In a semiconductor device that we are working on, we are observing that if we input an ac signal, the frequency of the signal can be doubled and tripled with some good efficiency. I have been wondering about the possible applications of such a harmonic generator. Since circuits is not really my forte, I would like to learn more about the potential of such a result. I have read that Phase-locked loops (PLL) have a good application for harmonic generation. What are the other applications where we can possibly try out this harmonic generation ability? And what would be the required quality of these harmonics if they are to be useful? Thanks in advance!