Small signal analysis of quadrature LC oscillator operating at 59–62.5 GHz

Abstract

In this study, a comprehensive small signal analysis method for quadrature oscillators based on cross-coupled LC-tuned oscillators and parallel coupling is proposed. The analysis is suitable for circuits operating up to millimetre wave frequencies since the key parasitics of all circuit elements such as core amplifiers, coupling amplifiers, buffers, inductors and varactors are considered. This allows for efficient circuit and device optimisations. The proposed method is based on the consequent parallelisation of all individual network elements into a simple overall equivalent RLC tank. The analysis is employed for CMOS but can also be adapted to other technologies. To verify the analysis, a circuit with oscillation frequency around 61 GHz was designed in 90 nm silicon on insulator (SOI) CMOS. Frequency control is enabled by adjusting the gain in the feedback amplifiers. Hence, no lossy varactors are necessary. At 50 Ω terminations, a supply voltage of 1.5 V and a total supply current of less than 56 mA, a tuning range above 3 GHz, an output power per channel of −19 dBm±1.5 dB and a phase noise of better than −80 dBc/Hz at 2 MHz offset are measured.