"With the rapid development of wireless communication with higher frequency and wider bandwidth, combined with the integration of multiple radio frequency (RF) interfaces and antennas, the traditional RF switching method has reached the limit. The RF switch based on MEMS technology has developed into a feasible and easy-to-use solution, which can help designers solve the problems of space, switching speed, front-end filtering and flexibility in advanced wireless systems.
This paper first introduces the traditional RF switching methods such as traditional electromechanical switches, various solid-state analog switches and PIN diodes, and then takes the product of analog devices as an example to discuss the key attributes of MEMS based RF switches. In addition, the performance characteristics and available development support will be discussed to help designers understand how to use MEMS RF switches to ensure long-life and reliability of operation.
RF switch applications and options
In addition to using a single antenna to support multiple radio integration, the RF switch also needs to support multiple antennas in multiple input multiple output (MIMO) configuration, guide the signal to the required internal path, or manage the switch matrix related to automatic test equipment (ATE). The RF switch action may include selecting one of a plurality of input signals to guide it to a single output path; Or, on the contrary, direct a single input signal to one of the specified multiple output paths.
Until recently, RF switches have been implemented in the following main ways:
• traditional electromechanical RF switches: these switches are controlled manually or by motor; Remote operation is supported via a simple 12 / 24 V line or USB port. This kind of switch is easy to use (including coaxial connector), with switching speed of tens of gigahertz and excellent performance, but it is obviously not suitable for applications requiring small size, light weight or fast switching speed. Despite the old design, such switches are still widely used and often the only solution in many cases.
• PIN diode based switches: these switches have good RF performance and fast switching speed. However, relevant expertise is needed to realize its potential. As a dual terminal device without independent on / off control circuit, the related circuit of this kind of switch is relatively complex. The input needs to combine the DC control and RF path, while the output needs to separate it. Therefore, pin based RF switches are mostly provided in the form of complete modules including support circuits.
• field effect transistor (FET) and hybrid solid-state switch: This solid-state switch uses advanced semiconductor materials and processes to provide the RF equivalent circuit of basic low-frequency transistor switch. As electronic switches, these devices can realize fast on / off conversion (in a few microseconds) and are easy to design and import, but they are limited in isolation and other performance properties.
Recently, MEMS based RF switches have become a viable option, and standard products have been introduced. The switching mechanism of these devices is based on cantilever MEMS components. Although it is similar to some MEMS accelerometers, it adds the functions and characteristics required for electronic control switches to provide metal to metal contacts for RF signal paths.
For example, let's take a look at adgm1004 of analog devices, a single pole four throw (SP4T) switch from 0 Hz (DC) to 13 GHz, and adgm1304, a DC to 14 GHz SP4T switch of similar products (Figure 1).
Adgm1004 and adgm1304 can realize typical mechanical on / off and contact closing functions. They are in a small RF compatible 24 pin leadframe chip level package (lfcsp) with a size of 5 × four × 1.45 mm。 These two devices can realize fast switching within 30 µ s, and the bandwidth ranges from DC to 13 or 14 GHz respectively. Although the specifications are generally similar, there are slight but significant differences in on resistance (RON), third-order intermodulation intercept point (IIP3) and RF power (maximum) (Table 1).
The two mechanical switching devices have metal to metal contacts, allowing signal energy to flow in either direction, that is, the signal of any one of the four poles can be sent to the common end, and the signal of the common end can also be sent to any one of the four switching poles.
Principle and implementation of MEMS RF switch
With the continuous progress of various technologies, although the concept is simple and clear, the practical application is not always true, just like MEMS RF switch. MEMS RF switches use micromachined cantilever beams and metallized tips as switching elements. The design problem is how to "activate" the cantilever to move and contact the corresponding metal surface when it is turned on and disconnect when it is turned off. MEMS RF switch drives the cantilever to move through electrostatic drive (Fig. 2). Conventionally, although each switch terminal is called "source", "gate" and "drain", the device is still a mechanical contact switch rather than a FET switching device.
In many ways, MEMS RF switches are very similar to mechanical relays, but their armature with contacts is based on the micron scale. The cantilever is actuated by electrostatic force rather than magnetic force. The whole switch is manufactured by MEMS special silicon IC process, so as to make full use of the rich design and manufacturing expertise related to the process, improve the output and reduce the cost (Fig. 3).
In order to improve the performance and reduce the DC contact resistance and RF impedance, each contact is actually composed of a group of parallel contacts; This approach is quite practical due to the use of MEMS Technology (Figure 4).
Each electronic component has one or more quality factors (FOM) to describe its performance characteristics. One of the most important fom values for switching devices is Ron Multiply by the off capacitance (coff), often recorded as roncoff Product in femtosecond (FS). RonCoff Smaller means smaller on insertion loss and higher off isolation. These two properties are ideal. Of course, in DC and AC power lines and low-frequency switching applications, Ron Is the main factor, and coff To a large extent, it has little impact. Roncoff of analog devices MEMS switch The product less than 8 FS indicates that the RF performance of the switch is excellent in both on and off modes.
Drive and ESD will complicate the design, but will not affect the actual use
For some device categories, a major concern for designers is the driving and control of the device, as well as the many difficulties faced in its implementation. Ideally, only standard logic level signals need to be used for control（ Recall that the difficulty of connecting and driving a pin diode RF switch is one of its drawbacks.)
In view of the electrostatic effect of MEMS RF switch of analog devices, the electric field needs about 89 V DC voltage to move the switch cantilever. This may be the initial design challenge faced by the control driver and interface. In fact, this is not a problem: these 3.1 to 3.3 V MEMS switches contain independent chips with DC / DC boost circuits, so there is no need to use external high-voltage drivers or power supplies (Figure 5).
ESD sensitivity is a common problem faced by almost all solid-state devices. However, traditional mechanical RF switches do not need to worry about this problem, because such devices essentially have high ESD immunity. In order to solve the problem of ESD sensitivity, analog devices provides ESD protection elements. The third independent element in the adgm1004 package is installed on the MEMS chip and is transparent to the user. The ESD manikin (HBM) voltage rating of pole pins (RF1 to rf4) and common pins (RFC) is 5 kV, and all other pins are 2.5 kV. For applications that do not require ESD protection (indeed some applications), adgm1304 can remove this protection functional element, so the package is thinner and the bandwidth is wider.
As mentioned above, although these two switches contain two active chips, the package is still very small, which is always an advantage for GHz RF. In addition, the control signal is compatible with CMOS / LVTTL, so it is easy to use.
Operation, performance and reliability
Solid state RF switches using analog switch or pin diode technology can process signals as low as 10 MHz at most, while electromechanical switches and MEMS switches can process signals as low as DC. Since the correlation signal ranges from hundreds of MHz to thousands of MHz, this performance expansion seems unnecessary.
However, many RF applications require not only high-frequency performance, but also processing close to DC or even real DC signals. These include systems using low intermediate frequency (if) such as 455 kHz and software radio (SDR) with a wide range of RF frequency bands. In addition, in some designs of very small aperture terminal (VSAT) antenna and satellite TV / Internet access, the RF path also needs to provide DC power path for the antenna front-end preamplifier of low noise block (LNB). In these applications, switching and guiding DC power supply and RF signal through a single small component is a major design advantage.
Like all mechanical and electromechanical devices, the service life of the core mechanism is limited. For metal electromechanical RF switches, the rated service life is usually between 5 and 10 million times. Given its switching time of about tens of milliseconds, this rating is acceptable. However, the on / off conversion time of MEMS based RF switches is quite short (30 µ s for adgm1004 and adgm1304). For many target applications such as dynamic MIMO system configuration, the working life of 10 million times is too limited. However, as long as it is used within the specified signal level and power range, the rated service life of MEMS switch is 1 billion times. Compared with traditional mechanical and electromechanical switches, this service life level has been improved by two orders of magnitude.
In addition to the temperature cyclic stress related to electronic and electromechanical components, there are some factors that will affect the working life of MEMS and traditional electromechanical RF switches. One of them is the "hot" switch and the "cold" switch.
In the thermal switching mode, there is a voltage difference between the signal source and drain when the switch is closed, and there is a current between the two poles after the switch is disconnected. In contrast, there is no signal power in the cold switching mode. Thermal switching will shorten the switching life of the contact surface, which depends on the open circuit voltage between the source and drain. The chart in MEMS switch specification shows the influence of thermal switching on working life and switching times.
Another important parameter of the on / off cycle spectrum is the continuous on life (Col), that is, the switch is set to the continuous on state for a long time, which often occurs in the instrument system, but it will also shorten the working life of the switch contact. Through design and accelerated life test, the rated col mean time between failures (MTBF) of MEMS switches of analog devices is 7 years at 50 ℃ and 10 years at 85 ℃.
As a relatively new technology, potential users may be cautious about these MEMS based RF switches and worry about various short-term and long-term reliability problems due to electrical and mechanical stress, temperature and shock / vibration. This is especially true for MEMS RF switch applications in mission critical military / aerospace and automotive systems. To address these concerns, analog devices has completed many industrial tests and mil defined tests (Table 2).
MEMS switches are designed into circuits
Although MEMS based RF switches are easy to apply, they are slightly more complex than standard electromechanical switches. Several design considerations are given in the device specification, including that all switch terminals must be connected to DC voltage reference. The reference can be another active device with internal voltage reference or grounding impedance (similar to CMOS gate input or output can not be "suspended"). Otherwise, the terminal will store charge, and the accumulated voltage may rise to an unknown level, resulting in unreliable actuation behavior and damage to the switch.
The specification illustrates several ways of node suspension caused by accidents, and shows the solutions. For example, in a typical cascaded use case of two adgm1304 devices, potential problems can be minimized with the help of shunt resistors (Fig. 6).
There are many application opportunities for MEMS RF switches, some of which are obvious and important. The development trend of wireless communication such as mobile radio and smart phone requires increasing the number of frequency bands and modes supported by a single path; 5g standard further promotes this trend. The dynamically reconfigurable RF filter allows coverage of more frequency bands / modes, small size and high speed, so this problem can be easily solved.
The reconfigurable bandpass filter can be realized by using a pair of adgm1304 devices. The inductive coupling single ended topology composed of two parts is adopted, and the nominal center frequency is 400 MHz (ultra-high frequency (UHF) band), as shown in the figure below (Fig. 7). MEMS switches are connected in series with shunt inductors, which meet the application requirements in terms of low and flat insertion loss, wide RF bandwidth, low parasitic effect, low capacitance and high linearity.
Shunt inductors with a total inductance value of 15 NH to 30 NH are used to set the filter frequency, MEMS switches are used to turn on / off these shunt inductors, and the low Ron value of the switch can reduce the adverse effect of series resistance on the quality factor (q) of shunt inductors. In addition, according to the switch setting requirements, the design retains the key 50 Ω load at the input and output ports.
When designing RF in the frequency band above GHz and establishing models and S parameters for simulation, an appropriate evaluation board is a necessary design tool, because there is no perfect model and it is impossible to grasp all the trivial details in the actual design. Eval-adgm1304 launched by analog devices can shorten the time required for product launch, minimize the troubles of users, and provide comprehensive and fair design evaluation (Fig. 8).
The evaluation board includes SMA connector for RF signal, SMB connector for switch control signal, on-board "calibration on" transmission line for analyzer calibration, and detailed user guide (ug-644).
summary
With the rapid development of wireless applications, the requirements for size, cost and performance are becoming more and more stringent. With the advantages of fast switching speed, small size, long-term reliability and other advantages, MEMS based RF switch adds a practical tool to the designer's toolkit.
MEMS RF switches such as adgm004 and adgm1304 of analog devices can simplify the old design, enable designers to meet the new design requirements of high-frequency products and improve the circuit density. In order to help designers make full use of the functions of these devices, the company also launched evaluation boards, models and documentation to provide a wide range of applications
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