Phase plot for a simple digital filter looks wrong

I tried to plot the phase response of a digital filter with transfer function H(z) = 1.28* z^-1 /(1-0.8*z^-1)

To do that I enterd,

b=[1.28,0];
a=[1,-0.8];
fvtool(b,a)

After the fvtool graphic pops-up, I clicked the "Phase response" button and got a plot for phase response. The plot shows a phase of zero at (Frequency = 0) and (Frequency = pi). Also the phase plot has a single local min at (frequency = 0.205*pi), (phase = -0.9273 radians).

It's easy to work out the phase at frequency = 3.141 by hand by computing H(exp( 3.141*i )). It's easy to see that the (Arg of that number) = (direction in the complex plane) = -3.141...

However, that is not consistent with the plot created with 'fvtool' in MATLAB. I was expecting a phase plot that decreased monotonicly. Am I not using the right features in MATLAB or what?

-- Ted Ersek --

Answer 1

[Honglei Chen]

Hi Ted,

In your equation, it should be

b = [0 1.28]

HTH