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We present the stabilization of terahertz (THz) quantum cascade lasers (QCLs) to a harmonic of the repetition rate of femtosecond fiber lasers. The technique that we have developed is 1) inherently broadband, i.e., it allows the stabilization of virtually any THz QCL regardless its emission frequency, and 2) can be implemented using different detection techniques such as electrooptic or photoconductive detection. Using electrooptic detection, we demonstrate phase locking of a 2.5-THz QCL, producing an RF beatnote with 70 dB of dynamic range in a bandwidth of 100 Hz. In addition, we show how the broadband character of the phase-locking technique can be used to coherently sample the emission of a multimode THz QCL that is actively mode-locked by current modulation at the roundtrip frequency.