Surface Acoustic Wave(SAW) Filter

Speaking of surface acoustic wave filters, everyone may be unfamiliar with them, but they are all around us. There are many surface acoustic wave filters in the communication module of mobile phones. Surface acoustic wave filters are widely used in RF front-end modules of various electronic devices. The RF front-end module is the core component of the communication system. It mainly includes filters (Filters), low noise amplifier (LNA), power amplifier (PA), radio frequency switch (RF Switch), antenna tuning switch (RF Antenna Switch), duplexer (duplexer), etc. Filters are the products with the highest output value in all fields of RF front-end. As can be seen from the figure below, the proportion of filters in the RF front-end will reach 66% in 2023.

Filter is the proportion of output value in each field of RF front-end ( Data Sources: resonant，Qorvo，Yole )

The function of the filter is to filter out signals outside a specific frequency and retain signals within a specific frequency band through a combination of electrical components such as capacitors, inductors, and resistors. Since smartphones need to receive multiple frequency bands (5G/4G/3G/2G), they also need to process multiple signals, such as WIFI, BT, GPS signals, etc. Different communication modes have different operating frequency bands, so we need to use multiple filters in the transceiver link to avoid interference between signals.

Filters can be divided into LC filters, cavity filters, acoustic filters, dielectric filters, etc. according to different implementation methods. Different filters have different characteristics and are suitable for different application scenarios. In wireless communication applications of consumer electronics, due to the requirements of low power consumption and small size, smartphones currently use small-sized and high-performance acoustic filters, including surface acoustic wave (SAW) filters and bulk acoustic waves. (BAW) filters, both have different characteristics and application frequency bands. This article focuses on the former - SAW filters.

What is a saw Filter?

The surface acoustic filter is composed of a metal film evaporated on a material substrate with a piezoelectric effect, and then photolithographed to form a pair of interdigitated electrodes at both ends. When a signal voltage is applied to the transmitting transducer, an electric field is formed between the input interdigital electrodes, causing the piezoelectric material to mechanically vibrate (i.e., ultrasonic waves) and propagate to the left and right sides in the form of ultrasonic waves. The energy toward the edge is absorbed by sound-absorbing materials. At the receiving end, the mechanical vibration is converted into electrical signals by the receiving transducer and output by the interdigitated electrodes.

Surface acoustic wave filter parts diagram

A Surface Acoustic Wave (SAW) filter is an electronic component that filters and processes electrical signals by using surface acoustic waves on a piezoelectric material, often quartz. SAW filters are known for their compact size and high-frequency selectivity, making them valuable in applications where precise signal filtering and noise reduction are required, such as in wireless communication devices and radar systems. They effectively allow specific frequencies to pass through while attenuating or rejecting others, contributing to improved signal quality.

How Surface acoustic wave filters work

Let’s first look at what surface acoustic waves are.

Surface acoustic waves were first discovered by Rayleigh, so they are also called Rayleigh waves. In 1885, Lord Rayleigh discovered the propagation mode of surface sound waves and predicted the properties of these waves in his classic paper. Surface acoustic waves have longitudinal and vertical shear components, which can couple with the medium in contact with the device surface, and its energy is limited to propagation on the contact surface. Since the SAW on the contact surface substrate has electrostatic waves associated with it, we can perform electroacoustic conversion on the transducer. Because the transducer is shaped like a pair of crossed fingers, it is often called an interdigital transducer (IDT). The figure below shows an example of an interdigital transducer.

Schematic diagram of a SAW transducer consisting of crossed electrodes

The interdigital transducer converts electroacoustic signals through the piezoelectric effect. As shown in the figure above, a SAW signal can be excited by applying a suitable oscillating signal (AC voltage) to both ends of the grid of a set of specially designed interdigital transducers. As can be seen from the figure, the interdigital type is composed of multiple pairs of intersecting electrodes, forming a grating-like structure. The spacing between the gratings determines the SAW wavelength. The conversion of the electrical signal to the SAW acoustic signal is accomplished by grounding one side of the transducer and applying a signal at a frequency obtained by dividing the SAW speed (approximately 2700m/s for GaAs) by the spacing of the transducer.

A SAW filter consists of at least two transducers. The figure below shows a schematic diagram of a simple SAW bidirectional filter.

SAW filter diagram

On the transducer at one end, the excited SAW signal propagates forward on the substrate surface. At the same time, the elastic deformation generated by the piezoelectric substrate is converted into electrical energy through the piezoelectric effect, which can then be easily transmitted by the interdigitated fingers at the other end. type transducer capture. Because surface acoustic waves are strongly coupled to the substrate, their amplitude and speed during propagation will be affected by substrate parameters, such as substrate quality, width, thickness, dielectric coefficient, insertion loss, etc. When a SAW passes under a grating-like structure transducer with appropriate spacing, an alternating potential is generated at both ends of the transducer. At this point, the entire system has completed the transmission process of electrical signals - acoustic signals - electrical signals. The figure below shows the transmission spectrum plot between two SAW transducers.

Normalized frequency response of SAW filter

It can be seen that after a process of electro-acoustic conversion, SAW propagation and acoustic-electric conversion, the components of the transmitted signal outside the 250MHz adjacent frequency band will be greatly attenuated. Therefore, SAW assists in the task of band-pass filtering electrical signals.

Considering that the device system consisting of two transducers and a substrate has specific frequency response characteristics, and the frequency response characteristics can be controlled through the preparation of the transducer and the substrate, and the device has high stability, Due to its small size, high selectivity, and high Q value, the system has been successfully used as a filter with various functions.

Schematic diagram of the working principle of surface acoustic wave filter( Data Sources: ScienceDirect )

As you can imagine, the process and design of SAW filters are equally important as the manufacturing of chips. Because the parameters of the substrate and IDT transducer have a huge impact on the performance of the entire device. The process is particularly important for device performance!

Other Applications of SAW

In addition to being used as filters, SAW also has very important applications in other aspects. SAWs are widely developed as various sensors. SAW sensor is a sensor that uses SAW devices as sensing carriers to intuitively display the physical information to be measured through changes in SAW frequency or speed, and converts it into electrical signal output. It has high sensitivity, low cost, and low power. It has the advantages of low power consumption, miniaturization and direct frequency signal output.

Surface acoustic wave sensor

The transmission speed of surface acoustic waves is several orders of magnitude slower than electromagnetic waves. Therefore, surface acoustic waves during propagation are easy to be sampled, analyzed and processed. Therefore, SAW can simulate various functions of electronic devices and make electronic devices multi-functional. and develop in the direction of ultra-miniaturization. At the same time, the SAW device itself has outstanding advantages in signal acquisition and processing, frequency control and selection. Therefore, SAW equipment has been widely used in communications, radar and electronic countermeasures. Currently, surface acoustic wave technology has been widely used in many fields such as mobile communications, aerospace, environmental monitoring, and medical instruments. In the 5G era, the SAW market size is very considerable.

References:

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• [2] Pan Xiaoshan, Liu Ruitong, Wang Qin, et al. Review of the principles and applications of surface acoustic wave sensors [J]. Sensors and Microsystems, 2018, 37(04): 1-4.

• [3] Liu Xiaoqing. Research on new structural surface acoustic wave devices[D]. Nanjing University of Posts and Telecommunications, 2020.

• [4] Liu Jiuling. Theoretical analysis and experimental research on surface acoustic wave resonant filter [D]. Institute of Acoustics, Chinese Academy of Sciences, 2004.

• [5] Ruppel C C W. Acoustic wave filter technology–a review[J]. IEEE transactions on ultrasonics, ferroelectrics, and frequency control, 2017, 64(9): 1390-1400.