SAW Mass Loading

The simplest interaction, and the one most utilized for SAW sensor applications, is the response due to changes in the arealAnass density (mass/area) on the device surface. The harmonic motion of the crystal surface caused by the passing surface wave causes particles bound to the surface to be translated in an elliptical orbit in synchronism with the SAW surface displacement. The effect on wave velocity and attenuation of this interaction may be derived firom eneigy considerations. 

Due to the greater confinement of wave energy near the surface that occurs as operating frequency increases, surface particle velocities increase in proportion to (Pa ). Thus, the quantities in parentheses being inde- 

Note from Equation 3.48 the frequency dependence of the SAW mass sensitivity the fractional velocity change Av/vo varies with operating frequency fo. Because the mass layer is assumed (in this case) to be lossless. Equation 2.53 implies that attenuation is unchanged by mass loading. 

Example 3.5 (a) Calculate the mass sensitivity factor c for a 100-Af//z SAW device on ST-cut quartz, (b) If a SAW device is incorporated in an oscillator loop, so that fractional frequency changes track fractional velocity changes (i.e., Aflfr = Av/v ), calculate the sensitivity S = dp,. (c) Calculate the limit of mass resolution for a typical SAW oscillator stability of 1 Hz. 

The previous example illustrates the superior mass sensitivity of the SAW device in comparison with the TSM resonator sensitivity is some 200 times larger for the 100-MHz SAW device than for the 5-MHz TSM resonator. Part (b) of the Solution also reveals that mass sensitivity, when expressed in the form dfldp, increases with /.  

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