The effect of the application of an ac external electromagnetic field to a long Josephson junction is studied by analyzing the spectrum of the radiation emitted by the junction. In the absence of the external field the junction, biased on a dc current singularity of the current-voltage characteristic, emits radiation at a frequency ƒ0 in the X-band of the microwaves. The application of an external ac field having a frequency ƒm of the order of several megahertz gives rise to the appearance of spectral components at frequencies ƒ0±nƒm, with n integer. The experimental results are described reasonably well by classical FM modulation theory.
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