MATLAB: Phase-plot when using freqz

freqzMATLABphase

Hi all.

Why does these two methods not create the same plot?

figure()
G3=[0.0178750173569734]
num3=[1,-2.22044604925031e-16,-1.00000000000000]
den3=[1,-1.96419015432470,0.964249965286053]
freqz(G3*num3,den3,1000,44100);
ax = findall(gcf, 'Type', 'axes');
set(ax, 'XLim', [0 500]), ylim([-15 1])

Amplitude-plot is the same, but phae plot differ from this one:

[H,w]=freqz(G3*num3,den3,1000,44100);
figure(), subplot(2,1,1)
plot(w,20*log10(abs(H)))
title('Frekvenskarakteristik')
xlabel('Frekvens [Hz]'), ylabel('Amplitude [dB]')
axis([0 500 -15 1]), grid
subplot(2,1,2)
plot(w,angle(H))
xlabel('Frekvens [Hz]'), ylabel('Fase')
axis([0 500 -100 50]), grid

Best Answer

It is a matter of expressing angle in degrees or radians. Replace:

plot(w,angle(H))

with

plot(w, 360/(2*pi)*angle(H))

Related Solutions

MATLAB: How to plot the result of multiple freqs functions in the same grid

I suspect the issue it looks different is because your axes are using a linear scale instead of a logarithmic. Try this.

first plot uses loglog. Removed 20*log10 from Y values. Replaced the axes command with xlim.
Second plot uses semilogx

%Filter Butterworth
[zb,pb,kb] = butter(3,1,'s');
[bb,ab] = zp2tf(zb,pb,kb);
[h{1},w{1}] = freqs(bb,ab,1000);
figure
freqs(bb,ab,1000);
%Filter  Chebyshev
[z1,p1,k1] = cheby1(3,1,1,'s');
[b1,a1] = zp2tf(z1,p1,k1);
[h{2},w{2}] = freqs(b1,a1,1000);
figure
freqs(b1,a1,1000);
%Filter  Eliptic
[ze,pe,ke] = ellip(3,1,30,1,'s');
[be,ae] = zp2tf(ze,pe,ke);
[h{3},w{3}] = freqs(be,ae,1000);
figure
freqs(be,ae,1000);
%Filter  Bessel
[zs,ps,ks] = besself(3,1);
[bs,as] = zp2tf(zs,ps,ks);
[h{4},w{4}] = freqs(bs,as,1000);
figure
freqs(bs,as,1000);
figure
subplot(2,1,1)
for k = 1:4
    loglog(w{k},abs(h{k}))
    hold on
end
hold off
grid
xlim([0.01 10]);
xlabel('Frequency (rad/s)');
ylabel('Attenuation (dB)');
legend('Butterworth','Chebyshev','Elliptik','Bessel','Location',"southwest");
subplot(2,1,2)
for k = 1:4
    semilogx(w{k},angle(h{k})*180/pi)
    hold on
end
hold off
grid
xlabel('Frequency (rad/s)');
ylabel('Phase');
legend('Butterworth','Chebyshev','Elliptik','Bessel','Location',"southwest");

MATLAB: How does phasez work

Hi! Indeed 'phasez()' plots the unwrapped angle, but 'phasez()' acts on the coefficients of the filter and 'unwrap(angle())' acts on the frequency response of the filter. Thus, in your code you should employ 'timeFilter' as the argument in 'phasez()' and 'h' as the argument of 'unwrap(angle())'. Attached your sample code with this modification.

timeFilter = [1 2 3 4 5 6 7 8 9];
h = freqz(timeFilter, [1 zeros(1, length(timeFilter))]);
[phi, w] = phasez(timeFilter);
plot(w, phi);
hold on;
plot(w, unwrap(angle(h)),'o');
xlabel('frequency (rad/sample)');
ylabel('phase (radians)')
legend({'phasez', 'unwrap(angle)'});

Best Answer

Related Solutions

MATLAB: How to plot the result of multiple freqs functions in the same grid

MATLAB: How does phasez work

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