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We designed and successfully tested a broadband-tunable terahertz circular polarizer, which we developed for our magneto-optical measurements using the far-infrared/THz laser source in the range of 0.25–7.5 THz. We described in detail a new lock-in detection method of tuning a suitable polarization state of THz beam. This unique experimental setup allows us to measure the cyclotron resonance of electrons (in semiconductors) and vortices (in superconductors).