MATLAB® functions are available to extract the phase response of a filter. Given a frequency response, the function
abs returns the magnitude and
angle returns the phase angle in radians. To view the magnitude and phase of a Butterworth filter using
d = designfilt('lowpassiir','FilterOrder',9, ... 'HalfPowerFrequency',400,'SampleRate',2000); fvtool(d,'Analysis','freq')
You can also click the Magnitude and Phase Response button on the toolbar or select Analysis > Magnitude and Phase Response to display the plot.
unwrap function is also useful in frequency analysis.
unwrap unwraps the phase to make it continuous across 360° phase discontinuities by adding multiples of ±360°, as needed. To see how
unwrap is useful, design a 25th-order lowpass FIR filter:
h = fir1(25,0.4);
Obtain the frequency response with
freqz and plot the phase in degrees:
[H,f] = freqz(h,1,512,2); plot(f,angle(H)*180/pi) grid
It is difficult to distinguish the 360° jumps (an artifact of the arctangent function inside
angle) from the 180° jumps that signify zeros in the frequency response.
unwrap eliminates the 360° jumps:
Alternatively, you can use
phasez to see the unwrapped phase: