A phase detector (PD) based on quadrature modulation is presented, which is used to perform a phase delay measurement due to signal path at X-band. The X-band signal phase difference is converted to the baseband signal difference through complex frequency conversion. In this paper, an improved active balun and a two-stage tunable poly-phase filter (PPF) are used to generate broadband in-phase and quadrature (I/Q) signals. L-C resonance-based double-balanced Gilbert cells are used as mixers for modulation and demodulation. The proposed X-band PD is implemented in 28-nm CMOS technology and occupies 0.33mm2. The simulated maximal phase error is less than 0.75° over 8-12GHz. The PD consumes 3.5 mW with 0.9V power supply, achieving 0°-180° extended linear phase detection range.