Development of a low phase noise microwave voltage controlled oscillator

[摘要] The topic for this project entailed the development of a 'Low Phase Noise –Microwave – Voltage Controlled Oscillator' for use in radar applications.First of all, a low phase noise oscillator was designed. In order to minimisethe phase noise of the oscillator, a high-Q, transmission line – cavity resonator wasdeveloped. By derivation it was confirmed that an optimal point for minimum phasenoise does exist. The latter was done by evaluating the equation for the outputpower spectral density of the oscillator phase noise (as defined by Leeson's PhaseNoise Model) at its minimum point. Subsequently, the amount of power that neededto be dissipated inside the resonator could be compared to that dissipated in thesource and the load. This identified the amount of coupling to the resonator allowed,assuring minimum phase noise. Since a specific amount of coupling to the resonatorwas sought after, it had to be practically feasible. Therefore several couplingtechniques were investigated to ensure the most user-friendly way of tuning theamount of coupling to the resonator, and hence easily reaching the optimum point ofminimum phase noise.After successful completion of the low phase noise oscillator design, it wasmodified for voltage controlled oscillator (VCO) use by means of variable tuningdiodes. These varactor diodes were situated inside the cavity of the resonator.Again the most suitable position to place the diodes had to be determined. The latterwas done through considerably detailed transmission line theory; where the loadedQ, the tuning bandwidth (amount of change in frequency reached) and the amount ofpower dissipated inside the resonator were measured against each other.By means of the necessary phase noise measurements, it was confirmedthat in order to keep the phase noise to a minimum, the tuning bandwidth had to bekept small and the amount of power dissipated inside the resonator maximised; so asto keep the overall loaded Q-value of the circuit as high as possible.

[发布机构] Stellenbosch University