginary part >= 0 { float theta = (2.0f*j + 1.0f)*pi_2order; sP_[j] = Complex(-cosf(theta), sinf(theta)); } } // Bilinear transform // fc: Cutoff frequency void BiliearDesign::Bilinear(float fc) { float wc = tanf(fc*PI_FS_); for (int k=0; k<ORDER_/2; k++) zP_[k] = (1.0f + wc*sP_[k])/(1.0f - wc*sP_[k]); } // Convert to coefficients for cascade structure void BiliearDesign::ToCascade() { for (int j=0; j<ORDER_/2; j++) { ck_[j].a1 = 2.0f*real(zP_[j]); // a1m ck_[j].a2 = -norm(zP_[j]); // a2m ck_[j].b1 = (PB_ == LPF) ? 2.0f : -2.0f; // b1m } } // Calculate gain factor void BiliearDesign::GetGain(){ float u = (PB_ == LPF) ? 1.0f : -1.0f; float g0 = 1.0f; for (int k=0; k<ORDER_/2; k++) g0 = g0*(1.0f - (ck_[k].a1 + ck_[k].a2*u)*u)/ (1.0f + (ck_[k].b1 + u)*u); gain_ = g0; } // Get coefficients void BiliearDesign::GetCoefs(Coefs c[], float& gain) { for (int k=0; k<ORDER_/2; k++) { c[k].a1 = ck_[k].a1; c[k].a2 = ck_[k].a2; c[k].b1 = ck_[k].b1; } gain = gain_; } }