Oscillator design in III-V technologies

Doktorsavhandling, 2014

The thesis treats the design of low phase noise oscillators/VCOs in GaAs/InGaP HBT and GaN HEMT technologies. The covered topics are: active device modeling, noise characterization, passive structures, phase noise models, simulation/measurement tools, circuit topologies, and design techniques. The mature InGaP HBT technology is known to be good for design of low phase noise oscillators, thanks to its low flicker noise and high breakdown voltage. The emerging GaN HEMT technology with its higher breakdown voltage also indicates the potential of low phase noise oscillator design. Large signal models for both devices were extracted based on Chalmers’ in-house models. Investigation of devices’ low frequency noise (LFN) characteristics showed that GaN HEMTs have noise levels close to InGaP HBT at low power, nevertheless, noise levels are high at high bias power. The bias dependency of LFN was utilized to develop low phase noise oscillators in GaN HEMT technology. An accurate method for phase noise calculation is implemented. The method is integrated with a commercial simulator and the phase noise is calculated posteriorly based on LFN and Hajimiri’s impulse sensitivity function. The method is validated for three different InGaP HBT VCOs designed at 7 GHz using well-known topologies, i.e. balanced Colpitts, cross-coupled, and Gm-boosted balanced Colpitts. The method is also applied to a 9 GHz GaN HEMT balanced Colpitts oscillator and proven to predict phase noise accurately over a wide range of bias points. The selection of oscillator topology is a key step in VCO design. A number of different topologies, suitable for various applications have been investigated and analyzed in this work. A 9 GHz double-tuned balanced Colpitts VCO demonstrates a 14.4% tuning range and phase noise better than -102dBc/Hz@100kHz over the tuning range. A wide tuning range of 34.4% is demonstrated in a 14.3 GHz mixer-based VCO. It is shown how a pair of modified Darlington HBTs is used in a gm boosted balanced Colpitts VCO to lower phase noise. In GaN HEMT oscillator designs, biasing strategies to avoid LFN are proposed. Two designed oscillators demonstrate the-state-of-the-art low phase noise in GaN HEMT technology. A MMIC balanced Colpitts oscillator at 10 GHz demonstrates a minimum phase noise of -136dBc/Hz at 1 MHz. A hybrid negative resistance oscillator with switch mode waveform designed at 2 GHz demonstrates a phase noise of -149dBc/Hz at 1 MHz.