Return to Question

added image

edited Apr 12 at 7:42

a1 = 1; b1 = [-0.0228015616127155 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127155]; b2 = [-0.0228015616127150 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127150]; bode(G1 = filt(b1,a1),'-b',; G2 = filt(b2,a1); fmin = 1e-1; fmax = 0.5; Bodeoptions = bodeoptions; Bodeoptions.FreqUnits = 'Hz'; Bodeoptions.Xlim = [fmin,fmax]; Bodeoptions.Ylim = {[-100,10],[0,720]}; Bodeoptions.Grid = 'on'; bode(G1,'-b',G2,':g',Bodeoptions) h = findobj(gcf,'type','line'); set(h,'LineWidth',2);  wmin = 2*pi*fmin; H1 = freqresp(G1, wmin); H2 = freqresp(G2, wmin); gain1_dB = 20*log10(abs(H1)); gain2_dB = 20*log10(abs(H2)); phase1_deg = angle(H1)*180/pi; phase2_deg = angle(H2)*180/pi; fprintf('\nAt f = %.4f Hz:\n', fmin); fprintf('G1: Gain = %.16f dB, Phase = %.4f degrees\n', gain1_dB, phase1_deg); fprintf('G2: Gain = %.16f dB, Phase = %.4f degrees\n', gain2_dB, phase2_deg); [mag1, phase1, wout] = bode(G1, wmin); [mag2, phase2, ~] = bode(G2, wmin); gain1_dB = 20*log10(squeeze(mag1)); gain2_dB = 20*log10(squeeze(mag2)); phase1_deg = squeeze(phase1); phase2_deg = squeeze(phase2); fprintf('\nFrom bode():\n'); fprintf('G1: Gain = %.16f dB, Phase = %.4f degrees\n', gain1_dB, phase1_deg); fprintf('G2: Gain = %.16f dB, Phase = %.4f degrees\n', gain2_dB, phase2_deg);

Output:

At f = 0.1000 Hz: G1: Gain = 3.4199354940498718 dB, Phase = -72.0000 degrees G2: Gain = 3.4199354940498772 dB, Phase = -72.0000 degrees From bode(): G1: Gain = 3.4199354940498852 dB, Phase = 648.0000 degrees G2: Gain = 3.4199354940498634 dB, Phase = 288.0000 degrees

a1 = 1; b1 = [-0.0228015616127155 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127155]; b2 = [-0.0228015616127150 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127150]; bode(filt(b1,a1),'-b',filt(b2,a1),':g')

a1 = 1; b1 = [-0.0228015616127155 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127155]; b2 = [-0.0228015616127150 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127150]; G1 = filt(b1,a1); G2 = filt(b2,a1); fmin = 1e-1; fmax = 0.5; Bodeoptions = bodeoptions; Bodeoptions.FreqUnits = 'Hz'; Bodeoptions.Xlim = [fmin,fmax]; Bodeoptions.Ylim = {[-100,10],[0,720]}; Bodeoptions.Grid = 'on'; bode(G1,'-b',G2,':g',Bodeoptions) h = findobj(gcf,'type','line'); set(h,'LineWidth',2);  wmin = 2*pi*fmin; H1 = freqresp(G1, wmin); H2 = freqresp(G2, wmin); gain1_dB = 20*log10(abs(H1)); gain2_dB = 20*log10(abs(H2)); phase1_deg = angle(H1)*180/pi; phase2_deg = angle(H2)*180/pi; fprintf('\nAt f = %.4f Hz:\n', fmin); fprintf('G1: Gain = %.16f dB, Phase = %.4f degrees\n', gain1_dB, phase1_deg); fprintf('G2: Gain = %.16f dB, Phase = %.4f degrees\n', gain2_dB, phase2_deg); [mag1, phase1, wout] = bode(G1, wmin); [mag2, phase2, ~] = bode(G2, wmin); gain1_dB = 20*log10(squeeze(mag1)); gain2_dB = 20*log10(squeeze(mag2)); phase1_deg = squeeze(phase1); phase2_deg = squeeze(phase2); fprintf('\nFrom bode():\n'); fprintf('G1: Gain = %.16f dB, Phase = %.4f degrees\n', gain1_dB, phase1_deg); fprintf('G2: Gain = %.16f dB, Phase = %.4f degrees\n', gain2_dB, phase2_deg);

Output:

At f = 0.1000 Hz: G1: Gain = 3.4199354940498718 dB, Phase = -72.0000 degrees G2: Gain = 3.4199354940498772 dB, Phase = -72.0000 degrees From bode(): G1: Gain = 3.4199354940498852 dB, Phase = 648.0000 degrees G2: Gain = 3.4199354940498634 dB, Phase = 288.0000 degrees

Clarify what the OP is seeing.

Link

edited Apr 11 at 20:08

TimWescott

14.5k
1
14
29

$360^\circ$ Phase shift between Bode plots of two similar systems

Source Link

asked Apr 11 at 18:08

Neuling

Phase shift between two similar systems

I am trying to identify the characteristics of an FIR filter in an ideal case as a sanity check. I observe that there is no strange behaviour, but there is an unexplainable issue in the phase plot of the original and identified filters. To make it simple, I've provided the transfer function coefficients below:

a1 = 1; b1 = [-0.0228015616127155 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127155]; b2 = [-0.0228015616127150 0.445214656159413 0.776226769351428 0.445214656159413 -0.0228015616127150]; bode(filt(b1,a1),'-b',filt(b2,a1),':g')

The difference between b1 and b2 is only at the level of machine precision, yet I observe 360$^\circ$ phase shift in the phase plot. I understand that a 360$^\circ$ phase shift has no physical significance, but I would like to understand why this happens with the bode function in MATLAB. Also, it would be great if you could tell me how to correct this when plotting?