Spurious modes are inharmonic modes of vibration in the quartz plate and they are the result of a number of factors:
Flatness and parallelism
Quartz to electrode interface and bonding
Spurious modes are measured in two ways; either in terms of equivalent series resistance ratio in reference to the main mode or its equivalent in transmission response. The standard is 2:1 resistance ration or -6 dB.
Depending on the application, spurious or unwanted modes may be more or less important to the crystal design and performance. In the case of an oscillator, the spurious mode must be suppressed adequately in order to prevent the oscillator from oscillating on the spur rather than the main mode. Distribution of the driving energy between spurs and the main mode may also prevent the oscillator from starting or from maintaining oscillation.
Filter applications require that spurious modes be outside of the band-pass of the filter. Spurious modes outside the band-pass are typically reduced as much as the design will allow. Spurious displacement and level are controlled by a number of factors that include:
Electrode plate-back and adhesion
Spurious modes are inherent in every crystal design, however there are techniques utilized by the design and manufacturing process to suppress them. Spurious suppression techniques do affect other parameters of a crystal operation, typically motional capacitance, inductance and resistance. A delicate balance of these parameters is often required affecting yield and eventually the cost of the device.
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