SAW Filter Fundamentals - What is SAW?

SAW stands for Surface Acoustic Wave, which refers to the sound waves that propagate along the surface of an object. The discovery of surface acoustic waves was later than that of electromagnetic waves. In 1885, British physicist Rayleigh discovered a type of sound wave with energy concentrated on the surface of the earth while studying seismic waves, initially named Rayleigh waves. Haha, naming after the discoverer is the greatest tribute to the discoverer. The energy of this wave is concentrated on the surface of the object, with a wave velocity of one hundred thousandth that of electromagnetic waves, and the propagation attenuation is very small.

However, due to limitations in the level of scientific and technological development at that time, it did not receive widespread application. Until 1965, R.M. White and F.M. Voltmov utilized interdigital transducers (IDTs) deposited on quartz crystals to effectively excite and detect SAW. At the same time, due to the ability to mass produce high-quality IDTs using photolithography technology for semiconductor manufacturing, surface acoustic waves were widely used, and surface acoustic wave technology has rapidly developed since then. Various surface acoustic wave devices have been designed and manufactured successively. This research result was published in "A New Type of Surface Acoustic Wave Electro Converter". This research achievement has integrated acoustics and electronics into a relatively new interdisciplinary field.

Surface acoustic waves, strictly speaking, are various modes of waves that propagate along solid surfaces or interfaces. They are both elastic waves and mechanical waves, possessing all the characteristics of mechanical waves. In an ideal situation, the wave modes that exist on the surface of a semi wireless substrate include Rayleigh waves, Leaky SAW waves, Generalized Rayleigh waves, Horizontal Shear Waves (SH.SAW), Electroacoustic Waves (B.G waves), Lamb waves, etc. In layered substrates, there are Love waves, Sezawa waves, Stoneley waves, and so on. Seismic waves are also a type of surface acoustic wave.

Surface acoustic wave devices have the advantages of light weight, small size, high reliability, good consistency, flexible design, and the ability to be manufactured using microelectronic processing technology, making them suitable for mass production. They have been applied in many fields such as mobile communication, broadcasting and television, non-destructive testing, identification and positioning, navigation, and remote sensing. The operating frequency of SAW devices now covers 10MHz to 5GHz, making them an indispensable key component in the modern information industry.

Many friends may ask, how small is the volume of surface acoustic wave devices? We know that the size of microwave devices is related to wavelength. For many passive devices, many designs are directly related to wavelength, such as 1/4 wavelength resonators The wavelength of electromagnetic waves is the ratio of the speed of light to the frequency, and of course, there is also the relative dielectric constant er, which has been widely verified in the reduction of wavelength in dielectric waveguide filters.

Why is the wavelength of sound waves much shorter compared to electromagnetic waves? Because the propagation speed of sound waves in a specific crystal is fixed, such as the propagation value in quartz glass mentioned earlier: 5370m/s, the wavelength of sound waves is equal to the wave speed divided by the frequency. For example, when the frequency is 1G, this wavelength is very small, so sound wave devices can be made very small. However, wavelengths that are too small require high process precision, similar to millimeter wave devices. This may also limit the frequency application range of SAW devices. But it doesn't matter, with SAW, there are BAW, FBAR, and XBAR. We will gradually master them one by one.