SAW Filter: Introduction, Features and Applications

Catalog

I. General Introduction

Surface Acoustic Wave (SAW) technology is an emerging field of science and technology that was developed in the late 1960s. It is a fringe subject that combines acoustics and electronics.

The development of surface acoustic wave technology is quite rapid. Its application field has been developed from the beginning of military radar to almost the entire radio communication, especially the rapid development of mobile communication technology, which has further promoted the development of surface acoustic wave technology.

A SAW filter is the abbreviation of Surface Acoustic Wave filter. It is a special filtering device made of piezoelectric materials such as quartz crystal, piezoelectric ceramics, and its piezoelectric effect and surface acoustic wave propagation. It is widely used It is used in the intermediate frequency circuit of TV and video recorder to replace the LC intermediate frequency filter to greatly improve the quality of images and sound. The so-called piezoelectric effect is a phenomenon in which an electric field proportional to the pressure is generated when the crystal is mechanically acted on. The crystal with a piezoelectric effect will also produce elastic deformation and send out mechanical waves (sound waves) when subjected to electrical signals, which can convert electrical signals into acoustic signals. A surface acoustic wave is an elastic wave that is generated and propagated on the surface of piezoelectric substrate material, and its amplitude decreases rapidly as the depth of the substrate material increases.

The surface acoustic wave filter is plated with an interdigitated comb-shaped metal electrode (also called a transducer) on both ends of a piezoelectric crystal substrate. When the electrical signal is connected to the input transducer, the surface of the piezoelectric crystal will generate a surface acoustic wave (mechanical vibration wave) with the same frequency as the input electrical signal. The surface acoustic wave propagates along the surface of the piezoelectric substrate and passes through A certain delay time is transmitted to the output transducer, and the output transducer converts the surface acoustic wave into an electrical signal for output. In the conversion process, the useful components in the signal are selected, attenuated, and filtered out of useless interference signals.

The surface acoustic wave filter has the advantages of small size, lightweight, reliable performance, and does not require complicated adjustments. It is a key component for realizing adjacent frequency transmission in a cable TV system.

The transmitting transducer converts the RF signal into a surface acoustic wave and propagates on the surface of the substrate. After a certain delay, the receiving transducer converts the acoustic signal into an electrical signal for output. The filtering process is realized in the conversion from electricity to sound and sound to electricity, so the SAW filter can be equivalent to a two-port passive network, as shown in Figure 2. In the figure, H1(ω) is the frequency response of transmitting (or input) interdigital transducer IDT1, H2(ω) is the frequency response of receiving (or output) interdigital transducer IDT2, and H3 (ω) is the frequency response of SAW in two Transmission characteristics between interdigital transducers. Suppose the velocity of the surface acoustic wave is Vs. Since Vs is non-dispersive, it is obvious that H3(ω) can be equivalent to an all-pass delay network with a certain delay t0. If the distance between the center of the input and output interdigital transducer is L, then:

In the formula, A3 is a constant, generally recorded as 1. Therefore, the total transfer function (or frequency response) of the SAW filter is:

Applying the Fourier transform characteristics, consider 1|)(|3≈ωH in the analysis, so it can be omitted)(3ωH. The frequency response of the surface acoustic wave filter is:

II. Features

The main features of SAW filters are:

1. The frequency response is flat, the unevenness is only ±0.3-±0.5dB, and the group delay is ±30-±50ns.

2. The SAWF rectangular coefficient is good, and the out-of-band suppression can reach more than 40dB.

3. Although the insertion loss is as high as 25-30dB, an amplifier can be used to compensate for the level loss.

4. Large design flexibility, analog/digital compatibility, group delay time deviation, and excellent frequency selectivity (the selectable frequency range is 10MHz～3GHz),

5. Small input and output impedance error, low transmission loss

6. Good anti-electromagnetic interference (EMI) performance and high reliability

7. The manufactured device body is small and light and its volume and weight are about 1/40 and 1/30 of the ceramic dielectric filter and can realize a variety of complex functions.

The characteristics and advantages of SAW filters are adapted to the requirements of modern communication system equipment and portable phones for lightness, thinness, shortness, high frequency, digitization, high performance, and high reliability. The disadvantage is that the required substrate material is expensive, and the orientation, cutting, grinding, polishing, and manufacturing process requirements of the substrate are high. Affected by the harsh substrate crystallization process and strict manufacturing precision requirements, a few manufacturers such as Fujitsu, Sanyo Electric, Toyota, and other manufacturers that master the production technology of piezoelectric substrates have monopolized the world SAW filter market. Fujitsu controls about 40% of the global market for small radio frequency SAW filters for mobile phones. At present, its annual output is more than 150 million. The smallest product size has reached 2.5mm×2mm and weighs 22mg. The patented resonator-type filter is designed in one, making the filter performance a breakthrough leap. Sanyo Electric is one of the world's largest manufacturers of SAW filters for audio-visual home appliances. Toyota mainly produces SAW filters for mobile communications and can provide more than 30 standard products, all suitable for surface mounting.

III. Applications

SAW filter plays a good role in suppressing high-order harmonics, image information, emission of leaked signals, and various parasitic clutter interference of electronic information equipment, and can achieve filtering of amplitude-frequency and phase-frequency characteristics with any required precision. This is difficult to accomplish with other filters. In recent years, SAW filters have been made into chips in foreign countries, weighing only 0.2g; in addition, due to the use of new crystal materials and the latest fine processing technology, the upper limit frequency of SAW devices has been increased to 2.5GHz ~ 3GHz. This promotes SAW filters to gain wider applications in the field of anti-EMI.

The SAW filter realizes the adjacent frequency transmission of CATV with a very steep transition band. Compared with the frequency isolation transmission, the spectrum utilization rate is doubled. If the TV receiver does not use SAW filters, it is impossible to work stably and reliably. In fact, the main application areas of early SAW filters were audiovisual home appliances represented by televisions. In the late 1980s, due to the rapid development of electronic information, especially in the communications industry, SAW filters provided a broad market space. , Resulting in a linear upward trend in output and demand. At present, the annual output of SAW filters in the world is more than 600 million, of which 430 million are miniaturized RFSAW filters for mobile communications.

The transmitting end (TX) and receiving end (RS) of the mobile communication system must be filtered by the filter before they can play a role. Since its working frequency band is generally 800MHz～2GHz and the bandwidth is 17MHz～30MHz, the filter is required to have low insertion loss, High stop band suppression, and high image attenuation, withstand high power, low cost, miniaturization, etc. Due to the advantages of the working frequency band, volume, and cost performance, SAW filters dominate the applications of mobile communication systems, which are beyond the reach of piezoelectric ceramic filters and monolithic crystal filters.

In the wireless paging system, the RF signal received by the BP machine needs to be filtered and then amplified. The electrical characteristics of the filter directly affect the sensitivity and accuracy of the received signal. The early-produced BP machines generally use LC filters. However, due to the complex debugging of the LC filters, the selectivity and stability are poor, so they are gradually being used by SAW filters.

SAW filters are also used in mobile communication systems, whether digital or analog, the circuit structure of the functional modules for transmitting and receiving signals is basically the same. At the Tx end, the signal is modulated on the carrier, the power is amplified by the amplifier circuit, and then the signal is sent out by the antenna after being filtered by the SAW filter. This channel requires low filter loss and can withstand high power; at the Rx end channel After the weak signal received by the antenna is filtered by the SAW filter, it is amplified and demodulated to finally obtain the desired information. It requires low filter loss and high stopband suppression.