A 900 MHz CMOS LC-oscillator with quadrature outputs

The local oscillator (LO) in a wireless transceiver satisfies many exacting requirements. A variable frequency enables a phase-locked loop (PLL) to servo the LO to a stable lower frequency reference, or to correct frequency errors from measurements on the received signal. A low phase noise ensures little interference with nearby channels. A large LO voltage-swing means that it can drive a mixer with greater linearity. Finally, in single-sideband applications, the LO must supply precise quadrature phases. Low phase noise mandates use of a high-Q resonator to tune the LO, although most RF resonators are usually not integrable on ICs. Quadrature outputs are usually derived from RC phase-shift of a single-phase LO output, but this is susceptible to component inaccuracy and loss in LO amplitude. The authors present a 900 MHz oscillator circuit implemented in 1 /spl mu/m CMOS that affords modestly low-phase noise, has variable frequency with large output swing, and provides quadrature-phase outputs from two identical coupled oscillators, connected in such a way that they exert a mutual squelch when their relative phase is not in quadrature. The coupled oscillators synchronize to exactly the same frequency, in spite of mismatches in their resonant circuits.