how to constrain tfest to give minimum phase zeros

I am trying to apply the MATLAB ident toolbox function "tfest" to some data. It is returning a non-minimum phase zero, yet I get a better visual fit to the phase of the data if I force the zero to be minimum phase.

Why does "tfest" do this and how do I force the zeros to be minimum phase?

Answer 1

[Alan]

init_sys = idtf([NaN NaN NaN],[1 NaN NaN NaN]);
init_sys.Structure.num.Minimum = 0;
init_sys.Structure.den.Minimum = 0;

T=tfest(z,init_sys);

is the solution.