downsample_frd.m

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% This file is part of pichim's controller tuning framework.
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%%
function Gds = downsample_frd(G, Ts, freq)

    % the frequency response is infinite at some frequencies due to poles on the stability boundary. 
    g = squeeze(freqresp(G, 2*pi*freq(2:end)));
    g = [g(1); g]; % copy second value to dc in case it is an integrating tf
    
    % i think this is not necessary... can't think atm
    if(isinf(g(1)))
        g(1) = g(2);
    end
    
    g(freq > 1/2/Ts) = flipud(conj(g(freq > 0 & freq < 1/2/Ts)));
    Gds = frd(g, freq, Ts, 'Units', 'Hz');

end