dan@0: function [fk fk_ders] = fourier_kernel(frqs, N,fs)
dan@0: R = size(frqs,2);
dan@0: t = [-(N-1)/2:(N-1)/2]'/fs;
dan@0: 
dan@0: t_mat = repmat(t,[1,R]);
dan@0: frq_mat = repmat(frqs,[N,1]);
dan@0: fk = exp(1j .* t_mat .* frq_mat);
dan@0: fk_ders = 1j * fk .* frq_mat;
dan@0: end