f[t_] := 2*Cos[2 Pi 120 t] + 3*Cos[2 Pi 125 t];
Plot[f[t], {t, 0, 2}]
dt = 0.002; a = Table[f[k*dt], {k, 1000}];
fa = Fourier[a];
ListPlot[Abs[fa], PlotRange -> All]
Export["a.csv", a]


x = Import["b.csv"] // Flatten;
ListPlot[x]
fx = Fourier[x];
ListPlot[Abs[fx], PlotRange->All]