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Difference frequency generation is a flexible method for the generation of mid-infrared light. It allows downshifting a near-infrared optical signal to the mid-infrared region by means of a nonlinear process, enabling to take full advantage of the availability of optical communications components for the generation of the multi-tone signals that are characteristic of molecular dispersion spectroscopic methods. In this way, it is possible to avoid several issues associated with optical instruments in which directly modulated mid-infrared quantum cascade lasers are employed. In this paper, we take full advantage of this benefit to implement the first heterodyne phase sensitive dispersion spectroscopy gas sensor based on a difference frequency generation source. The performance of the instrument is validated by detecting with high sensitivity methane in the 3.4 mum mid-infrared band.