A subspace resonator was a technology capable of producing a warp field.

In 2368, an engineering team from the starship Enterprise modified a subspace resonator to act as a makeshift warp core, for use on a Romulan vessel which had suffered catastrophic damage to their own core. Commander Riker noted that the resonator would not be capable of producing speeds in excess of warp 2, but that it would be sufficient for the vessel to return to Romulan space. (TNG: "The Next Phase")