Design of Low Phase-Noise Microwave Oscillator and Wideband VCO Based on Microstrip Combline Bandpass Filters

This paper presents a new low phase-noise microwave oscillator and wideband voltage-controlled oscillator (VCO) based on microstrip combline bandpass filters. For this type of oscillator, the passband filter is embedded into the feedback loop to treat as a frequency stabilization element. Instead of designing the oscillator at the group-delay-peak frequency of the filter to achieve a good phase-noise performance, in this paper, the peak frequency of the complex quality factor Q_sc is adopted for oscillator design. To demonstrate the effectiveness of using Q_sc-peak frequency, two filter-based oscillators are implemented at the Q_sc-peak and group-delay-peak frequencies, respectively. The oscillator designed at the Q_sc-peak frequency improves the phase-noise about 10 dB as compared with that realized at the group-delay-peak frequency. The developed oscillator with the three-pole combline filter is experimentally demonstrated at 2.05 GHz with -148.3-dBc/Hz phase noise at 1-MHz offset frequency. Moreover, by attaching a varactor on each resonator of the combline filter, the oscillator can be extended to a wideband VCO. The developed VCO has a frequency tuning range from 1.3 to 2.2813 GHz with a 54.8% bandwidth. Over this frequency range, all the phase noises measured at 1-MHz offset frequency are better than -117.19 dBc/Hz.