Error in discrete-time transfer function bode plot

Question

Arpit Aggarwal 2018-4-16

回答： Star Strider 2018-4-16

Hello,

I am having an error in the bode plot for a discrete-time transfer function. The entire plot has been shifted to the left on the frequency axis.

I use Ubuntu 16.04 (subsequently the Linux version of MATLAB 2017a)

The code:

sys = tf([0 0 0 0.10276 0.18123], [1 -1.99185 2.20265 -1.84083 0.89413], -1, 'variable', 'z^-1');
bode(sys);
grid on

The plot that I get is attached below:

Where as the right plot is (Got by running the commands on a Mac & Windows PC):

The difference can be seen on the frequency axis.

Does anyone know what is wrong?

Answer 1

Star Strider 2018-4-16

There is nothing wrong. The upper plot begins at 1E-3 r/s, and the lower plot begins at 1E-2 r/s.

If you want them all to look the same, you can specify the frequency vector as for example:

w = logspace(-2, 3);

They should then all have the same frequency axis.