Low Phase Noise GaN HEMT Oscillator Design based on High-Q resonators

Doktorsavhandling, 2017

The thesis considers the design and optimization of oscillators targeting low phase noise, given boundary conditions from the technology. Crucial technology figures are power capability, RF noise figure, low-frequency noise and the quality factor (Q-factor) of the resonator. Parameters that can be optimized from a design perspective are the resonator coupling, bias point and waveforms. The technology used in this study is GaN-HEMT, due to its low RF noise figure, high power capability and good DC to RF efficiency. The focus has been on the resonator coupling which is an essential part of the oscillator design. Strong coupling with high power transfer to the resonator improves the phase noise. Contradictory it will also decrease the loaded Q-factor of the resonator. The optimum coupling factor is found to be between ß=1/2 and ß=1, defined as the ratio of power dissipated in the resonator compared to the total power delivered by the active device. Several designs in various resonator technologies have been investigated. For example, an oscillator based on an aluminum cavity connected to a GaN-MMIC reflection amplifier has a phase noise of -145 dBc/Hz at 100 kHz offset from a 9.9 GHz carrier. The analysis of the coupling’s effect to the cavity shows the optimum phase noise occurs for ß close to unity, which is equivalent to an open loop gain close to 0 dB. A MMIC oscillator based on the same reflection amplifier and a quasi-lumped on-chip-resonator has a phase noise of -106 dBc/Hz at 100 kHz offset from a 15 GHz carrier, which clearly shows that the phase noise scales with the Q-factor of the resonator. A reflection amplifier with an electronically controlled gain is also designed for control of the resonator coupling. Varactors in the termination network perform a gain adjustment without changing the bias point of the active transistor. The phase noise of a cavity oscillator based on this reflection amplifier is -136 dBc/Hz at 100 kHz offset from 8.5 GHz. A similar oscillator with a mechanically tuned cavity has about 3 dB better phase noise. Despite a small degradation in phase noise, the simplicity facilitated with electronic tuning motivates this design for practical applications. High-Q tunable elements are key components for frequency control. This work reports an ohmic cantilever radio frequency electromechanical system (RF-MEMS) integrated on a PCB forming a tunable ground plane inside a cavity. Vertical and horizontal positions of the MEMSs are investigated for trade-offs between tuning-range, frequency resolution and phase noise. Placing the PCB at 1 mm depth from the cavity wall, 5 % tunability around 10 GHz is reached, with 100 kHz phase noise ranging from -140 dBc/Hz to -129 dBc/Hz. Placing the PCB deeper into the cavity, at 2.5 mm, the tuning range can be increased to 12.3 %, with 100 kHz phase noise varying from -133 dBc/Hz to -123 dBc/Hz. A varactor-tuned cavity oscillator has been implemented using the same PCB. It presents a tuning range of 1.6 %. The optimum phase noise at 100 kHz is ranging from -111 dBc/Hz to -118 dBc/Hz. At 1 MHz offset the phase noise is varying from -138 dBc to -146 dBc/Hz, versus the tuning-range. GaN-HEMT devices from different commercial vendors have been used for the designs. For modeling purposes, low-frequency noise is measured for all devices. A special high-voltage and current low-frequency noise test setup was developed and used for benchmarking of different GaN HEMTs versus other technologies, e.g., GaAs InGaP HBTs and GaAs-HEMTs.