An apparatus comprises a ring oscillator comprising a plurality of delay stages connected in cascade and an injection apparatus comprising a plurality of injection devices, wherein the injection devices receive a reference clock from their inputs and outputs of the injection devices are coupled to respective outputs of the delay stages, and wherein each injection device comprises a polarity selection stage having inputs coupled to the reference clock and an adjustable gain stage having inputs coupled to outputs of the polarity selection stage and outputs coupled to outputs of a corresponding delay stage.