Surface acoustic wave (SAW) filters are piezoelectric acoustic wave devices that operate on an electric-acoustic-electric basis. These SAW filters are used, among other things, as bandpass filters and are characterized by a high quality factor (Q) with steep filter edges.
In principle, SAW filters convert the electrical input signal into an acoustic signal in an input circuit. The input circuit consists of an interdigital transducer (IDT). These are two nested comb structures made of quartz whose crystal lattices contract and expand due to the piezo effect when a voltage is applied. The resulting acoustic signal propagates through the substrate of the SAW filter and impinges on the IDT output transducer, where it is in turn converted into an electrical signal. Substrates of quartz or lithium tantalate are used as piezoelectric materials. Any change along the propagation path alters the propagation speed and amplitude of the acoustic wave, and determines the frequency and phase characteristics of the surface acoustic wave filter.
The wavelength and selectivity of the acoustic signal can be influenced by the structure and dimensions of the piezo material. This is then converted back into an electrical voltage. The design of a piezo transducer can be such that only one acoustic wave from a broadband electrical input signal is allowed to pass through to the output electrode and be converted into a highly selective frequency.
Surface acoustic wave filters are available in the frequency range from about 10 MHz to the gigahertz range of 3 GHz. They are used in radio and television reception technology and in other RF-oriented reception technologies such as in cell phones, WLAN receivers and base stations. But they are also used as surface wave sensors in automotive technology, building automation or in SAW touchscreens to determine the position of the cursor.