Formulation of a Dual-Tapped Microstrip Resonator VCO

This paper provides a formulation of the dual-tapped microstrip resonator. The formulation derives the Q-factor of the resonator, and the maximum value of the Q-factor while still satisfying oscillation conditions. It is shown that the optimized Q-factor can be determined without changing the resonant frequency bandwidth. The phase-noise performance of a voltage-controlled oscillator (VCO) using a dual-tapped microstrip resonator can therefore be designed independent of the design of the oscillation bandwidth. This feature results in simple design of low phase-noise VCOs while maintaining the desired oscillation bandwidth. Experimental results of an X-band VCO employing the dual-tapped microstrip resonator verify the theoretical findings. The fabricated X-band VCO achieves an oscillation bandwidth of 11.86-12.09 GHz and phase noise lower than -114.4 dBc/Hz at 100-kHz offset from the carrier. This VCO is one of the most low phase-noise planar X-band VCOs published to date.