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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
Diff: BilinearDesignLpfHpf/BilinearDesignLH.cpp
- Revision:
- 0:a9412b9e85b7
- Child:
- 5:b291c4653eb9
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/BilinearDesignLpfHpf/BilinearDesignLH.cpp Thu Oct 23 06:44:40 2014 +0000 @@ -0,0 +1,80 @@ +//------------------------------------------------------------------------------ +// 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_; + } +} + +