A 27-GHz Quad-Core CMOS Oscillator With No Mode Ambiguity

There is a lower bound on the size of a planar inductor below which significant Q-degradation occurs. While this bound can limit the absolute performance of an LC oscillator at any frequency, it is usually only encountered when designing at mm-wave and above. The classic way to mitigate the issue is through the use of NMOS-only and/or multi-core designs. Such techniques, however, are often not suitable for mm-wave commercialization; NMOS-only designs require thick-oxide devices, while multi-core designs need to be carefully controlled at startup to avoid unwanted oscillation modes. The circular geometry topology, first proposed in 2004, appears to overcome these issues, but, surprisingly, it has not yet been adapted to mm-wave frequencies. In this paper, we show how the topology is well suited to high-frequency applications. A quad-core 27-GHz LC oscillator is presented that has a single highQ mode. The suppression of the three unwanted modes removes any mode ambiguity and enhances the Q of the wanted mode. The design is fully complementary and, so, is compatible with the use of high fT thin-oxide devices. The near mm-wave prototype achieves a figure of merit of 187 dBc/Hz and a tuning range of 26%. The measured phase noise is -110 dBc/Hz@1 MHz, which is among the lowest reported for a fully complementary near mm-wave design.