Hi On page 11 of this book, http://books.google.dk/books?id=Wv2OIC_SaDUC&printsec=frontcover&hl=da#v=onepage&q&f=false, it is stated that: "The phase modulation has an additional advantage: The first two sidebands at frequencies ω+Ω and ω-Ω have equal amplitudes, but opposite phases. A lock-in detector tuned to the modulation frequency Ω therefore recieves the superposition of the two beat signals between the carrier and the two sidebands, which cancel to zero if no absorption is present". I have been trying to show that the signal is 0 when there is no absorption. What I have done is to plot the two functions [tex] \cos((\omega - \Omega)t) + \cos(\omega t) \\ -\cos((\omega + \Omega)t) + \cos(\omega t) [/tex] for some ω and Ω, but it is clear that these two functions when added do not cancel out. Have I misunderstood something here? Thanks for any help in advance. Best regards, Niles.