Phase noise reduction in surface wave oscillators by using nonlinear sustaining amplifiers

Nonlinear sustaining amplifier operation has been investigated and applied to high-power negative resistance oscillators (NRO), using single-port surface transverse wave (STW) resonators, and two-transistor sustaining amplifiers for feed-back-loop STW oscillators (FLSO) using lossy two-port STW devices. In all cases, self limiting Si-bipolar sustaining amplifiers which operate in the highly nonlinear AB-, B- or C-class modes are implemented. Phase noise reduction is based on the assumption that a sustaining amplifier, operating in one of these modes, uses current limiting and remains cut off over a significant portion of the wave period. Therefore, it does not generate 1/f noise over the cut-off portion of the RF cycle and this reduces the close-in oscillator phase noise significantly. The proposed method has been found to provide phase noise suppression levels in the -111 to -119 dBc/Hz range at 1 KHz carrier offset in 915 MHz C-class power NRO and FLSO generating up to 23 dBm of RF-power at RF/dc efficiencies exceeding 40%. C-class amplifier design techniques are utilized for adequate matching and high RF/d.c. efficiency.