Problem Calculating an instantanous frequency

Hi,

I'm trying to calculate an instantanous frequency in the (x,t) coordinates

 signal -->W=f(t)+i*h(t)

 inst. freq--> Wt=(f*h'-h*f')/(h^2+f^2);

the code I'm using (this is an example with a chirp signal);

    dt=0.002;
    nt=1001;
    tvec=linspace(0,2,nt);
    f=chirp(tvec,40,2,80);
    h=imag(hilbert(f));
    n=(f.*gradient(h)-h.*gradient(f))/dt;
    d=f.^2+h.^2;
    D=diag(d);
    w=D\n';
    figure(1)
    plot(t_vec,ww/2/pi);

the result becomes less and less acurate as the frequency increases and the increase is in a certain rate (which I think concernes the sampling rate).

u can try tt even with a wavelet containing one frequency:

    f=cos(2*pi*freq); %change values from 10 to 80

and notice the increase in the error.

I can not upsample the signal and must do the calculation in these coordinates for further work.

anyone have any ideas how to work it out if possible how to get the correct inst. frequency?