A reflective metasurface enabling linear-to-circular polarization (LP-to-CP) conversion is proposed. The metasurface polarizer is composed of active unit cells incorporating voltage-controlled varactor diodes, where the dispersion responses of the cells can be tailored. When illuminated by an incident wave, the metasurface converts the LP wave from the source to a CP one. Moreover, in a single voltage configuration, two circular polarized waves with different handedness can be obtained at distinct frequencies. Such operation can be tuned in frequency by changing the applied voltage along the metasurface. Simulations are performed to verify the concept, and measurements done on a fabricated prototype validate the polarization conversion performances at microwave frequencies.