FBAR filter technology for 4G/LTE smartphones

While the global mobile phone market is still growing at a single-digit rate each quarter, smartphone shipments continue to surge. With the introduction of new generations of mobile phones, smartphones continue to provide more functions without significantly increasing size and weight. The driving force behind these new products comes from the introduction of new mobile communication networks, such as 4G and LTE technologies. Most people have equated faster download speeds and better mobile Internet experience with the latest services. As expected, almost all industry news revolves around the features of the latest mobile phones, such as battery life, display brightness, color and resolution, etc. But what kind of semiconductor building blocks bring such amazing products?

A very important part of modern smartphones is the radio frequency (RF) filter. As its basic principle, the filter is mainly used to pass the required frequencies and reject the unwanted frequencies so that the many receivers in the mobile phone can only process the expected signals. In the past, mobile phones usually only worked in a few frequency bands in specific regions of the world. However, for modern mobile phones, they basically work in multiple wireless frequency bands at the same time, including mobile communications, Bluetooth, WiFi and GPS, etc. Manufacturers also hope to design products that can work in different regions and different telecom operators around the world. To make mobile phones work in more frequency bands and regions means that the requirements for RF filtering are getting higher and higher.

In previous generations of wireless technology, filtering requirements were not difficult to achieve and could be achieved using surface acoustic wave filters. However, as operator networks evolved to CDMA and 3G, Avago Technologies found that its film cavity acoustic resonator (FBAR) technology was well suited to the challenging filtering requirements of PCS and some other frequency bands in the United States. In order to take advantage of current 4G/LTE services, smartphones themselves have become more complex, so mobile phone manufacturers have begun to expand the use of Avago's FBAR technology to solve the unique problems faced by 4G/LTE, which will be discussed below.

RF filter challenge 1: 4G/LTE mobile phones operating in multiple frequency bands

The latest smartphone products must be designed to work in multiple frequency bands around the world. The overall size of multi-band smartphones is not larger than the previous generation. Therefore, if more filters are to be added in the same space reserved for the RF front-end circuit, then it is very obvious that the filter itself must be very small. With the help of Microcap micro-packaging technology, FBAR filters can meet most space-constrained applications through chip-level packaging. Since FBAR is a matrix material, it can provide very good power handling capabilities without the need to use parallel structures such as those commonly found in SAW filters. In addition, the size of FBAR devices will also shrink as the frequency increases, making FBAR very suitable for the current 2300MHz to 2700MHz and the future 3.5GHz new 4G/LTE band applications.

Perhaps the most important features of FBAR technology are the steep filtering curves that support challenging frequency band assignments and excellent out-of-band rejection. Figure 1 shows the situation for the US PCS application mentioned earlier, where the gap between the upper transmit frequency and the lower receive frequency is very narrow. This feature is even more important for 4G/LTE applications because the gap between the transmit and receive frequencies is even narrower. In addition, today's smartphones must also operate in a crowded spectrum next to existing 2G/3G RF services, and avoid interference that may affect or even interrupt data services. Typically, the transition frequency between bands is on the order of a few MHz, and sometimes there is no guard band at all.

Figure 1 FBAR filter response characteristics

One of the value advantages brought by FBAR filters can be seen in the Band 13 environment used in the United States. This frequency band used by Verizon for LTE service is only 2MHz away from the new public safety radio service band used by police, fire and other emergency response agencies. In order to avoid interfering with PSR operation, the LTE standard requires that mobile phones operating on the Band 13 band must significantly reduce transmission power, but the reduction in power will significantly affect network efficiency, reduce the number of users that can be served and the quality of service, significantly reduce data throughput and even cause dropped calls. By combining ultra-fast turning FBAR duplexers with temperature compensation and highly linear power amplifiers in an integrated front-end module with a well-controlled environment, Avago has created a product that allows Band 13 band mobile phones to operate at full power without interfering with PSR operation. The performance of the FBAR duplexer can be said to be one of the keys to the success of such products.

RF Filter Challenge 2: 4G/LTE Smartphones Operating at Higher Data Rates

Compared to 3G services, 4G/LTE can download about 10 times faster for the same amount of data, which means that 10 times more data can be downloaded in the same amount of time. There are several ways to achieve higher data rates. 4G/LTE uses different modulation methods depending on the detected signal strength. In simple terms, the higher the signal-to-noise ratio, the higher the data rate, such as switching from QPSK to QAM16/64 modulation. On multi-band 4G/LTE phones that use a single-pole, multi-throw switch combined with multiple duplexers, the detected signal may be too low to affect the data rate. The low insertion loss of FBAR helps maximize the input signal strength, resulting in higher data throughput, which results in a better user experience and higher data capacity. Phones that use frequency division multiplexing modulation use duplexers that allow simultaneous signal transmission and reception. Since the transmit and receive filters are connected to the same antenna port, filter isolation between them is very important. Higher isolation minimizes noise in the receive band, which, as discussed earlier, can improve SNR and data rates.

Figure 2 Guard band for 4G/LTE services in the 2.3GHz to 2.7GHz band

Another way to increase data rates is through carrier aggregation, which increases download data rates by operating more than one frequency band at the same time. Some new LTE bands occupy relatively small spectrum, such as Band 25 (5MHz), so this is an effective way for network operators to increase communication capacity. Since the transmission and reception of each band will work at the same time, switches cannot be used, so multiplexers are used to combine various transmit and receive filters to the same antenna port. When combined in a multiplexer configuration, Avago's FBAR filters can provide low signal loss paths, helping to maximize data rates.

RF filter challenge 3: Smartphones use multiple wireless signals simultaneously

It is difficult to find a smartphone without Wi-Fi connection function. Depending on the operating frequency of the mobile phone, if the signal is not properly filtered, it may interfere with the normal operation of Wi-Fi. As an example, the transmission frequency of Europe's LTE Band 7 (2500MHz~2570MHz) is just above the frequency used by European Wi-Fi (2401MHz~2488MHz), see Figure 2.

When using a smartphone as a Wi-Fi hotspot, Wi-Fi will work simultaneously with 4G/LTE wireless signals. Without excellent filtering capabilities, the Wi-Fi transceiver may be obscured or affected by LTE signal transmission on Band 7. Avago's ACMD-6107 duplexer provides sufficient protection to allow Wi-Fi channels running at higher frequencies to not interfere. Other competing filter products cannot effectively provide the required out-of-band attenuation capabilities, which may cause the upper Wi-Fi channels to be unusable. When combined with Avago's ACPF-7124 Wi-Fi coexistence filter, it can provide outstanding performance that meets or even exceeds system requirements.

Today, most mobile phones also support GPS and even GLONASS services. Since GPS/GLONASS signals are usually very low power, about -125dBm ~ -150dBm, all transmitted signals close to the GPS frequency may affect the sensitivity of the GPS/GLONASS receiver. The AGPS-F001 pre-filter plus LNA module has steep filtering and broadband attenuation capabilities, so it can provide excellent out-of-band shielding capabilities and good linear performance for mobile network, PCS and WiFi signals.

FBAR Technology Advantages

Battery life is an important feature that is often used to test and compare mobile phone performance. On the receiving side, we discussed how the lower insertion loss of FBAR supports higher data rates of 4G/LTE mobile phones by compensating for the higher loss caused by combining multiple bands in the RF front end. Another benefit is that it expands the coverage of mobile communications by enabling mobile phones to detect weaker signals, avoiding poor reception or even dropped calls. On the transmitting side, lower transmit filter insertion loss means lower output power required by the power amplifier at the same antenna transmit power. Compared with other filter technologies, Avago's Band 4 duplexer brings about an insertion loss improvement of about 0.2dB ~ 0.5dB, which is equivalent to saving up to 50mA of current consumption, thus providing longer battery life and talk time.

When most applications were still based on 3G services, only a few frequency bands could benefit from FBAR technology. With the popularity of 4G/LTE multi-band smartphones, the characteristic advantages of FBAR technology, such as low insertion loss, steep filtering curve, high isolation and extremely small size, have become the reasons why all major smartphone manufacturers have quickly introduced this technology. Currently, filters, duplexers and multiplexers using FBAR technology have been introduced into smartphone designs in 15 different operating frequency bands in the United States, Europe and Asia. As new filtering challenges emerge, FBAR technology will continue to be the preferred choice for providing solutions. In other words, FBAR technology has become mainstream.