Low phase noise microwave oscillator with greater than 60 dB second‐harmonic suppression

This study presents a microwave oscillator in which an innovative hybrid microstrip and substrate integrated waveguide filter is proposed for phase noise reduction. By virtue of equivalent circuit, the specific topology of this filter is clearly revealed and verified by both the simulation and the experiment. It can realize a third‐order elliptic bandpass response with two transmission zeros near the passband. Specific phase response can be formed, in which a high group delay can be achieved within a narrow frequency range around the centre frequency. This feature makes the filter performance very flexible for its application in microwave oscillator, and then reduces its phase noise. For demonstration, an oscillator example at 9.0 GHz was deigned, fabricated and measured. The measured phase noise at 100 kHz away from the oscillation frequency is as low as −120.06 dBc/Hz. Furthermore, the proposed oscillator is featured by greater than 60 dB second‐harmonic suppression and at least the third harmonic is suppressed.