This paper presents a 56 GHz Sub-Sampling Phase-Locked Loop (SSPLL) for an optical transceiver system. It employs an LC oscillator without frequency multiplier featuring a novel combined resonator for high purity signals, a differential track-and-hold with dummy samplers, a charge pump with feedback amplifiers and specialized input pairs for high voltage operation. The sub-sampling architecture allows for ultra-low phase noise at low offset frequencies, while far-out phase noise is minimized due to the used high-purity oscillator. The phase noise at an 1 MHz amounts to -127 dBc/Hz resulting in a total integrated jitter of 19 fs (range from 1 kHz to 1 GHz). This 56 GHz SSPLL enables next-generation wire-line optical communication standards with over 100 Gbit/s.