BAW (Bulk Acoustic Wave) oscillators are a new entry to oscillator technology. Crystal-based oscillators have existed for many years, even before the introduction of MEMS technology. BAW technology brings benefits to reliability over Crystal-based and MEMS-based oscillators. The high grade reliability of BAW in terms of improved vibration, mechanical shock and mean time between failure (MTBF) performance, alleviates system designer concerns about oscillator performance in harsh environments, allowing designers to focus more on the overall system performance. Unlike quartz or mechanical elements used in the quartz oscillators, TI’s piezoelectric BAW resonator is designed using a semiconductor process.
Mechanical shock and vibration can damage oscillators and degrade performance. Vibrations can result in increased phase noise or jitter and mechanical shocks can result in frequency shifts or spikes. This application note provides more details on BAW oscillator performance under stringent sinusoidal, random vibration, and mechanical shock conditions and describes various MIL-STD-883 test methods, test setup, and performance results.
VibrationVIEW® is a registered trademark of Vibration Research Corp.
All trademarks are the property of their respective owners.
The sensitivity to vibration and shock is a key consideration when designing systems with a crystal- or MEMS-based clock oscillator. Devices with high sensitivity to vibration can have a detrimental impact on the overall system performance, affecting the phase noise and jitter, frequency stability, and long-term reliability. The clock oscillator needs to provide a stable clock with strong resistance against acceleration forces, vibration, and shock, as resistance provides stability throughout the product life cycles under process and temperature variations.
The two important parameters for quantifying vibration are the acceleration force and vibration frequency applied to the devices. To quantify shock, acceleration force and the time duration for which the peak acceleration is applied are used. Vibrations and mechanical shock affect resonators by inducing noise and frequency drift, degrading system performance over time. In oscillators, vibration and shock are common causes of elevated phase noise and jitter, frequency shifts and spikes, or even physical damage to the resonator and resonator package. These degradations of phase noise and jitter directly impact the system performance. Typically, external disturbances couple into the micro resonator through the package. Since crystal oscillators fundamentally rely on the vibration and mechanical resonance of a piezoelectric material, external disturbances can couple into the device and degrade oscillator performance. Mechanical shocks of sufficient magnitude can also cause irreversible frequency shifts at the output of the crystal oscillator.
TI BAW oscillators fare better, when compared to quartz-based oscillators. TI’s BAW oscillators are more immune to vibration and mechanical shock due to the smaller mass (by orders of magnitude) of the resonator and higher resonance frequency. The force applied to the device from external acceleration is much smaller due to smaller mass. The immunity of the device is further enhanced by the semiconductor manufacturing process of the BAW resonator. The BAW piezo and metal layers are surrounded by Bragg mirrors, which shield the resonator from environmental stresses. The BAW oscillator also includes a wafer-level encapsulation for making the oscillator a robust and reliable product. TI’s dual-Bragg BAW resonators contains no moving parts, which provides resilience against environmental stress with improved device reliability.
Vibration sources are present in many end-applications including hand-held mobile devices, cooling fans in equipment chassis, factory automation equipment, construction equipment, moving vehicles or aircraft. The following table provides examples of the vibration levels at different environment conditions.
Environment(1) | Typical acceleration (g) |
---|---|
Buildings | quiescent 0.02 rms |
Tractor-trailer | (3 to 80 Hz) 0.2 peak |
Armored personnel carrier | 0.5 to 3 rms |
Ship - calm seas | 0.02 to 0.1 peak |
Ship - rough seas | 0.8 peak |
Railroads | 0.1 to 1 peak |
Propeller aircraft | 0.3 to 5 rms |
Helicopter | 0.1 to 7 rms |
Jet aircraft | 0.02 to 2 rms |
Missile - boost phase | 15 peak |
A LMK6x oscillator from Texas Instruments is used to quantify the vibration and shock performance of BAW oscillators. The devices are subjected to sinusoidal vibrations at various frequencies along the X, Y and Z directions. The tests are repeated along each axis with random vibration profiles. The final test measures the transient frequency deviation of the units during operation in response to mechanical shock. Phase noise (including spurs) and frequency shift data are then recorded during these tests.