Injection locked coupled VCOs for low phase noise and high accuracy quadrature generation

Coupled VCOs for quadrature generation allow very accurate 90 degrees phase shifted output signal but suffer from a trade-off between phase noise and phase accuracy, leading to poor noise times power product. In this work, we propose to synchronize them by means of an additional VCO. In this way, the coupled VCOs guarantee the quadrature accuracy between output signals, whereas the synchronizing VCO sets phase noise. The design is tailored to a DCS-1800 system. Prototypes, realized in a 0.18 mu m CMOS technology, show the following: 285MHz tuning band, 45dB minimum Image Band Rejection, -127 dBc/Hz Phase Noise at 600kHz offset, while drawing 10mA from 1.8V. The Phase Noise figure of merit is 185dB, 20dB better than free running quadrature VCOs.