Non linear optimization problem in a range of frequencies

Hello, I need to optimize the phase difference of two transfer functions, such as:

    f = phase(1/(1+tau1*j*w)) - phase(1/(1+tau2*j*w))

The variables to optimize are tau1 and tau2, "j" is the imaginary number and "w" is a vector of frequencies. I need to optimize the function in a range of frequencies (specifically between 100 and 4000 Hz) so function "f" stays within a lower and upper bound (-10 to 10) in the range of 100 Hz to 4000 Hz.

Im using the optimization tool, my objective function is

    function f = objecfun(tau)

    freq = 100:0.1:4000;   %Frequency vector

    w = 2*pi*freq;         %Frequency in radians per seccond

    trans_func = @(taux) 1./(1+1i*w*taux);   % Anonymous transfer function

    trans_func_1=trans_func(tau(1));         % Calculates first transfer function
    trans_func_2=trans_func(tau(2));         % Calculates second transfer function

    f = phase(trans_func_1)-phase(trans_func_2); %Takes phase of each transfer function and substracts
    end

And my non-linear constraints function is

    function [c,ceq] = nonlconstr(tau)

    freq = 100:0.1:4000;   %Frequency vector

    w = 2*pi*freq;         %Frequency in radians per seccond

    trans_func = @(taux) 1./(1+1i*w*taux);   % Anonymous transfer function

    trans_func_1=trans_func(tau(1));         % Calculates first transfer function
    trans_func_2=trans_func(tau(2));         % Calculates second transfer function

    f = phase(trans_func_1)-phase(trans_func_2); %Takes phase of each transfer function and substracts

    c = [f -10; -f 10]; %Lower and upper bounds
    ceq = [];
    end

However when I run the optimization tool, It gives me an error that the "Supplied objective function must return a scalar value.", I know that it is because im using a vector of frequencies "w" instead of using a single frequency, so my question is: How can I run the optimization tool so the phase difference of my transfer functions (f) stays between -10 to 10 in the frequency range of interest?

[Joe]

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