Multiphase Reference Signal Generation Using Coupled Oscillators

Book excerpt: The continuing trend toward higher frequencies of operation poses formidable challenges in the design of multiphase reference signals at mm-wave frequencies and beyond. Conventional multiphase reference generation techniques face serious implementation or performance challenges when scaled to microwave and mm-wave frequencies. Ring oscillators suffer from poor phase noise, and hence fail to fulfill the stringent requirements of most wireless applications. Generating multiphase signals by dividing the output of an oscillator operating at multiples of the intended frequency of operation is impractical when frequencies approach the mm-wave range. Cross-coupled LC oscillators have been explored as a promising alternative for multiphase and, in particular, quadrature generation. However, the frequency ambiguity that results from multiple modes of operation, as well as the severe phase noise degradation due to their inherent off-resonance operation, has inhibited their utilization in practice. This work introduces a new topology for coupled oscillators that solves the frequency ambiguity issue and mitigates phase noise degradation in coupled oscillators by employing an array of LC oscillators that are coupled in a bidirectional fashion. The proposed bidirectional coupling enforces operation at the resonance frequency of the LC tanks of the oscillator in the loop, a property that proves to be key in solving both the aforementioned issues. A quadrature frequency doubling topology using bidirectionally-coupled oscillators is also presented. The proposed approach relaxes the linearity requirements on the mixers employed in the circuit, thus allowing the frequency doubler to use highly nonlinear mixers. An experimental prototype integrated in a 90-nm CMOS technology provides output phases in increments of 45 degrees and achieves a phase noise of −101 dBc/Hz at 1- MHz offset from a 19.6-GHz carrier. The quadrature 40-GHz signal generated on chip drives a single-sideband transmitter that achieves a sideband suppression of better than 45 dB.