A Phase modulator was a primary component of phase-modulated energy weapons, including phase pistols and phase cannons, utilized by Earth Starfleet in the 22nd century. The modulator is the component which shifts the intensity of the weapon's energy beam.
In 2151, the phase modulator of the recently-installed phase cannons aboard the NX-01 Enterprise overloaded due to interference from a hostile alien race, and the energy output of the cannons was ten times what was expected. The overload also caused a plasma recoil, which blew out relays on decks "C" and "D".
When the aliens returned and attacked the Enterprise, the phase cannons were ineffective against their vessel. Chief engineer Tucker and armory officer Reed then intentionally overloaded the modulators a second time, using the gravity plating of the affected decks to absorb the recoil. The overload once again increased the weapon's output, allowing Enterprise to cripple the enemy vessel. (ENT: "Silent Enemy")