Low Phase Noise Oscillator Incorporating a Quarter-Wavelength Resonators-Based Network With Source/Load Coupling

In a feedback-type microwave oscillator, the frequency selection network plays an important role in affecting phase noise. This letter proposes a quarter-wavelength resonators-based network with source/load coupling (QWRNSLC), applies it in an $S$ -band oscillator, and successfully achieves the lowest phase noise up to now. It consists of two $\lambda $ /4 short-ended resonators in which stepped impedance is introduced for pushing up the second harmonic. By introducing direct source/load coupling, the QWRNSLC is featured by a specific coupling schematic. Two transmission zeros are generated near the passband so that the group delay can be adjusted by flexibly shifting them. At the same time, the specific feedlines will generate extra two transmission zeros for efficient harmonic suppression in the stopband. For demonstration, an oscillator example was fabricated and measured. It works at 1.989 GHz and the output power is 8.35 dBm. The phase noise is as low as −127.76 dBc/Hz at 100 kHz.