...
SAW sensors may also include a filtering element as a first step to sensing, for example, a particular chemical or biological compoundan additional filtering or packaging element to sense many types of phenomena indirectly. The sensor in this case is not directly sensing the compoundphenomenon, but instead sensing the response of the filtering or packaging element to the its presence of the compound.
The piezoelectric substrate
...
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)
An interdigital transducer consists of a series of comb-like conductive structures with an interleaving pattern that resembles fingers, or "digits" (see Figure 3). Using the piezoelectric effect, the IDTs convert the electromagnetic current of the impulse signal into acoustic waves which propagate in both directions of each "digit," creating constructive and destructive interference among the wavesand vice versa.
Figure 3: A generic IDT with pitch p (left); Frequency response of a generic IDT (right). Adapted from source9.
Where The acoustic waves propagate in both directions of each "digit," creating constructive and destructive interference among the waves. Where constructive interference (and therefore wave amplitude) is maximized, the wave is said to be at the synchronous frequency, or fs (also termed the characteristic frequency). The period length, or pitch, between an IDT's fingers affects this frequency respective to the velocity of the waves along the propagation area. This relationship is expressed as:
...