Vibration-induced PM Noise in Oscillators and its Suppression

High-precision oscillators have significant applications in modern communication and navigation systems, radars, and sensors mounted in unmanned aerial vehicles, helicopters, missiles, and other dynamic platforms. These systems must provide their required performance even when subject to mild to severe dynamic environmental conditions. Oscillators often can provide sufficiently low intrinsic phase modulation (PM) noise to satisfy particular system requirements when in a static environment. However, these oscillators are sensitive to acceleration that can be in the form of steady acceleration, vibration, shock, or acoustic pickup. In most applications the acceleration experienced by an oscillator is in the form of vibration, which can introduce mechanical deformations that deteriorate the oscillator’s otherwise low PM noise (Filler, 1988; Vig et al., 1992; Howe et al., 2005). This degrades the performance of the entire electronic system that depends on this oscillator’s low phase noise. For example, when radars and sensors mounted on helicopters are subjected to severe lowand medium-frequency vibration environments, the vibration noise induced into the system’s reference oscillator translates to blurring of targets and possibly false detection. 1 This sensitivity to vibration originates most commonly from phase fluctuations within the oscillator’s positive-feedback loop, due usually to the physical deformations in the frequency determining element, the resonator. Factors that lead to high acceleration sensitivity of the resonator include nonlinear or sensitive mechanical coupling effects and lack of mechanical symmetry that serve to cancel frequency changes in the resonator. Vibration also causes mechanical deformations in non-frequency-determining electronic components that then cause phase fluctuations (Steinberg, 2000). Because these fluctuations are inside the oscillator feedback loop and are integrated according to Leeson’s model (Leeson, 1966), they can become large at Fourier, or offset, frequencies close to carrier frequency. An oscillator’s sensitivity to vibration is characterized traditionally by acceleration sensitivity, which is the normalized frequency change per g (1 g is the acceleration of gravity near the earth’s surface, approximately 9.8 m/s2). Typically, frequency shifts in oscillators are on the order of 10-8 to 10-10 per g, primarily because of the physical deformations.