The Prospect and Development Trend of SAW Filters
The surface acoustic wave SAW is an elastic wave that is generated and propagated on the surface of a piezoelectric substrate material and whose amplitude rapidly decreases as the depth of the substrate material increases. The basic structure of the SAW filter is to fabricate two acoustic-electric transducers, the Interdigital Transducer (IDT), on the polished surface of the substrate material having piezoelectric properties. It adopts the planar process of semiconductor integrated circuit, vapor-deposits a certain thickness of aluminum film on the surface of the piezoelectric substrate, and then deposits two mask patterns of IDT deposited on the surface of the substrate by photolithography, respectively as input transducer and output transducer. The working principle is that the input transducer turns the electrical signal into an acoustic signal and propagates along the surface of the crystal, and the output transducer converts the received acoustic signal into an electrical signal output.
The main features of SAW filter are: large design flexibility, analog/digital compatibility, excellent group delay time deviation and frequency selection characteristics (optional frequency range 10MHz ~ 3GHz), small input and output impedance error, low transmission loss, anti-electromagnetic interference (EMI) has good performance, high reliability, and the device body is small and light (the volume and weight are about 1/40 and 1/30 of the ceramic dielectric filter respectively), and various complex functions can be realized. The features and advantages of the SAW filter are adapting to the requirements of modern communication system equipment and portable telephones in terms of thinness, shortness, high frequency, digitization, high performance, and high reliability. The disadvantage is that the required substrate material is expensive, and the substrate orientation, cutting, grinding, polishing and manufacturing processes are required to be high.
The SAW filter plays a good role in suppressing high-order harmonics, image information, emission and leakage signals, and various types of spurious noise interference of electronic information equipment, and can realize filtering of amplitude-frequency characteristics and phase-frequency characteristics of arbitrary precision required. This is difficult to do with other filters. In recent years, SAW filters have been sliced and weigh about 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 is increased to 2.5 to 3 GHz, which further promotes SAW filtering. The device is widely used in the field of anti-EMI.
The SAW filter enables the adjacent frequency transmission of CATV with extremely steep over-bands, which doubles the spectrum utilization compared to the frequency-separated transmission. If the TV receiver does not use a SAW filter, it is impossible to work so stable and reliable. In fact, the main application field of early SAW filters is audio-visual home appliances represented by TV sets. After the end of the 1980s, due to the rapid development of electronic information, especially the communication industry, SAW filters have embraced a broad market and a linear upward trend in production and demand. At present, the annual output of SAW filters in the world is more than 600 million, of which 430 million are small RF SAW filters for mobile communication.
In the radio paging system, the RF signal received by the BP machine needs to be filtered before being amplified, and the electrical characteristics of the filter directly affect the sensitivity and accuracy of the received signal. The BP machines produced in the early days generally use LC filters. Due to the complicated debugging of the LC filters, the selectivity and stability are poor, so they have been gradually replaced by SAW filters.
With the rapid development of the Internet, there are more and more users accessing the Internet worldwide. However, the biggest disadvantage of using the Internet through the phone is that the bandwidth is too narrow (tens of thousands of frequencies), the download speed is extremely slow, and the network frequency resources of CATV are abundant. Therefore, merchants are developing broadband multimedia data broadcasting systems based on CATV networks (such as VOD, etc.), and the information transmission speed can be increased by several dozen times by CATV access. In these systems, high-performance SAW filters are used to solve adjacent frequency suppression problem. It can be seen that the market prospect of SAW filters is very impressive.
Currently, the development trend of SAW filters is mainly reflected in the following aspects:
The small slice of SAW filters is a basic requirement for mobile communications and other portable products. In order to reduce the size of the SAW filter, three measures are usually taken: one is to optimize the chip for designing the device, and try to make it smaller; the second is to improve the package form of the device, which has been changed from the traditional round metal case package. It is a square or rectangular flat metal package or LCCC (lead-free ceramic chip carrier) surface mount form; the third is to package different functional SAW filters together to form a combined device to reduce the area occupied by the PCB, such as the dual-band SAW filter used on 1.9GHz PCS terminal's 60MHz bandwidth and the recently adopted dual-band (both analog and digital mode) portable mobile phone SAW filters developed by Fujitsu, which both are equipped with two filters.
This article is from Allicdata Electronics Limited. Reprinted need to indicate the source: