Effects of crystal mount resonance, under random vibration, on the close to carrier phase noise

The phase noise of quartz crystal oscillators under vibration is degraded, mainly due to the acceleration sensitivity of the quartz crystal element. It is shown that if a crystal mount resonance is excited by a random vibration profile then the phase noise close to the carrier, outside the random vibration profile, can be further substantially degraded. A consequence is that the vibration sensitivity would seem to increase rapidly at low vibration frequencies. For example, a crystal oscillator with a resonance at 1950 Hz subjected to a random vibration profile of 0.01 g²/Hz extending from 100 Hz to 3000 Hz compared with a profile of 100 to 500 Hz, where the resonance is not excited, can have over 40 dB degradation in the phase noise at 20 Hz from the carrier. The resonance level is shown to become non-linear with vibration power, possibly due to the large acceleration amplification of the resonance, 100 to 300 times. The vibration level where the resonance becomes non-linear varies by two orders of magnitude and starts as low as 0.0003 g²/Hz. This phenomenon is demonstrated for four oscillators and investigated under varying acceleration levels in an attempt to quantify the nonlinear behavior. The nonlinear behaviour of the resonance is believed to produce intermodulation of the mechanical vibration which leads to the degradation of the close to carrier phase noise