A GaN HEMT X-band cavity oscillator with electronic gain control

This paper reports on a very low phase-noise GaN HEMT cavity oscillator at 8.5 GHz based on a reflection amplifier with electronic gain control. The gain control functionality is essential in order to control the open loop gain, which is critical for the phase noise performance. A large loop gain forces the oscillator in deep compression, resulting in increased noise conversion and degraded phase noise. On the other hand, a sufficient gain margin is mandatory to ensure satisfaction of the oscillation condition with margin that covers temperature drift and individual spread. The electronic gain control uses varactors to change the output termination of a reflection amplifier. In this way the loop gain can be set independently of the bias point of the active device and the position of the metal cavity. A minimum phase noise of -136 dBc/Hz@ 100 kHz off-set is achieved, which is comparable to what is reached for a mechanically tuned oscillator in the same process