Energy Trapping in Low Phase Noise Bulk Acoustic Wave Oscillators

In the design of resonators in low phase noise bulk acoustic wave (BAW) oscillators, maximization of quality factor is the primary target while energy trapping is not typically of concern. Analysis shows that although energy-trapping mode energy outside the electroded region decreases exponentially with distance away from the electrode edge of the wafer, the decaying wave will reflect at the wafer edge to the electroded region and generate a wave with same frequency but different phase which generates mutual modulation with resonant frequency. It is a source of phase noise and mainly affects the near-carrier-frequency phase noise. Two 120 MHz SC-cut 5^th overtone UM-1 crystals with similar dynamic equivalent parameters and different shunt capacitances are compared using the same circuit. Experimental results show that energy trapping also needs to be considered in the design of resonators in low phase noise BAW oscillators.