This paper describes an on-chip instrument for jitter estimation of clock signals in the GHz range with a sub-picosecond resolution. A self-referenced technique is used to remove the need of a very clean external reference clock. The instrument has been designed in STMicroelectronics 28 nm FDSOI technology. By exploiting the fine delay control which can be achieved with this technology, simulation results haven shown a resolution down to 100 fs for GHz clock signals with a simple calibration procedure.