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A two-frequency microchip laser based on highly doped ceramic Nd:YAG and twisted polarization modes is presented. The frequency difference between modes was tunable continuously over a frequency range of 150 GHz, from radio frequency to terahertz frequencies. This tuning is limited by the gain bandwidth of the neodymium-doped YAG laser medium. The long-term frequency stability of the resulting narrow-linewidth beat-note signal is very good. This offers a simple, yet widely tunable terahertz source with potential for portable frequency reference applications.