Phase noise is a critical impairment in all oscillators, and may be the bottleneck for data capacity in modern communication systems, or limit the range and resolution in radar applications, etc. It increases the system noise floor, which for a communication systems will restrict the modulation complexity potential and thus spectral efficiency due to reduced margins for signal detection.
The performance of the parts included in an oscillator, as the power capacity, the RF noise floor and low-frequency noise for the amplifying part, and the unloaded quality-factor and tunability of the resonator circuitry set boundaries of the theoretically achievable performance. To utilize the components’ best performance an optimized and well-designed oscillator is required.
High-performance oscillators can be implemented by using high-quality resonators, connected to active devices with high power capacity as for GaN-HEMT technology. By using a flexible setup the active part and the resonator can be separately characterized, and then be tuned together for best oscillator performance. In particular, the coupling factor between the resonator and the active device can be considered. High power coupling to the resonator is beneficial for the phase noise, but contradictory a strong coupling loads the resonator and degrades its quality factor. For different conditions, like bias, frequency offset, and waveform, the optimum coupling factor can be investigated.
The flicker noise behavior of a GaN-HEMT is disadvantageous for the noise level near the carrier, and its current dependency restricts to low power handling. Far out noise, which is less affected by flicker-noise, is improved by the increased power relative to the noise floor. The noise floor is critical, in particular, for broadband systems.
In this work, a GaN-HEMT MMIC-reflection amplifier is connected to different high-Q metal-cavity resonators. Tunable cavities are realized by integrating RF-micromachined systems (RF-MEMS)-switches, and solid state varactors inside the cavity to enable digital and analog tuning, respectively.
Several state-of-the-art oscillators are reported in this work. Further, methods and strategies are demonstrated to build optimized oscillators, which are needed to meet the demands of cutting edge technology in future communication systems.