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Basic operation
An acoustic wave sensor uses mechanical (acoustic) waves to sense multiple phenomena from the device's environment, which are registered as changes in the wave's phase, amplitude, and/or frequency relative to some reference. For surface acoustic wave (SAW) sensors, the device operation itself is fairly simple:
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A piezoelectric substrate which generates electrical charges from mechanical force, and vice versa
- At least one interdigitalA transducer (IDT) to convert electromagnetic waves to acoustic waves, and vice versa
- An area of propagation, oftentimes in some cases conceived as a delay line (see below), through which the acoustic wave propagates
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Figure 2: Diagram of a surface acoustic wave sensor using a delay line. Source: http://en.wikipedia.org/wiki/File:Surface_Acoustic_Wave_Sensor_Interdigitated_Transducer_Diagram.png
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Piezoelectric substrate
The device uses the piezoelectric effect to respond to mechanical forces by generating a voltage, and vice versa. This voltage is proportional to the amount of force applied to the device as well as the type of force applied (i.e. tension and compression produce opposite polarities). Furthermore, this effect is reciprocal, so the device will also respond to an electric field by generating a mechanical response that is proportional to the field's strength and polarity.
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 longer delay along a shorter delay line. The most common piezoelectric substrate materials are quartz, lithium niobate, and lithium tantalate, zinc oxide, and bismuth germanium oxide.
IDTs and areas of propagation
Typically, SAW devices use either a one-port resonator or a two-port delay line configuration. The two-port delay line configuration (pictured above) consists of one input IDT, one output IDT, and an area of propagation in between called the delay line.
The one-port resonator configuration includes only Instead of multiple IDTs (as shown in the figures above), many acoustic wave sensors include only one IDT element for transducing both impulse and response signals. These devices use In these devices, the area of propagation leads to a reflector element to reflect , which reflects the acoustic wave back into the same IDT that produced it. The slow propagation speed of the mechanical wave (compared to the electromagnetic impulse) along the delay line allows sufficient time for a short electromagnetic impulse input to dissipate before the reflected response is captured by the single IDT.
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SH-SAW sensors employing acoustic waves without a shear vertical component (e.g. Love surface transverse waves) are better suited for operation in liquid environments since their shear horizontal components do not lose much energy into liquids external to the device.
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