How does MEMS technology meet antenna tuning? Mobile terminal antenna challenge

Antenna designs for mobile applications are becoming more complex, requiring flexible MEMS technology to meet the requirements of integrating new features and new applications.

New applications and new features such as TV reception are continually integrated into wireless devices. In addition, wireless technologies, including local area networks (Wi-Fi), broadband wireless access (WiMAX), digital television (DVB-H), global positioning system (GPS), ultra-wideband (UWB), and multiple input multiple output (MIMO) It is expected to improve performance, expand the frequency range and enhance throughput, but it also poses a major challenge to the cost, size and performance of the required antenna. As customers' demand for wireless devices grows, market research firm Frost & Sullivan expects that by 2012, the mobile antenna market in North America alone will reach $905.3 million.

Each wireless standard operating in a specific frequency range requires their own antenna design to provide optimal performance. In addition to standard cellular radios, these separate wireless networks are closely adjacent, causing an increase in self-interference, degradation in performance, sound quality or data rate, or loss of line. This problem is exacerbated by the slender form factor, which puts the antennas together intensively and delivers them to the customer, while the specific absorp ratio (SAR) limits affect the radiation pattern and output energy of the same antenna ( figure 1).

The solution is to make the antenna dynamically tunable. The benefits of this approach are many, including the ability to reduce the size of the individual antennas and make a single antenna a multi-band antenna, ie a single antenna can support multiple frequencies, and/or near-field interference from other RF devices in the phone to the antenna. The impact is smaller. Even better, the mobile terminal architecture requires additional antennas to support the new features and functions that customers are constantly demanding, while the tunable antennas remove the need for additional antennas.

RF Micro-Electro-Mechanical Systems (RF-MEMS) is an emerging technology that provides designers with the major components needed to make tunable antennas. With RF-MEMS-based active antenna systems, future multi-band, multi-featured wireless devices will become smaller, perform better, and consume less power than devices implemented using traditional techniques.

Advances in wireless communications have led to more research on antenna performance in different work environments. With the introduction of new communication systems with wider frequency bands, single-band wireless terminals have evolved into multi-band, multi-mode and current multi-function devices. In addition, the size of the mobile terminal has been reduced to be smaller than a deck of cards. At the same time, the built-in antenna is the first choice for transmitting equipment because of its small size. All of these changes, combined with the strict limitations of mobile phone users on absorbing energy, have created the need to improve antennas to meet increasingly smaller platforms. To meet these challenges, antenna and mobile terminal vendors are constantly breaking these limits, allowing these small antennas to efficiently transmit and receive RF waves in different frequency bands and communication modes (Figure 2).

The choice of antenna, whether built-in or external, small or large, will drive different considerations for the design of the backplane. In general, larger antenna elements can provide greater gain and wider frequency band response. And antennas of the same size can work well in dynamic environments, such as when a wireless device is close to a person's head. In this case, the ideal 50 ohm transmit impedance will vary slightly, and the output of the power amplifier can be compensated for by a small change in the corresponding overall output energy. However, when the antenna element is small, it may perform well under normal operating conditions, but at the expense of lower gain and poor environmental sensitivity for a given bandwidth. Taking the human head as an example, the feedback impedance of a small-sized antenna may vary by more than 5 times, which forces the system to drive the power amplifier to compensate more than usual, resulting in increased power consumption and reduced battery life.

In addition, when a small-sized antenna is placed on a PCB of a mobile phone, the size, shape, and position of the antenna can greatly affect its performance. The backplane affects bandwidth, radiation efficiency, and the specific absorption rate of the built-in mobile phone antenna. Therefore, in addition to the impedance bandwidth, the radiation efficiency and absorption rate of the talk position strongly depend on the parameters of the telephone floor. This often causes people to be forced to redesign at the end of the design cycle, sometimes even reworking after type testing, delaying the production time of the product.

For a long time, mobile phone designers have been looking for ways to make the phone antenna work well under all conditions. However, because of the limitations of SAR requirements, for example, the radiation limit in the United States is no more than 1.6 W/kg per gram of human tissue, which means that most of the transmitted energy must be radiated away from the head, and if the antenna is sent omnidirectionally Then its overall energy must be very low. In order to compensate for this, the antenna designer must carefully define the shape or pattern of the metal that makes up the antenna so that the antenna radiates along the vertical and parallel planes to reduce the effects on human tissue and from human tissue. They use computer models to predict the performance of mobile phones under different conditions or locations when people talk. In short, SAR limitations result in design considerations for the radiant energy of the antenna and affect its shape, size, and position in the phone.

An ideal antenna should have a 50Ω impedance in all frequency bands in any environment, but this is rarely the case, as a single antenna is used to support up to six different frequency bands. Antenna designers are often constrained by efficiency and size in the operating band. On the other hand, power amplifier (PA) manufacturers always design their amplifiers to drive 50Ω loads. This presents the mobile phone designer with the task of matching the antenna load to the PA. Impedance matched to fixed passive components such as capacitors and inductors, mainly used

To solve the problem of mismatch between antenna and PA. Such passive networks are compromised and their performance is lower than the design value because the matching network is fixed, which limits their ability to match in all frequency bands. Matching the network can also result in loss of energy. Designers compensate by using two or more narrowband amplifiers, each with its own antenna matching network.

When an antenna transmits an unwanted signal (regardless of the frequency), interference occurs, but it is still received by the other antenna anyway. Interference occurs when mobile phones are close to each other, and mobile phone designers have long observed this phenomenon and used a variety of circuits and control techniques to solve the interference problem. Until recently, the phone used only one antenna for cellular calls, and perhaps a second antenna for Bluetooth, and now there may be a third for GPS. These versatile cellular phones typically have a cellular antenna that is isolated from Bluetooth and GPS antennas. With the advent of new standards and other features (Figure 2), cellular phones carry broadband and even competing technologies such as CDMA and GSM become necessary. Coupled with Bluetooth, WiFi and WiMAX, which will soon be adopted, mobile phone designers will have more and more antennas on their mobile terminals. With advances in personal and LAN technologies such as UWB and MIMO transceivers, cell phone developers have realized that serious interference problems are waiting for them.

Very early in the fixed system that sends signals to all directions

Interference is observed in the system, such as WiFi, when two transmitters with the same energy and frequency are close together. Time allocation, directional transmission, diversification, and/or frequency hopping are often used to solve this problem. But the design of mobile phones is far less flexible because of device mobility and other unpredictable sources of interference, such as mobile phones, local area networks and Bluetooth, as well as small-sized mobile phones and high receiver sensitivity requirements. . Ultra-high Q filtering (SAW, FBAR, and/or cavity) is used to protect the receiver from interference, and the transmission is cleaned to minimize the possibility of interference. Unfortunately, this requires at least one additional fixed filter per operating mode/band, which increases the cost and size of the mobile phone. The signals to be filtered by these filters may come from other antennas on the same platform, but mutual interference may occur between adjacent antennas (Figure 3). Dense antennas can cause direct antenna-to-antenna coupling, which requires tighter filter specifications, which increases filter complexity and cost. If the antenna is coupled to the RF path, the fixed filter may be difficult to function because the filter is not isolated from the RF path, but the PCB shield helps to minimize this coupling effect.

Filters and shields can effectively reduce interference and antenna coupling, but they increase losses and cost, which affects the transmit performance of the antenna. The loss on each transmit path can be as high as 1-2dB. This again requires the PA to be more difficult to compensate.

A wideband antenna is designed to operate optimally in the desired frequency band, operates and is optimized in the target band, but is less efficient than a single band antenna. In addition, the use of a small-sized internal antenna for a mobile phone having an elongated form factor can further degrade performance. Placing a small-sized antenna in a tuning network is one way to improve its multi-band performance. The antenna can be tuned in a certain frequency band using a variable device such as a varactor. By testing and modeling, you can get the optimal value of the varactor operating in multiple bands, and then you can use software to extract these values ​​and put them into the desired network.

Antenna tuning can improve the performance of the wideband. But unfortunately, tuning can degrade performance because the antenna is not as effective as an antenna that naturally resonates. This is because tuning the network itself introduces additional losses.

All in all, interference coupling, slender form factor, built-in antennas, SAR limitations, and ever-increasing multi-band/multi-function selection make the development of cellular phone antenna solutions a very complex challenge. Designers of wireless mobile phones have developed terminals under these constraints and will continue to develop. At the same time, the prospects for wireless applications clearly require a new approach to meet the needs of customers and carriers.

The use of MEMS began in 1970, when the automotive industry first used them as pressure sensors. Later, the automotive industry used MEMS as an acceleration sensor for collision airbags. Today, MEMS-based devices have found their way into widescreen TVs, mobile phone microphones, and GPS tools.

For RF applications, MEMS devices are promising as a small, high-performance alternative to existing solutions, reducing the cost of materials and being a way to achieve more intensive functional integration. The growing mobile phone market, along with evolving multi-band/multi-modal telephony issues, has given versatile wireless device designers a keen interest in using MEMS to solve the serious problems they face.

RF-MEMS devices come in many shapes and forms. Some are like miniature versions of cantilever bridges, while others may mimic trampolines (Figure 4). Their geometric dimensions are typically tens to hundreds of microns. Such dimensions make these devices very attractive because they make it possible to implement complex circuit solutions in a space of 1 mm2 or less. In addition, MEMS can be changed to implement a variety of different tunable micro-applications, such as filters and amplifiers.

Discrete fixed antennas are typically tuned for use in mobile terminals, making a given antenna suitable for each mobile phone platform, but often it is not possible to optimally match loads in different frequency bands and operating environments. This causes the maximum efficiency of the antenna to decrease, the transmission power consumption to rise, and the reception sensitivity to decrease. Variable RF components such as GaAs varactors have long performed well in certain applications.

However, because of the unacceptable insertion loss and the resulting linearity of the circuit, they cannot be applied to the tunable front end and the antenna. So for all components directly coupled to the antenna, linearity is critical because there is no filter to clean the transmitted signal or prevent the received signal from being modulated by the transmitted signal.

By using digitally selected MEMS capacitive devices, MEMS digitally tunable ICs offer a high performance alternative to varactors. The construction of these adjustable capacitors begins with two metal plates, one of which is on the surface of the silicon chip, and the other is suspended a few microns above it. By changing the distance between the two metal plates, the capacitance between them can be adjusted, and by applying the attractive force of the electrostatic field to move the suspended metal on and off, the capacitance value can be changed very accurately. The array of capacitive elements forms a tuning matrix that can be used to control the capacitance system very accurately. In fact, accurate digital approximation of varactors has become possible, with near-perfect linearity and a wider tuning range than traditional analog solutions.

One application of antenna tuning devices is to use a resonant or impedance-tunable (RLC) circuit to allow the variable capacitor to adjust the complex impedance between the power amplifier and the antenna. The basic method is shown in Figure 5, where the tunable capacitor and the precision inductor are integrated together by RF-MEMS technology.

RF-MEMS developers are working hard to prove that RF-MEMS-based tunable ICs will become mobile phones

The main components. The tunable antenna device is one of its key applications because of the advantages of RF-MEMS performance and ease of integration. This solution helps to reduce the effects of interference while allowing a smaller multi-band antenna to work well in most cases.

RF-MEMS products are expected to make a huge difference in the way mobile phones are designed. RF-MEMS technology enables the production of small, low-cost, high-performance tunable RF ICs that perform a variety of functions, including high-efficiency tunable amplifiers, tunable filters, and smart antennas.