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Generation of cw-THz by photomixing is an attractive solution for high resolution spectroscopy, nevertheless it has suffered from a lack of frequency metrology to determine the line centre frequencies. The phase locking of two extended cavity laser diodes (ECLD) to a frequency comb (FC) has allowed THz frequencies to be synthesized and an accuracy of ΔνTHz/νTHz = 10-8 obtained, however, the continuous tuning range is limited to 10 MHz. A new configuration based on a single FC and three cw-lasers has been implemented and provides a tuning range in excess of 300 MHz. The use of a single FC eliminates the need to measure and stabilize the carrier envelope offset frequency of the FC. The system also has the advantage of establishing a direct link between the THz frequency and any microwave frequency standard without the need of any calibration steps.