We report on a compact ( footprint) silicon photonics integrated system performing high-sensitivity monitoring of in-band optical signal-to-noise ratio (OSNR). The system, including a thermally tunable racetrack resonator filter and an unbalanced Mach-Zehnder interferometer, performs the autocorrelation measurement of a filtered fraction of the noisy signal spectrum. Monitor performance is evaluated on a 10 Gb/s ON/OFF keying nonreturn to zero signal, demonstrating an accuracy of 0.4 dB over a wide OSNR range from 8 to 28 dB. We also demonstrate that the proposed system induces a tiny distortion of the optical signal, making it suitable for in-line monitoring of signal quality.