DescriptionButterworth phases 3.svg	English: phase response of a 3rd order lowpass Butterworth filter
Date	7 June 2012
Source	Own work
Author	Fcorthay
SVG development InfoField	The SVG code is valid.  This vector image was created with Inkscape, or with something else.

This graphics has been realized with the help of the following Octave script:

#-------------------------------------------------------------------------------
# Specifications
#
order = 3;
fc = 1;
pointNb = 100;
axesLength = 1.25;
textOffset = 0.05;
wLog = logspace(-2, 2, pointNb);
AdbMax = 10;
AdbMin = -60;

makeGraphics = 1;

figureIndex = 0;

#===============================================================================
# Butterworth filter
#
[zeroes, poles, gain] = butter(order, fc, 's');

[num, den] = butter(order, fc, 's');
while ( length(num) < length(den) )
  num = [0, num];
endwhile;

#-------------------------------------------------------------------------------
# Poles plot
#
figureIndex = figureIndex+1;
figure(figureIndex);
angle = linspace(pi/2, 3*pi/2, pointNb);
plot(cos(angle), sin(angle));

hold on;
plot([-axesLength, axesLength-1], [0, 0], 'k');
plot([0, 0], [-axesLength, axesLength], 'k');
plot(axesLength-1+textOffset*[0, 2, 0, 0], textOffset*3/4*[-1, 0, 1, -1], 'k');
plot(textOffset*3/4*[-1, 0, 1, -1], axesLength+textOffset*[0, 2, 0, 0], 'k');
plot(poles, 'xr');
hold off;

axis("equal", "off");
text(axesLength-1+textOffset, 1.5*textOffset, "re(s)");
text(textOffset, axesLength+1.5*textOffset, "im(s)");

for index = 1:order
  if imag(poles(index)) > 0
    text(real(poles(index))-textOffset, imag(poles(index))+textOffset, ...
    sprintf("p%i", index), "horizontalalignment", "right");
  elseif imag(poles(index)) == 0
    text(real(poles(index))-textOffset, imag(poles(index))+textOffset, ...
    "pr", "horizontalalignment", "right");
  endif
endfor

if (makeGraphics != 0)
  print -dsvg butterworth_poles_3.svg
endif

#-------------------------------------------------------------------------------
# Amplitude plot
#
Adb = 20*log10(abs(freqs(num, den, wLog)));

figureIndex = figureIndex+1;
figure(figureIndex);
semilogx(wLog, Adb);
hold on;
for index = 1:order
  if imag(poles(index)) > 0
    numP = [0, 0, 1];
    denP = [1, -2*real(poles(index)), abs(poles(index))^2];
    Adb = 20*log10(abs(freqs(numP, denP, wLog)));
    semilogx(wLog, Adb, 'g');
  elseif imag(poles(index)) == 0
    numP = [0, 1];
    denP = [1, -real(poles(index))];
    Adb = 20*log10(abs(freqs(numP, denP, wLog)));
    semilogx(wLog, Adb, 'g');
  endif
endfor
hold off;

axis([wLog(1), wLog(length(wLog)), AdbMin, AdbMax]);
xlabel('Frequency');
ylabel('Amplitude responses');
grid;

if (makeGraphics != 0)
  print -dsvg butterworth_amplitudes_3.svg
endif

#-------------------------------------------------------------------------------
# Phase plot
#
phase = arg(freqs(num, den, wLog));

figureIndex = figureIndex+1;
figure(figureIndex);
semilogx(wLog, phase);
hold on;
for index = 1:order
  if imag(poles(index)) > 0
    numP = [0, 0, 1];
    denP = [1, -2*real(poles(index)), abs(poles(index))^2];
    phase = arg(freqs(numP, denP, wLog));
    semilogx(wLog, phase, 'g');
  elseif imag(poles(index)) == 0
    numP = [0, 1];
    denP = [1, -real(poles(index))];
    phase = arg(freqs(numP, denP, wLog));
    semilogx(wLog, phase, 'g');
  endif
endfor
semilogx([wLog(1), wLog(length(wLog))], -pi/2*[1, 1], 'c');
semilogx([wLog(1), wLog(length(wLog))],   0  *[1, 1], 'c');
semilogx([wLog(1), wLog(length(wLog))],  pi/2*[1, 1], 'c');
hold off;

axis([wLog(1), wLog(length(wLog)), -pi, pi]);
xlabel('Frequency');
ylabel('Phase responses');

if (makeGraphics != 0)
  print -dsvg butterworth_phases_3.svg
endif

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