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The material of the device's piezoelectric substrate determines the velocity of the acoustic wave, which is in the range of 1500-4800 m/s. This is 105 times slower than the electromagnetic wave velocity, allowing for a long delay along a relatively short area of propagation. The most common piezoelectric substrate materials are quartz, lithium niobate, and lithium tantalate.
Interdigital transducers (IDTs)
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While the magnitude of the delay line transfer function depends on the characteristics of the individual IDTs, the phase of the response is only dependent on the delay and signal and synchronous frequencies:
Some sample frequency responses of various SAW sensors are given in the figures below:
Figure 6: This SAW Hg sensor responds to Mercury concentrations with varying frequency response slopes. Plotting the initial frequency slopes results in a fairly linear response that can be used to accurately determine Mercury concentration.1
Figure 7: This SAW NO sensor responds to Nitric Oxide concentraions with varying frequency response magnitudes. As with the example above, the sensor's response can be approximated linearly to an extent1.
Sensitivity
SAW sensor sensitivities are also dependant on their wavetype, configuration, components, materials, and applications. Some typical sensitivities are listed below:
| Physical quantity | Linear coefficient |
|---|---|
| Temperature | up to 100 ppm*/K |
| Pressure, stress | 2 ppm/kPa |
| Force | 10 ppm/kN |
| Mass loading | 30 ppm/μg·cm2 |
| Voltage | 1 ppm/V |
| Electric field | 30 ppm/V·μm−1 |
Figure 6: Linear coefficients for physical effects on SAW sensors6. *ppm = parts per million. For example, when measuring pressure, a change in SAW frequency of 2x10-6 corresponds to a change in pressure of 1 kPa.
SAW sensors are often valued for their high degree of sensitivity due to the concentration of energy at the device’s surface, where the external environment can have a greater effect. However, this is oftentimes a design challenge. For example, whereas surface acoustic waves with shear vertical components are very sensitive to changes in gaseous environments, they can undergo severe damping in liquid environments. Furthermore, in environments with large temperature fluctuations, a SAW sensor’s piezoelectric substrate can be affected by these fluctuations, often necessitating an additional “reference” configuration to control for such effects.
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- Electromagnetic feedthrough between IDTs, causing amplitude and phase ripple
- “Triple-transit interference” associated with SAW reflections, causing ripple effects
- Mass-loading by IDT digits, causing SAW velocity changes
- Unwanted bulk wave emissions accompanying SAW emissions, causing passband distortion
- Finite source and load impedances, causing frequency-dependent voltages across IDTs
- IDT diffraction similar to optical systems, causing changes in transition band and shape
- Harmonic frequencies generated by the input IDT (may be desirable or undesirable, depending on application)
Manufacturing materials and processes
| Substrate material | Crystal cut | Linear TK* |
|---|---|---|
| Lithiumniobate LiNbO3 | rotated 128 Y/X cut Y/Z standard cut | 72 ppm**/K 92 ppm/K |
| Lithiumtantalate LiTaO3 | X/112Y 36 Y/X rotated cut | 18 ppm/K 30 ppm/K |
| Quartz (SiO2) | ST-X cut | 0 ppm/K |
Figure 7: Temperature coefficients at room temperature for SAW substrate materials. *TK = temperature coefficient. **See note on figure 6 above for explanation of ppm.
Applications
Acoustic wave sensors are very versatile in that they may be used alone or as part of a filtered sensor to measure many phenomena, including:
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