An X-band varactor-tuned cavity oscillator
Paper in proceedings, 2017
This paper reports on an X-band varactor-tuned cavity oscillator. The varactors are mounted on a low loss printed circuit board (PCB) that is intruded inside the cavity, which enables efficient coupling to the RF-field. The varactors' positions are changed by adjusting the intrusion depth of the PCB as well as the horizontal positions of the varactors on the PCB. This compromises between the tuning range and the resonator Q-factor. The active part, i.e., the reflection amplifier, is implemented in GaN-HEMT MMIC technology. A microstrip line couples the cavity to the reflection amplifier. The coupling factor can be controlled by mechanically adjusting the cavity's position. A lateral displacement changes the amplitude coupling, while a displacement along the microstrip line changes the phase condition. A tuning range of 1.6 % about 10 GHz is reached with the PCB placed at 1 mm depth from the cavity wall. The measured phase noise at 100 kHz and 1 MHz offset, respectively, ranges from -111 dBc/Hz to -118 dBc/Hz, and -138 dBc/Hz to -146 dBc/Hz over the tuning range. Increased tuning range can be reached if the varactors are placed deeper into the cavity, but then the phase noise degrades due to the increased tuning sensitivity that causes modulation noise and degradation of large-signal quality factor as the varactors are exposed to a stronger RF-field.