High purity signal generation is a key issue in wireless communication systems and radar systems. The voltage controlled oscillator (VCO) is one of the bottle necks in modern systems using advanced modulation formats like higher order QAM or OFDM. Currently state-of-the art low-phase noise VCOs are designed in InGaP HBT technology, with key properties for low phase noise such as low flicker noise and relatively high breakdown voltage. A technology with significantly higher breakdown voltage is GaN HEMT. Thus, a voltage-controlled oscillator based on a GaN HEMT can potentially reach very low phase noise.Objectives
The objective is to forward state-of-the-art for oscillators and VCOs in terms of phase noise and tuning range. Output power should be sufficient with reasonable variations over the tuning range. The emphasis is on implementations that are cost effective in production, thus integrated oscillators are to be preferred, despite relatively poor Q factor. A strategy to overcome the relatively poor Q for MMICs is to use GaN HEMT technology with high power handling capability, increasing signal-to-noise ratio and lowering phase noise according to Leeson’s equation.
A potential show-stopper for GaN HEMT based oscillators is low-frequency noise and its upconversion. It will be studied how the phase noise is scaling with DC power to ensure that the increased oscillation power Psig is not counteracted by an increase in effective noise figure F. Low-frequency noise measurements and residual phase noise measurements of HEMTs will be carried out.
High-Q external resonators may be considered if the performance requires so. However, the improvement in performance must not be to the price of a too complex realization. The focus will be on technologies and topologies with potentials to be cost effective in production.
For wideband designs, reduction of the varactor modulation noise is important. Two strategies are foreseen to limit the influence from the varactor.
- Wide-tuning varactors with low tuning sensitivity [Hz/V] but high breakdown voltage
- A small varactor in parallel with a bank of fixed capacitors chosen by switches
Both tracks are compatible with GaN HEMT technology. For initial investigations, track 1 is easiest studied in hybrid technology enabling the use of hyperabrupt SiC varactors. Track 2 is suitable for MMIC technology, where GaN HEMTs may be used as switches.
The GaN HEMT oscillator (hybrid or MMIC) is a relatively immature component but may enable a paradigm shift if successful in terms of phase noise etc for robust communication links. A vision for exploitation is therefore to see the results from GANOSC to bring novel component generations for space satellite communications and terrestrial communication also at higher frequencies (E-band).