"The frequency of frequency and broader bandwidth is widely developed, and the integration of multiple radio frequency (RF) interfaces and antennas makes the traditional RF switch method to achieve the limit. The radio frequency switch based on micro-electromechanical system (MEMS) technology has developed Become a feasible and easeful solution that helps designers solve problems such as space, switching speed, front-end filtering and flexibility in advanced wireless systems.
This paper first introduces traditional radio frequency switching methods such as traditional electromechanical switches, various solid state analog switches and PIN diodes, and then discusses the key attributes based on MEMS RF switches as analog products in ANALOG DEVICES. In addition, this paper will discuss performance characteristics and available development support, helping designers understand how to use MEMS RF switch to ensure long life and reliability of operations.
RF switch application and option
In addition to supporting multiple radio integrations using single antennas, RF switches need to support multi-antenna configured for multi-input Multi-Output (MIMO), boot signals to the desired internal path, or manage automatic test equipment (ATE) switch matrices. The RF switch action can include one of the plurality of input signals and directs it to a single output path; or, in contrast, direct a single input signal to one of the specified plurality of output paths.
Until recently, the RF switch has always been implemented in the following main way:
• Traditional Mechanical RF Switch: This type of switch is manually controlled; remote operation is supported by simple 12/24 V line or USB port. Such switches are easy to use (including coaxial connector), and the switching speed reached a few Jigpeh, which is excellent in performance, but it is obviously not suitable for applications with small size, light weight or fast switching speed. Although designed, such switches are still widely used, and in many cases often a unique solution.
• Switch based on PIN Diodes: These switches have good radio frequency performance and faster switching speed. However, relevant expertise can play its potential. As a dual-terminal device without independently / off control line, the related circuits of such switches are more complex, and the input needs to be merged with the DC control and RF paths, while the output needs to be separated. Therefore, most of the radio frequency switch based on the PIN is provided in the form of a full module containing support circuits.
• Field Effect Transistor (FET) and a mixed solid state switch: This solid state switch adopts advanced semiconductor materials and processes to provide radio frequency equivalent circuits of basic low-frequency transistor switches. As an electronic switch, these devices can realize fast on / off (within a few microcodies), which is easy to design import, but is limited in terms of isolation and other performance properties.
Recently, MEMS-based RF switches have become a feasible option, and standard products are now launched. The switching mechanism of these devices is based on the cantilever MEMS component, although similar to some MEMS accelerometers, the function and characteristics needed to add electronic control switches, providing metal to metal contacts for radio frequency signal paths.
For example, let's take a look at the ADGM1004 of Analog Devices, a single-knife (SP4T) switch (SP4T) switch (SP4T) switch, as well as the similar product ADGM1304, a SP4T switch (Figure 1).
The ADGM1004 and ADGM 1304 can realize typical mechanical on / off, contact closing functions, small RF compatible 24-pin lead frame chip-level package (LFCSP), 5 × 4 × 1.45 mm. These two devices can achieve rapid switch in 30 μs, with bandwidth from DC to 13 or 14 GHz, respectively. Although the specifications are generally similar, there is a different difference in the onpic resistance (RON), third-order interception point (IIP3) and RF power (maximum), etc. (Table 1).
These two mechanical switching devices have metal contacts, allowing signal energy to flow in either direction, i.e., the signals of any of the four poles can be sent to the public, and the signal of the public can also be sent to four switch poles. Any one.
Principle and implementation of MEMS RF switch
With the continuous improvement of various technologies, the concept is simple, the actual application is not exhausted, the MEMS RF switch is the case. The MEMS RF switch uses a micromechanical cantilever beam and a metallized tip as a switching element. The design problem is how to "activate" the cantilever to move it in turn and contact the corresponding metal surface and disconnected at the time of shutdown. The MEMS RF switch drives the cantilever movement by an electrostatic drive (Fig. 2). It is accustomed to the "source", "gate" and "drain", but the device is still a mechanical contact switch, not FET switching device.
For many aspects, MEMS RF switches are extremely similar to mechanical relays, but their armature with contacts is built on a micron scale. The cantilever is actuated by electrostatic power rather than magnetic field strength. The entire switch adopts the MEMS special silicon IC process, which is made to make full use of rich design and manufacturing expertise related to the process, increase production, and reduce costs (Figure 3).
In order to improve performance and reduce DC contact resistance and radio frequency impedance, each contact is actually constructed of a set of parallel contacts; this method is quite practical due to MEMS technology (Figure 4).
Each electronic component has one or more quality factors (FOM) for describing its performance characteristics. For switching devices, one of the most important FOM values is RON multiplied by shutdown capacitance (COFF), often written as RonCoff product, and unit is femtosecond (FS). The smaller the RONCOFF, the smaller the conduction insertion, the higher the shutdown, and the two properties are ideal. Of course, in DC and AC power lines and low frequency switch applications, RON is the main factor, and COFF has a large extent. The RONCOFF product of the Analog Devices MEMS switch is less than 8 fs, indicating that the switch is quite good in turnover in turn-on and off mode.
Drivers and ESDs will make the design complex, but does not affect the actual use
For some devices categories, a major concern is that the drivers and control of the device, as well as many difficulties achieved. Ideally, just use standard logic level signals. (Recalling that the difficulty of connecting and driving the PIN diode RF switch is one of its defects.)
Given the MEMS RF switch of Analog Devices, the electric field requires approximately 89 V DC voltage to move the switch creating arm, the original design introduction challenge that controls the driver and the interface may be in this. In fact, this is not a problem: these MEMS switches of these 3.1 to 3.3 V include a separate chip with a DC / DC boost circuit, so there is no need to use an external high pressure drive or power supply (Figure 5).
ESD sensitivity is almost common in all solid-state devices. However, traditional mechanical RF switches do not need to worry about this problem, as such devices have high ESD immunity. To resolve ESD sensitivity, Analog Devices offers an ESD protection component. This third independent component in the ADGM1004 package is mounted on the MEMS chip and transparent to the user. The ESD Human Model (HBM) rated voltage of the extreme pin (RF1 to RF4) and common pin (RFC) is 5 kV, and all other pins are 2.5 kV. For applications that do not require ESD protection (in some applications), the ADGM 1304 removes this protection functional component, so the package is thinner and bandwidth.
As mentioned above, although these two switches contain two active chips, the package is still small, which is always a major advantage for Gigabo RF. In addition, the control signal is compatible with CMOS / LVTTL and is therefore easy to use.
Operation, performance and reliability
The signal frequency of the solid-state radio frequency switch in which an analog switch or PIN diode technology can only be processed can only be as low as 10 MHz, while the electromechanical switch and the MEMS switch can process the signals of the DC. This performance extension seems to be unnecessary due to the scope of the relevant signals from hundreds to thousands of gabt.
However, many RF applications not only require high frequency performance, but also process proximity DC or even real DC signals. This includes a system of 455 kHz and other low-frequency (IF), as well as software radio (SDR) with a wide range of radio frequency band range. In addition, in some of the aperture terminal (VSAT) antenna and satellite TV / Internet access, the RF path also provides a DC power path for the antenna front end amplifier for low noise block (LNB). In these applications, you can switch and direct the DC power supply and the radio frequency signal without a large design advantage through a single small component.
Like all mechanical and electromechanical devices, the core mechanism has limited service life. For metal electromechanical radio frequency switches, rated work life is typically between 500 and 10 million times. In view of its switching time of approximately tens of milliseconds, this rated value is still acceptable. However, the on / off switching time based on MEMS-based RF switch is relatively short (ADGM1004 and ADGM1304 are 30 μs). For many target applications such as dynamic MIMO system configuration, 10 million work life is too limited. However, as long as it is used within the specified signal level and power range, the rated workout of the MEMS switch is 1 billion. This work life level has increased two orders of magnitude compared to traditional machinery and electromechanical switches.
In addition to the temperature cycle stress associated with electronics and machinery components, there are still some factors that will affect MEMS and traditional mechanical radio frequency switches. One of them is "hot" switching and "cold" switching.
In the heat switched mode, there is a voltage difference between the closing timing signal source and the drain, and there is a current between the two poles after the switch is disconnected. In contrast, the cold switch mode does not exist signal power. The heat exchange will shorten the switch life of the contact surface, depending on the opening voltage between the source and the drain. The chart in the MEMS Switch Specification shows the impact of thermal switching on operating life and switching.
Another important parameter of the on / off period is a continuous conduction life (COL), that is, in a long time, set the switch to a continuous conduction state, which often occurs during the instrument system, but will shorten the switch Contact work life. Designed and accelerated life test, the ANALOG DEVICES MEMS switch rating COL The average non-fault time (MTBF) was 7 years at 50 ° C, 10 years at 85 ° C.
As a relatively new technique, potential users may carefully see these MEMS-based radio frequency switches, which are worried that various short-term and long-term reliability problems due to electrical and mechanical stress, temperature and impact / vibration. This is especially true for MEMS RF switch applications in mission-critical military / aerospace and automotive systems. In order to relieve these concerns, Analog Devices completed many industrial tests and MIL definitions (Table 2).
Design MEMS switch to the circuit
Although MEMS-based RF switches are easy to apply, it is slightly complicated compared to standard electromechanical switches. Several design considerations are given in the device specification, including all switch terminals must be connected to DC voltage reference. The baseline may be an active device with an internal voltage reference or a ground impedance (similar to the CMOS gate input or output cannot be "floating)). Otherwise, the terminal is stored, and the accumulated voltage may rise to an unknown level, resulting in unreliable actuation behavior and damage the switch.
The specification explains several ways to accidentally cause the node floating and demonstrate the solution. For example, in a typical cascading case of two ADGM1304 devices, it is only necessary to minimize potential problems with a shunt resistor (Fig. 6).
MEMS RF switch has not lack of application opportunities, some of which have been obvious and quite important. The development trend of wireless communications such as mobile radio and smartphones requires increased number of frequency bands and patterns supported by single paths; 5G standards have further promoted this trend. Dynamically reconfigurable RF filter allows you to override more bands / modes, and small size, fast, so you can easily solve this problem.
Using a pair of ADGM1304 devices, you can realize the reconfigurable bandpass filter, which uses a two-part inductive coupling single-ended topology, nominal center frequency of 400 MHz (ultra-high frequency (UHF) band), as shown below (Figure 7). The MEMS switch is in series with each shunt inductor, which meets the application requirements in low, flat insertion loss, width frequency bandwidth, low parasitic effect, low capacitance, and high linearity.
Substream inductance of 15 NH to 30 NH is used to set the filter frequency, and the MEMS switch is used to turn on / disconnect these shunt inductors, and the low RON value of the switch can reduce the series resistance to shunt inductor quality factor The adverse effects of (q). Further, in accordance with the switching setting requirements, the design retains a key 50 Ω load in the input and output ports.
The radio design of the Gigabole is used, and the model and S parameters are used for simulation. The appropriate evaluation board is the necessary design tool, because there is no perfect model, it is impossible to seize all trivial details in the actual design. The EVAL-ADGM1304 launched by Analog Devices can shorten the time required for product listing, minimizing user worries, and provide a comprehensive and fair design assessment (Figure 8).
The evaluation board includes an SMA connector for a radio frequency signal for a SMB connector for a switching control signal, an analyzer calibrated onboard "calibration" transmission line, and a detailed user guide (UG-644).
Summarize
With the rapid development of wireless applications, the requirements for size, cost, and performance are increasing, and the MEMS-based RF switch is fast, small size, long-term reliability and other advantages, to design personnel toolbags. .
The MEMS RF switch like Analog Devices and the ADGM1304 can make designers to meet the new design requirements of high-frequency products while simplifying old-fashioned design, and increase circuit density. To help designers make full use of these devices, the company has also launched an evaluation board, model, and documentation to provide extensive.support. "
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