Low Phase Noise GaN HEMT Oscillators With Excellent Figures of Merit

This letter presents guidelines for the design of low phase noise oscillators in GaN high electron mobility transistor (HEMT) technology. The design starts from bias-dependent low-frequency (LF) noise measurements. Oscillator topology and bias point are then chosen for operation in regions where LF noise is low. The best LF noise properties are obtained for low drain voltage and current. Thus, the low phase noise can be achieved at low dc power which also means that power normalized phase noise figure of merit (FOM) will be good. Two different oscillators have been designed and measured. A 9.9 GHz common-gate balanced Colpitts oscillator operating in class C presents a phase noise of -136 dBc/Hz@1 MHz. The result is achieved for Vd =6 V and Id = 30 mA, giving FOM = 193 dBc/Hz. A 1.95 GHz negative-resistance oscillator operating in switched mode presents a phase noise of of -149 dBc/Hz@ 1 MHz offset. With drain voltage and current of Vd = 4 V and Id = 100 mA, this oscillator presents FOM = 189 dBc/Hz. To the best of the author's knowledge, these two oscillators present the highest reported FOM for GaN HEMT oscillators.