The core function of a filter is to remove high frequency noise from power supply lines, whether that noise is generated by the supply itself or comes from external electromagnetic interference. Filters are usually built from inductors, capacitors and resistors, and in combination these suppress high frequency noise effectively, allow low frequency signals to pass and thereby stabilise the output voltage and protect the circuit from damage. Filters play a key role in rejecting other bands and interfering signals, and the main types are surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters. A SAW filter consists mainly of two interdigital transducers on a piezoelectric substrate, and the SAW signal travels along the surface of the piezoelectric material, whereas a BAW filter controls frequency by adjusting film thickness and can be used at higher frequencies. Key parameters include passband insertion loss, stopband rejection, bandwidth, power handling, temperature coefficient of frequency and size. Spectrum congestion and the growing number of filters place new demands on overall filter performance.

01 Introduction

With the rapid progress of wireless technologies such as fifth generation mobile communications (5G), millimetre wave radar detection and terahertz communication, and with the expansion of applications such as autonomous vehicles, the internet of things and artificial intelligence, wireless RF systems face ever tougher requirements for integration, miniaturisation, light weight, low power consumption and low cost. Performance requirements for RF transistor technology are also rising, calling for higher power, higher frequency and higher efficiency while keeping cost and size low.

Since its arrival, microwave wireless communication has been widely used in mobile communications, global positioning and data transmission, greatly improving convenience and becoming an important driver of social progress. The spread of 5G has significantly raised transmission rates and data capacity. Traditional RF system approaches such as discrete modules and planar integration can no longer meet rising integration needs, so RF systems are moving towards higher levels of integration, including system on chip (SoC), multi-chip module (MCM), system in package (SiP) and system on package (SoP).

In the RF front end, filters are core components in both the receive and transmit chains, and their performance directly affects communication quality. From 2G to 5G the number of filters in the RF front end has kept rising. In a modern smartphone front end, for example, the filter count has grown from single digits to more than 100. There are two main reasons for this surge. First, mobile terminals must support more and more functions such as GPS, Bluetooth, WiFi and 3G, 4G and 5G, each using different spectrum, and typically each function needs at least two filters or one duplexer. Second, a single filter has limited bandwidth and usually cannot cover an entire communication band, so the RF front end stacks filters at different frequencies to meet wideband requirements.

02 Filter technology principles

To carry more information per unit time, the data rate of a communication system must increase. Increasing bandwidth is one of the most effective ways to do so. The data rate of 5G (1 Gbps) is an order of magnitude higher than 4G, thanks mainly to the increase in the number of bands - from a dozen in 4G to more than 30 - and to multiple input multiple output, carrier aggregation and diversity reception, which combine several bands and raise instantaneous bandwidth from tens of MHz to hundreds of MHz. In smartphones, besides the rapid growth in communication bands, WiFi, Bluetooth and navigation bands are also being added, making the spectrum increasingly congested and turning interference between bands into a key problem that communication systems must solve.

As communication technology advances, demand for frequency and bandwidth keeps growing, posing new challenges for multifunction and integration in communication equipment and further increasing the difficulty of RF device design.

Filters play a key role in rejecting other bands and interfering signals. Key parameters include passband insertion loss, stopband rejection, bandwidth, power handling, temperature coefficient of frequency and size. Filters are now used not only widely in consumer applications but also in radio stations, guidance, telemetry and remote control, phased array radar and electronic countermeasures. To guarantee communication quality, every band in a smartphone needs a filter. Spectrum congestion and the rising filter count place new demands on overall performance: higher Q, a smaller temperature coefficient of frequency and a smaller footprint. Higher Q gives a narrower transition band and therefore better filtering; a smaller temperature coefficient reduces the effect of interfering signals; and a smaller footprint helps make phones smaller and lighter and frees space for the battery.

Acoustic filters convert electrical signals into acoustic signals, perform signal processing in the acoustic domain and then convert the acoustic signals back into electrical signals, thereby filtering the electrical signal. Compared with conventional LC, dielectric and cavity filters, micro-acoustic filters introduce multiple transmission zeros into the transfer function, giving higher Q and better filtering performance. Because the wavelength of an acoustic wave is only one hundred thousandth that of an electrical signal, micro-acoustic filters are far smaller than electrical filters, combining high performance with a small footprint.

Micro-acoustic filters mainly comprise surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters. A SAW filter consists mainly of two interdigital transducers on a piezoelectric substrate, with the SAW signal travelling along the surface of the piezoelectric material. A BAW filter controls frequency by adjusting film thickness and can cover higher frequencies, such as the 18 GHz K band. BAW filters meet both low and high frequency needs and, thanks to a continuous electrode structure, can handle higher power, giving them broad prospects in both commercial and military applications.

Surface electrode morphology of different acoustic filters: (a) SAW filter, (b) BAW filter

Wide bandwidth filters cover more bands and significantly reduce the number of filters needed in the RF front end, which matters for shrinking the front end. Raising BAW bandwidth has therefore long been a research focus in the filter industry. In the 5G era the RF front end urgently needs BAW filters with high frequency, wide bandwidth and small size. However, BAW structures are far more complex than SAW structures, making the technology harder. Domestic research on BAW filters also started later, and the domestic BAW industry lacks drive, with market share largely monopolised by US and Japanese companies such as Broadcom, Qorvo and Taiyo Yuden. By the time domestic companies began to focus on BAW filters, overseas suppliers had already completed industrial production and built a complete patent barrier. Decades of gap have created high technical barriers in the BAW filter industry.

BAW filters mainly comprise solidly mounted resonator (SMR) and film bulk acoustic resonator (FBAR) types. In both, the core is a sandwich transducer in which a piezoelectric thin film is clamped between two metal electrodes. When an electrical signal is applied, the inverse piezoelectric effect converts it into an acoustic signal that propagates upwards and downwards, and reflecting interfaces formed by air or other structures reflect it back. The structural parameters of the transducer mean only acoustic signals satisfying the standing wave condition build up, while other signals cancel each other through multiple reflections with different phases, giving selective response to a particular acoustic signal. Finally, the piezoelectric effect converts the standing wave acoustic signal back into an electrical signal at the same frequency, filtering the electrical signal. The SMR and FBAR structures are shown below.

BAW filters: SMR structure and FBAR/XBAR air gap structure

The structural difference between SMR (solidly mounted resonator) and FBAR (film bulk acoustic resonator) leads to different performance. In the SMR structure the upper reflector is air and the lower reflector is a stack of alternating low and high impedance layers, each a quarter of an acoustic wavelength thick, working on the Bragg reflector principle. Because the layers in the lower reflector reflect acoustic waves with relatively low efficiency, some acoustic energy leaks along the reflector and substrate, so devices using this structure have higher losses. The advantage of SMR is that its heat path is a solid medium, so it can handle higher power. In the FBAR structure both upper and lower reflectors are air, achieving almost total reflection, so losses are lower, but the heat path is air and power handling is weaker. As a result, SMR filters are mainly used in base station applications with high power requirements, while FBAR filters suit handsets and military equipment where low loss matters most.

Market applications and frequency band allocation of RF filters

SAW filters have advantages in low frequency and cost and are mainly used in 2G, 3G and 4G mobile communications. BAW filters offer higher Q, operating frequency, steepness and power handling, a lower temperature coefficient of frequency and easier integration, so they are mainly used in high frequency bands with demanding steepness requirements.

As communication systems and military equipment evolve, BAW filters are required to operate at higher frequencies, in smaller packages and with higher temperature stability. FBAR filters, as a type of BAW filter, hold an important position in mobile communications and military equipment thanks to their advantages in high frequency operation and miniaturisation. However, the temperature coefficient of frequency of FBAR filters is relatively large (-25 to -30 ppm/℃), which for narrowband filters in harsh environments can increase the system guard band, weaken out-of-band rejection and let more interfering signal into the receiver, reducing overall sensitivity and making it hard to meet the high sensitivity requirements of communication equipment and military systems in harsh environments. A temperature compensation layer is therefore added to hold the temperature coefficient within a lower range.

03 Automotive applications and market size

Filters are used in vehicles mainly in the following areas.

In-vehicle network noise suppression. Modern vehicles carry many sensors and high speed networks such as CAN bus and automotive Ethernet, and these networks need efficient noise filters to guarantee data communication quality. Filters remove unwanted signals (noise) from the network, ensuring accurate and reliable data communication.

Electromagnetic compatibility (EMC). Filters also serve an EMC role in vehicles, helping to reduce electromagnetic interference and improve electromagnetic compatibility. This is particularly important for electric vehicles and other vehicles using high voltage technology.

Power line noise suppression. In vehicle power lines, filters suppress noise generated by the line itself, and their role becomes more important as DC-DC converters operate at higher switching frequencies.

Active filters. Active filters in the automotive industry help improve product quality, meet environmental requirements, raise production efficiency and drive technical innovation. They actively compensate harmonics and electromagnetic interference in the supply network, improving the stability and reliability of the electrical system.

Common mode filters. Common mode filters in vehicles mainly eliminate common mode noise and safeguard communication quality. They suit high speed networks such as automotive Ethernet and CAN bus, ensuring accurate and complete data transmission.

Filter applications in vehicles are therefore broad and important, from improving in-vehicle network performance to ensuring power line stability to driving technical progress across the industry.

Filter market keeps growing

The global filter market has expanded steadily in recent years, showing strong momentum. According to the China Filter Industry In-depth Analysis and 15th Five-Year Plan Guidance Report, the global filter market reached USD 11.74 billion in 2023 and is expected to climb to USD 29.71 billion within the next decade. The growth is driven mainly by thriving 4G and 5G smartphone markets and by fast growing areas such as the internet of things and vehicle connectivity. As communication technology advances, demand for filters as the core of the RF front end rises with it.

Technically, the filter industry is moving up from low end to mid and high end products. As 4G and 5G phones support more bands, the number of filters per handset and their value both increase. High end filters such as BAW and temperature compensated SAW (TC-SAW) are becoming mainstream choices thanks to their performance and stability. The global high end filter market, however, is still dominated by a small number of US and Japanese suppliers, leaving domestic filter makers considerable room to improve both technology and market share.

Import substitution is under way

In the global BAW filter market, Asia-Pacific holds a dominant position with a 75% share. As demand from 5G communications and smartphones grows, the BAW filter market keeps expanding. According to the latest QYResearch report, Global BAW Filter Market Report 2024-2030, the global BAW filter market is expected to reach USD 13.61 billion by 2030, with a compound annual growth rate of 8.7%. FBAR BAW filters are the largest sub-segment with about 90% of the market, and smartphones are the largest application area with 85%.

In the Chinese market, domestic production of BAW filters is accelerating. Although the global BAW filter market is still dominated by leading overseas companies, the Chinese government has made BAW filters a priority area, using policy and funding support to encourage companies and research institutes to achieve breakthroughs in technology and product development. According to East Money data, the China BAW filter market is expected to reach CNY 63.3 billion by 2027. As domestic R&D investment keeps rising, some domestic BAW filters have already entered mass production.

Policy support and supply chain optimisation

To make 5G and future communication technology autonomous and controllable, the Chinese government attaches great importance to domestic production of RF devices. Policies such as the New Infrastructure Construction Action Plan and the 14th Five-Year Plan for the Information Industry explicitly support domestic RF devices and step up efforts to break through high end filter technology bottlenecks. These policies give domestic filter makers strong support and accelerate industrialisation of domestic BAW filters.

On the supply chain side, demand for BAW filters keeps growing with 5G and future 6G technologies. To reduce dependence on overseas suppliers, China is actively promoting import substitution for filters. Although overseas companies hold advantages in core technology and patents, domestic makers are narrowing the gap with international leaders through continuous innovation and patent filings. According to Sina Finance data, more than 2,000 BAW filter related patent applications have been filed in China, over 60% of them by domestic applicants, which shows that independent R&D capability in BAW filters is improving and that China is expected to play a more important role in the global market.

04 Filters must pass automotive AEC-Q200 qualification

The AEC-Q series is the recognised general test standard for automotive components. For a filter company to enter automotive electronics and the automotive component supply chain, AEC-Q200 is one of the qualifications it must obtain.

(1) AEC-Q is the entry threshold for the international automotive electronics industry

AEC stands for the Automotive Electronics Council, an American organisation founded in 1994 by Chrysler, Ford and General Motors. Its membership now spans major vehicle makers, automotive electronics companies and semiconductor manufacturers worldwide. Components meeting AEC specifications can be adopted by all three founding carmakers at the same time, which encouraged component makers to exchange product characteristic data and promoted standardisation of automotive parts, laying the foundation for rapid growth of the automotive component market. AEC-Q is the automotive reliability test standard issued by the AEC organisation and is an important ticket for component makers entering automotive electronics and the tier-one supply chain.

(2) AEC-Q200 is the entry threshold for automotive-grade passive components

AEC-Q200 is the quality and reliability verification standard issued by the Automotive Electronics Council for passive components used in vehicles, and it is the threshold for passive components entering the automotive field. A series of tests verifies whether passive components can withstand extreme temperature, humidity, vibration and ageing in automotive applications. The standard is mainly intended to prevent performance variation or potential fault conditions in passive components, ensure their quality and reliability, and keep function and performance consistent and stable after continuous use.

(3) Product scope of AEC-Q200

Tantalum and niobium capacitors, supercapacitors, ceramic capacitors, aluminium electrolytic capacitors, film capacitors, electromagnetic devices such as inductors and transformers, resistor-capacitor and capacitor-resistor networks, resistors, thermistors, variable capacitors and variable resistors, varistors, ferrite EMI suppressors and filters, quartz crystals, ceramic resonators, fuses and polymeric resettable fuses.

(4) NTEK testing capability against AEC-Q200 requirements for filters

AEC-Q200 qualification for filters comprises 20 tests, and the latest edition redefines sample size by product size. Sample numbers are set according to three categories: less than 10 cm3, 10 cm3 to 330 cm3, and more than 330 cm3, with larger components needing fewer samples. Taking temperature cycling as an example, a product smaller than 10 cm3 needs 77 samples, a product between 10 cm3 and 330 cm3 needs 26 samples, and a product larger than 330 cm3 needs only 10 samples.

About NTEK

NTEK Group was founded in 2009 and is headquartered in Shenzhen. The group provides quality research and technical services covering R&D verification, inspection and testing, failure analysis, simulation and market access for intelligent connected vehicles, electronic communications and new energy.

NTEK has extensive experience in automotive-grade electronic certification and has helped more than 100 companies pass AEC-Q series certification. Driven by the need of vehicle makers to localise automotive-grade components and supported by the domestic semiconductor industry, NTEK provides comprehensive testing and certification services, applying AEC-Q automotive standards to control the safety and quality of automotive components, supporting the growth of domestic automotive-grade components and adding new momentum to the intelligent vehicle safety system.