what is the difference between "unwrap" and "phasez" function

Hello, I'm trying to analyze the phase response of S-parameter.

And there are 2 functions in Matlab in term of unwrapped phase, which are "unwrap" and "phasez".

When I plot three case below, it shows difference result.

test1 = angle(S21)*180/pi;

test2 = unwrap(angle(S21)*180/pi);

test3 = phasez(angle(S21)*180/pi);

I understood the how the "unwrap" function work. (It just make it flat.)

But, I couldn't get How the matlab create the "phasez" plot. And, I found out the "phasez" function make the matrix size to be 512*1. Is there specific reason for this?

In addition,in principle, the unwrapped phase need to start from 0 in DC.

But, the plot of test2 doesn't. So, that's also confusing.

Is there anyone to explain this question?

Answer 1

[Rachana Reddy Mamilla]

Unwrap function is used to make the phase continuous and it’s not necessary for it to Start from 0.It depends on the input you give. phasez returns the n-point unwrapped phase response vector. If n is not defined ,It would consider the default value to be 512(which is an intermediate value using which one can hopefully get most of the information). If n is not defined , it would consider the default value to be 512 (which is an intermediate value using which one can hopefully get most of the information). For best results, set n to a value greater than the filter order. You can check this for more info.

[Nathan Jessurun]

Hi Rachana,

The answer you linked no longer exists. Would you mind updating the reference?

Also, I attempted to plot the phase with a greater n than the filter order in this example (https://www.mathworks.com/matlabcentral/answers/435035-how-does-phasez-work) but it didn't make any difference. Do you have any other suggestions?

Thanks!