Sub-GHz Transceiver Crystal Passivation under Extended High Current Transmit Pulses
Sub-GHz transceiver crystal passivation prevents drive-level dependency shifts and frequency pulling during extended high-current power amplifier transmit pulses.
Piezoelectric Shear Strain
Meaning
Mechanical deformation that occurs parallel to the surface of a crystal blank when an electric field is applied across its thickness drives the operation of high-frequency quartz resonators. This phenomenon, known as piezoelectric shear strain, relies on the AT-cut or SC-cut crystal orientations to generate thickness-shear vibration modes. The magnitude of this strain is proportional to the strength of the applied electrical field.
This mechanical motion forms the basis of stable clock signals in wireless systems.
Shear Mode
Thickness-shear vibration is particularly suited for high-frequency applications because the resonance frequency is determined by the thickness of the crystal blank. Applying an alternating voltage across the electrodes of the crystal generates continuous piezoelectric shear strain. This oscillation is highly stable and exhibits low sensitivity to temperature changes.
Strain Coefficient
The efficiency of this mechanical conversion is governed by the piezoelectric strain coefficients of the material. Quartz has a lower strain coefficient than materials like lead zirconate titanate, but it offers much higher stability and lower mechanical loss.
Resonator Design
Controlling the distribution of strain across the crystal blank is essential to prevent energy loss to the mounting structure. Designers thin the edges of the blank to confine the acoustic energy to the center of the quartz crystal.