Radio-frequency microelectromechanical system

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Fig. 1: (a) A capacitive fixed-fixed beam RF MEMS switch, connected in shunt to a CPW line. (b) An ohmic cantilever RF MEMS switch, connected in series to a microstrip line. RF MEMS.png
Fig. 1: (a) A capacitive fixed-fixed beam RF MEMS switch, connected in shunt to a CPW line. (b) An ohmic cantilever RF MEMS switch, connected in series to a microstrip line.

A radio-frequency microelectromechanical system (RF MEMS) is a microelectromechanical system with electronic components comprising moving sub-millimeter-sized parts that provide radio-frequency (RF) functionality. [1] RF functionality can be implemented using a variety of RF technologies. Besides RF MEMS technology, III-V compound semiconductor (GaAs, GaN, InP, InSb), ferrite, ferroelectric, silicon-based semiconductor (RF CMOS, SiC and SiGe), and vacuum tube technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency, gain, large-scale integration, lifetime, linearity, noise figure, packaging, power handling, power consumption, reliability, ruggedness, size, supply voltage, switching time and weight.

Contents

Components

There are various types of RF MEMS components, such as CMOS integrable RF MEMS resonators and self-sustained oscillators with small form factor and low phase noise, RF MEMS tunable inductors, and RF MEMS switches, switched capacitors and varactors.

Switches, switched capacitors and varactors

The components discussed in this article are based on RF MEMS switches, switched capacitors and varactors. These components can be used instead of FET and HEMT switches (FET and HEMT transistors in common gate configuration), and PIN diodes. RF MEMS switches, switched capacitors and varactors are classified by actuation method (electrostatic, electrothermal, magnetostatic, piezoelectric), by axis of deflection (lateral, vertical), by circuit configuration (series, shunt), by clamp configuration (cantilever, fixed-fixed beam), or by contact interface (capacitive, ohmic). Electrostatically actuated RF MEMS components offer low insertion loss and high isolation, linearity, power handling and Q factor, do not consume power, but require a high control voltage and hermetic single-chip packaging (thin film capping, LCP or LTCC packaging) or wafer-level packaging (anodic or glass frit wafer bonding).

RF MEMS switches were pioneered by IBM Research Laboratory, San Jose, CA, [2] [3] Hughes Research Laboratories, Malibu, CA, [4] Northeastern University in cooperation with Analog Devices, Boston, MA, [5] Raytheon, Dallas, TX, [6] [7] and Rockwell Science, Thousand Oaks, CA. [8] A capacitive fixed-fixed beam RF MEMS switch, as shown in Fig. 1(a), is in essence a micro-machined capacitor with a moving top electrode, which is the beam. It is generally connected in shunt with the transmission line and used in X- to W-band (77 GHz and 94 GHz) RF MEMS components. An ohmic cantilever RF MEMS switch, as shown in Fig. 1(b), is capacitive in the up-state, but makes an ohmic contact in the down-state. It is generally connected in series with the transmission line and is used in DC to the Ka-band components.

From an electromechanical perspective, the components behave like a damped mass-spring system, actuated by an electrostatic force. The spring constant is a function of the dimensions of the beam, as well as the Young's modulus, the residual stress and the Poisson ratio of the beam material. The electrostatic force is a function of the capacitance and the bias voltage. Knowledge of the spring constant allows for hand calculation of the pull-in voltage, which is the bias voltage necessary to pull-in the beam, whereas knowledge of the spring constant and the mass allows for hand calculation of the switching time.

From an RF perspective, the components behave like a series RLC circuit with negligible resistance and inductance. The up- and down-state capacitance are in the order of 50 fF and 1.2 pF, which are functional values for millimeter-wave circuit design. Switches typically have a capacitance ratio of 30 or higher, while switched capacitors and varactors have a capacitance ratio of about 1.2 to 10. The loaded Q factor is between 20 and 50 in the X-, Ku- and Ka-band. [9]

RF MEMS switched capacitors are capacitive fixed-fixed beam switches with a low capacitance ratio. RF MEMS varactors are capacitive fixed-fixed beam switches which are biased below pull-in voltage. Other examples of RF MEMS switches are ohmic cantilever switches, and capacitive single pole N throw (SPNT) switches based on the axial gap wobble motor. [10]

Biasing

RF MEMS components are biased electrostatically using a bipolar NRZ drive voltage, as shown in Fig. 2, in order to avoid dielectric charging [11] and to increase the lifetime of the device. Dielectric charges exert a permanent electrostatic force on the beam. The use of a bipolar NRZ drive voltage instead of a DC drive voltage avoids dielectric charging whereas the electrostatic force exerted on the beam is maintained, because the electrostatic force varies quadratically with the DC drive voltage. Electrostatic biasing implies no current flow, allowing high-resistivity bias lines to be used instead of RF chokes.

Fig. 2: Electrostatic biasing of a capacitive fixed-fixed beam RF MEMS switch, switched capacitor or varactor. RF MEMS BIASING.png
Fig. 2: Electrostatic biasing of a capacitive fixed-fixed beam RF MEMS switch, switched capacitor or varactor.

Packaging

RF MEMS components are fragile and require wafer level packaging or single chip packaging which allow for hermetic cavity sealing. A cavity is required to allow movement, whereas hermeticity is required to prevent cancellation of the spring force by the Van der Waals force exerted by water droplets and other contaminants on the beam. RF MEMS switches, switched capacitors and varactors can be packaged using wafer level packaging. Large monolithic RF MEMS filters, phase shifters, and tunable matching networks require single chip packaging.

Wafer-level packaging is implemented before wafer dicing, as shown in Fig. 3(a), and is based on anodic, metal diffusion, metal eutectic, glass frit, polymer adhesive, and silicon fusion wafer bonding. The selection of a wafer-level packaging technique is based on balancing the thermal expansion coefficients of the material layers of the RF MEMS component and those of the substrates to minimize the wafer bow and the residual stress, as well as on alignment and hermeticity requirements. Figures of merit for wafer-level packaging techniques are chip size, hermeticity, processing temperature, (in)tolerance to alignment errors and surface roughness. Anodic and silicon fusion bonding do not require an intermediate layer, but do not tolerate surface roughness. Wafer-level packaging techniques based on a bonding technique with a conductive intermediate layer (conductive split ring) restrict the bandwidth and isolation of the RF MEMS component. The most common wafer-level packaging techniques are based on anodic and glass frit wafer bonding. Wafer-level packaging techniques, enhanced with vertical interconnects, offer the opportunity of three-dimensional integration.

Single-chip packaging, as shown in Fig. 3(b), is implemented after wafer dicing, using pre-fabricated ceramic or organic packages, such as LCP injection molded packages or LTCC packages. Pre-fabricated packages require hermetic cavity sealing through clogging, shedding, soldering or welding. Figures of merit for single-chip packaging techniques are chip size, hermeticity, and processing temperature.

Fig. 3: (a) Wafer-level packaging. (b) Single chip packaging of an ohmic cantilever RF MEMS switch. RF MEMS PACKAGING.png
Fig. 3: (a) Wafer-level packaging. (b) Single chip packaging of an ohmic cantilever RF MEMS switch.

Microfabrication

An RF MEMS fabrication process is based on surface micromachining techniques, and allows for integration of SiCr or TaN thin film resistors (TFR), metal-air-metal (MAM) capacitors, metal-insulator-metal (MIM) capacitors, and RF MEMS components. An RF MEMS fabrication process can be realized on a variety of wafers: III-V compound semi-insulating, borosilicate glass, fused silica (quartz), LCP, sapphire, and passivated silicon wafers. As shown in Fig. 4, RF MEMS components can be fabricated in class 100 clean rooms using 6 to 8 optical lithography steps with a 5 μm contact alignment error, whereas state-of-the-art MMIC and RFIC fabrication processes require 13 to 25 lithography steps.

Fig. 4: RF MEMS switch, switched capacitor, or varactor fabrication process RF MEMS FABRICATION PROCESS.png
Fig. 4: RF MEMS switch, switched capacitor, or varactor fabrication process

As outlined in Fig. 4, the essential microfabrication steps are:

With the exception of the removal of the sacrificial spacer, which requires critical point drying, the fabrication steps are similar to CMOS fabrication process steps. RF MEMS fabrication processes, unlike BST or PZT ferroelectric and MMIC fabrication processes, do not require electron beam lithography, MBE, or MOCVD.

Reliability

Contact interface degradation poses a reliability issue for ohmic cantilever RF MEMS switches, whereas dielectric charging beam stiction, [12] as shown in Fig. 5(a), and humidity induced beam stiction, as shown in Fig. 5(b), pose a reliability issue for capacitive fixed-fixed beam RF MEMS switches. Stiction is the inability of the beam to release after removal of the drive voltage. A high contact pressure assures a low-ohmic contact or alleviates dielectric charging induced beam stiction. Commercially available ohmic cantilever RF MEMS switches and capacitive fixed-fixed beam RF MEMS switches have demonstrated lifetimes in excess of 100 billion cycles at 100 mW of RF input power. [13] [14] Reliability issues pertaining to high-power operation are discussed in the limiter section.

Fig. 5: (a) [Bottom] Dielectric charging induced beam stiction. (b) [Top] Humidity induced beam stiction. RF MEMS RELIABILITY.png
Fig. 5: (a) [Bottom] Dielectric charging induced beam stiction. (b) [Top] Humidity induced beam stiction.

Applications

RF MEMS resonators are applied in filters and reference oscillators. [15] RF MEMS switches, switched capacitors and varactors are applied in electronically scanned (sub)arrays (phase shifters) and software-defined radios (reconfigurable antennas, tunable band-pass filters). [16]

Antennas

Polarization and radiation pattern reconfigurability, and frequency tunability, are usually achieved by incorporation of III-V semiconductor components, such as SPST switches or varactor diodes. However, these components can be readily replaced by RF MEMS switches and varactors in order to take advantage of the low insertion loss and high Q factor offered by RF MEMS technology. In addition, RF MEMS components can be integrated monolithically on low-loss dielectric substrates, [17] such as borosilicate glass, fused silica or LCP, whereas III-V compound semi-insulating and passivated silicon substrates are generally lossier and have a higher dielectric constant. A low loss tangent and low dielectric constant are of importance for the efficiency and the bandwidth of the antenna.

The prior art includes an RF MEMS frequency tunable fractal antenna for the 0.1–6 GHz frequency range, [18] and the actual integration of RF MEMS switches on a self-similar Sierpinski gasket antenna to increase its number of resonant frequencies, extending its range to 8 GHz, 14 GHz and 25 GHz, [19] [20] an RF MEMS radiation pattern reconfigurable spiral antenna for 6 and 10 GHz, [21] an RF MEMS radiation pattern reconfigurable spiral antenna for the 6–7 GHz frequency band based on packaged Radant MEMS SPST-RMSW100 switches, [22] an RF MEMS multiband Sierpinski fractal antenna, again with integrated RF MEMS switches, functioning at different bands from 2.4 to 18 GHz, [23] and a 2-bit Ka-band RF MEMS frequency tunable slot antenna. [24]

The Samsung Omnia W was the first smart phone to include a RF MEMS antenna. [25]

Filters

RF bandpass filters can be used to increase out-of-band rejection, in case the antenna fails to provide sufficient selectivity. Out-of-band rejection eases the dynamic range requirement on the LNA and the mixer in the light of interference. Off-chip RF bandpass filters based on lumped bulk acoustic wave (BAW), ceramic, SAW, quartz crystal, and FBAR resonators have superseded distributed RF bandpass filters based on transmission line resonators, printed on substrates with low loss tangent, or based on waveguide cavities.

Tunable RF bandpass filters offer a significant size reduction over switched RF bandpass filter banks. They can be implemented using III-V semiconducting varactors, BST or PZT ferroelectric and RF MEMS resonators and switches, switched capacitors and varactors, and YIG ferrites. RF MEMS resonators offer the potential of on-chip integration of high-Q resonators and low-loss bandpass filters. The Q factor of RF MEMS resonators is in the order of 100–1000. [15] RF MEMS switch, switched capacitor and varactor technology, offers the tunable filter designer a compelling trade-off between insertion loss, linearity, power consumption, power handling, size, and switching time. [26]

Phase shifters

Fig. 6: EIRP x Gr/T RF MEMS EIRP TIMES GT VERSUS N 1.png
Fig. 6: EIRP x Gr/T
Fig. 7: EIRP versus number of antenna elements in a passive subarray. RF MEMS EIRP VERSUS N.png
Fig. 7: EIRP versus number of antenna elements in a passive subarray.

Passive subarrays based on RF MEMS phase shifters may be used to lower the amount of T/R modules in an active electronically scanned array. The statement is illustrated with examples in Fig. 6: assume a one-by-eight passive subarray is used for transmit as well as receive, with following characteristics: f = 38 GHz, Gr = Gt = 10 dBi, BW = 2 GHz, Pt = 4 W. The low loss (6.75 ps/dB) and good power handling (500 mW) of the RF MEMS phase shifters allow an EIRP of 40 W and a Gr/T of 0.036 1/K. EIRP, also referred to as the power-aperture product, is the product of the transmit gain, Gt, and the transmit power, Pt. Gr/T is the quotient of the receive gain and the antenna noise temperature. A high EIRP and Gr/T are a prerequisite for long-range detection. The EIRP and Gr/T are a function of the number of antenna elements per subarray and of the maximum scanning angle. The number of antenna elements per subarray should be chosen in order to optimize the EIRP or the EIRP x Gr/T product, as shown in Fig. 7 and Fig. 8. The radar range equation can be used to calculate the maximum range for which targets can be detected with 10 dB of SNR at the input of the receiver.

in which kB is the Boltzmann constant, λ is the free-space wavelength, and σ is the RCS of the target. Range values are tabulated in Table 1 for following targets: a sphere with a radius, a, of 10 cm (σ = π a2), a dihedral corner reflector with facet size, a, of 10 cm (σ = 12 a4/λ2), the rear of a car (σ = 20 m2) and for a non-evasive fighter jet (σ = 400 m2).

Table 1: Maximum Detectable Range
(SNR = 10 dB)
RCS (m2)Range (m)
Sphere0.031410
Rear of Car2051
Dihedral Corner Reflector60.967
Fighter Jet400107
Fig. 8: EIRP x Gr/T versus number of antenna elements in a passive subarray. RF MEMS EIRP TIMES GT VERSUS N 2.png
Fig. 8: EIRP x Gr/T versus number of antenna elements in a passive subarray.

RF MEMS phase shifters enable wide-angle passive electronically scanned arrays, such as lens arrays, reflect arrays, subarrays and switched beamforming networks, with high EIRP and high Gr/T. The prior art in passive electronically scanned arrays, includes an X-band continuous transverse stub (CTS) array fed by a line source synthesized by sixteen 5-bit reflect-type RF MEMS phase shifters based on ohmic cantilever RF MEMS switches, [27] [28] an X-band 2-D lens array consisting of parallel-plate waveguides and featuring 25,000 ohmic cantilever RF MEMS switches, [29] and a W-band switched beamforming network based on an RF MEMS SP4T switch and a Rotman lens focal plane scanner. [30]

The usage of true-time-delay TTD phase shifters instead of RF MEMS phase shifters allows UWB radar waveforms with associated high range resolution, and avoids beam squinting or frequency scanning. TTD phase shifters are designed using the switched-line principle [8] [31] [32] or the distributed loaded-line principle. [33] [34] [35] [36] [37] [38] Switched-line TTD phase shifters outperform distributed loaded-line TTD phase shifters in terms of time delay per decibel NF, especially at frequencies up to X-band, but are inherently digital and require low-loss and high-isolation SPNT switches. Distributed loaded-line TTD phase shifters, however, can be realized analogously or digitally, and in smaller form factors, which is important at the subarray level. Analog phase shifters are biased through a single bias line, whereas multibit digital phase shifters require a parallel bus along with complex routing schemes at the subarray level.

Related Research Articles

<span class="mw-page-title-main">MEMS</span> Very small devices that incorporate moving components

MEMS is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size, and MEMS devices generally range in size from 20 micrometres to a millimetre, although components arranged in arrays can be more than 1000 mm2. They usually consist of a central unit that processes data and several components that interact with the surroundings.

<span class="mw-page-title-main">Microwave</span> Electromagnetic radiation with wavelengths from 1 m to 1 mm

Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ranges as microwaves; the above broad definition includes UHF, SHF and EHF bands. A more common definition in radio-frequency engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

<span class="mw-page-title-main">Phased array</span> Array of antennas creating a steerable beam

In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around 20 kHz to around 300 GHz. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies. These are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves, so they are used in radio technology, among other uses. Different sources specify different upper and lower bounds for the frequency range.

In electrical circuits, reactance is the opposition presented to alternating current by inductance and capacitance. Along with resistance, it is one of two elements of impedance; however, while both elements involve transfer of electrical energy, no dissipation of electrical energy as heat occurs in reactance; instead, the reactance stores energy until a quarter-cycle later when the energy is returned to the circuit. Greater reactance gives smaller current for the same applied voltage.

<span class="mw-page-title-main">Wireless power transfer</span> Transmission of electrical energy without wires as a physical link

Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, a transmitter device, driven by electric power from a power source, generates a time-varying electromagnetic field, which transmits power across space to a receiver device, which extracts power from the field and supplies it to an electrical load. The technology of wireless power transmission can eliminate the use of the wires and batteries, thus increasing the mobility, convenience, and safety of an electronic device for all users. Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.

<span class="mw-page-title-main">Electronic component</span> Discrete device in an electronic system

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements.

<span class="mw-page-title-main">Variable capacitor</span> Capacitor whose capacitance can be changed

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio, or as a variable reactance, e.g. for impedance matching in antenna tuners.

A bias tee is a three-port network used for setting the DC bias point of some electronic components without disturbing other components. The bias tee is a diplexer. The low-frequency port is used to set the bias; the high-frequency port passes the radio-frequency signals but blocks the biasing levels; the combined port connects to the device, which sees both the bias and RF. It is called a tee because the 3 ports are often arranged in the shape of a T.

Radar engineering details are technical details pertaining to the components of a radar and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This includes field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.

RF microwave CAE CAD is computer-aided design (CAD) using computer technology to aid in the design, modeling, and simulation of an RF or microwave product. It is a visual and symbol-based method of communication whose conventions are particular to RF/microwave engineering.

<span class="mw-page-title-main">Applications of capacitors</span> Uses of capacitors in daily life

Capacitors have many uses in electronic and electrical systems. They are so ubiquitous that it is rare that an electrical product does not include at least one for some purpose. Capacitors allow only AC signals to pass when they are charged blocking DC signals. The main components of filters are capacitors. Capacitors have the ability to connect one circuit segment to another. Capacitors are used by Dynamic Random Access Memory (DRAM) devices to represent binary information as bits.

<span class="mw-page-title-main">Metamaterial antenna</span>

Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often microscopic, structures to produce unusual physical properties. Antenna designs incorporating metamaterials can step-up the antenna's radiated power.

<span class="mw-page-title-main">Tunable metamaterial</span>

A tunable metamaterial is a metamaterial with a variable response to an incident electromagnetic wave. This includes remotely controlling how an incident electromagnetic wave interacts with a metamaterial. This translates into the capability to determine whether the EM wave is transmitted, reflected, or absorbed. In general, the lattice structure of the tunable metamaterial is adjustable in real time, making it possible to reconfigure a metamaterial device during operation. It encompasses developments beyond the bandwidth limitations in left-handed materials by constructing various types of metamaterials. The ongoing research in this domain includes electromagnetic materials that are very meta which mean good and has a band gap metamaterials (EBG), also known as photonic band gap (PBG), and negative refractive index material (NIM).

Microelectromechanical system oscillators are devices that generate highly stable reference frequencies to measure time. The core technologies used in MEMS oscillators have been in development since the mid-1960s, but have only been sufficiently advanced for commercial applications since 2006. MEMS oscillators incorporate MEMS resonators, which are microelectromechanical structures that define stable frequencies. MEMS clock generators are MEMS timing devices with multiple outputs for systems that need more than a single reference frequency. MEMS oscillators are a valid alternative to older, more established quartz crystal oscillators, offering better resilience against vibration and mechanical shock, and reliability with respect to temperature variation.

<span class="mw-page-title-main">Reconfigurable antenna</span> Antenna capable of modifying its frequency and radiation properties dynamically

A reconfigurable antenna is an antenna capable of modifying its frequency and radiation properties dynamically, in a controlled and reversible manner. In order to provide a dynamic response, reconfigurable antennas integrate an inner mechanism that enable the intentional redistribution of the RF currents over the antenna surface and produce reversible modifications of its properties. Reconfigurable antennas differ from smart antennas because the reconfiguration mechanism lies inside the antenna, rather than in an external beamforming network. The reconfiguration capability of reconfigurable antennas is used to maximize the antenna performance in a changing scenario or to satisfy changing operating requirements.

<span class="mw-page-title-main">Frequency selective surface</span> Optical filter

A frequency-selective surface (FSS) is any thin, repetitive surface designed to reflect, transmit or absorb electromagnetic fields based on the frequency of the field. In this sense, an FSS is a type of optical filter or metal-mesh optical filters in which the filtering is accomplished by virtue of the regular, periodic pattern on the surface of the FSS. Though not explicitly mentioned in the name, FSS's also have properties which vary with incidence angle and polarization as well - these are unavoidable consequences of the way in which FSS's are constructed. Frequency-selective surfaces have been most commonly used in the radio frequency region of the electromagnetic spectrum and find use in applications as diverse as the aforementioned microwave oven, antenna radomes and modern metamaterials. Sometimes frequency selective surfaces are referred to simply as periodic surfaces and are a 2-dimensional analog of the new periodic volumes known as photonic crystals.

A piezoelectric microelectromechanical system (piezoMEMS) is a miniature or microscopic device that uses piezoelectricity to generate motion and carry out its tasks. It is a microelectromechanical system that takes advantage of an electrical potential that appears under mechanical stress. PiezoMEMS can be found in a variety of applications, such as switches, inkjet printer heads, sensors, micropumps, and energy harvesters.

<span class="mw-page-title-main">Transmitarray antenna</span>

A transmitarray antenna is a phase-shifting surface (PSS), a structure capable of focusing electromagnetic radiation from a source antenna to produce a high-gain beam. Transmitarrays consist of an array of unit cells placed above a source (feeding) antenna. Phase shifts are applied to the unit cells, between elements on the receive and transmit surfaces, to focus the incident wavefronts from the feeding antenna. These thin surfaces can be used instead of a dielectric lens. Unlike phased arrays, transmitarrays do not require a feed network, so losses can be greatly reduced. Similarly, they have an advantage over reflectarrays in that feed blockage is avoided.

<span class="mw-page-title-main">Reflectarray antenna</span> Beam focusing, typically horn-fed planar array of unit cells

A reflectarray antenna consists of an array of unit cells, illuminated by a feeding antenna. The feeding antenna is usually a horn. The unit cells are usually backed by a ground plane, and the incident wave reflects off them towards the direction of the beam. A phase distribution of concentric rings is applied to focus the wavefronts from the feeding antenna into a plane wave . A progressive phase shift can be applied to the unit cells to steer the beam direction. It is common to offset the feeding antenna to prevent blockage of the beam. In this case, the phase distribution on the reflectarray surface needs to be altered. A reflectarray focuses a beam in a similar way to a parabolic reflector (dish), but with a much thinner form factor.

References

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  22. ↑ G. H. Huff and J. T. Bernhard: “Integration of Packaged RF MEMS Switches with Radiation Pattern Reconfigurable Square Spiral Microstrip Antennas,” IEEE Trans. Antennas Propag., vol. 54, no. 2, pp. 464–469, Feb. 2006
  23. ↑ N. Kingsley, D. E. Anagnostou, M. Tentzeris, and J. Papapolymerou: “RF MEMS Sequentially Reconfigurable Sierpinski Antenna on a Flexible Organic Substrate with Novel DC-Biasing Technique,” IEEE/ASME J. Microelectromech. Syst., vol. 16, no. 5, pp. 1185–1192, Oct. 2007
  24. ↑ K. Van Caekenberghe and K. Sarabandi: "A 2-Bit Ka-Band RF MEMS Frequency Tunable Slot Antenna," IEEE Antennas and Wireless Propagation Letters, vol. 7, pp. 179-182, 2008
  25. ↑ "WTF is... RF-MEMS?"
  26. ↑ R. M. Young, J. D. Adam, C. R. Vale, T. T. Braggins, S. V. Krishnaswamy, C. E. Milton, D. W. Bever, L. G. Chorosinski, Li-Shu Chen, D. E. Crockett, C. B. Freidhoff, S. H. Talisa, E. Capelle, R. Tranchini, J. R. Fende, J. M. Lorthioir, A. R. Tories: “Low-Loss Bandpass RF Filter Using MEMS Capacitance Switches to Achieve a One-Octave Tuning Range and Independently Variable Bandwidth,” IEEE MTT-S International Microwave Symposium Digest, vol. 3, pp. 1781-1784, Jun. 2003
  27. ↑ J. J. Lee, C. Quan, and B. M. Pierce: “Low-Cost 2-D Electronically Scanned Array with Compact CTS Feed and MEMS Phase Shifters,” U.S. Patent 6 677 899, Jan. 13, 2004
  28. ↑ C. Quan, J. J. Lee, B. M. Pierce, and R. C. Allison: “Wideband 2-D Electronically Scanned Array with Compact CTS Feed and MEMS Phase Shifters,” U.S. Patent 6 822 615, Nov. 23, 2004
  29. ↑ J. J. Maciel, J. F. Slocum, J. K. Smith, and J. Turtle: “MEMS Electronically Steerable Antennas for Fire Control Radars,” IEEE Aerosp. Electron. Syst. Mag, pp. 17–20, Nov. 2007
  30. ↑ J. Schoebel, T. Buck, M. Reimann, M. Ulm, M. Schneider, A. Jourdain, G. J. Carchon, and H. A. C. Tilmans: "Design Considerations and Technology Assessment of Phased Array Antenna Systems with RF MEMS for Automotive Radar Applications," IEEE Trans. Microwave Theory Tech., vol. 53, no. 6, pp. 1968-1975, Jun. 2005
  31. ↑ G. L. Tan, R. E. Mihailovich, J. B. Hacker, J. F. DeNatale, and G. M. Rebeiz: “Low-loss 2- and 4-Bit TTD MEMS Phase Shifters Based on SP4T Switches,” IEEE Trans. Microw. Theory Tech., vol. 51, no. 1, pp. 297–304, Jan. 2003
  32. ↑ C. D. Nordquist, C. W. Dyck, G. M. Kraus, I. C. Reines, C. L. Goldsmith, W. D. Cowan, T. A. Plut, F. Austin, P. S. Finnegan, M. H. Ballance, and C. T. Sullivan: “A DC to 10 GHz 6-Bit RF MEMS Time Delay Circuit,” IEEE Microw. Wireless Compon. Lett., vol. 16, no. 5, pp. 305–307, May 2006
  33. ↑ N. S. Barker and G. M. Rebeiz, “Optimization of distributed MEMS phase shifters,” in IEEE MTT-S Int. Microw. Symp. Dig., pp. 299–302, 1999
  34. ↑ A. S. Nagra and R. A. York, “Distributed Analog Phase Shifters with Low Insertion Loss: ” IEEE Trans. Microw. Theory Tech., vol. 47, no. 9, pp. 1705–1711, Sep. 1999
  35. ↑ J. Perruisseau-Carrier, R. Fritschi, P. Crespo-Valero, and A. K. Skrivervik: “Modeling of Periodic Distributed MEMS Application to the Design of Variable True-Time-Delay Lines,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 1, pp. 383–392, Jan. 2006
  36. ↑ B. Lakshminarayanan and T. M. Weller: “Design and Modeling of 4-Bit Slow-Wave MEMS Phase Shifters,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 1, pp. 120–127, Jan. 2006
  37. ↑ B. Lakshminarayanan and T. M. Weller: “Optimization and Implementation of Impedance-Matched True-Time-Delay Phase Shifters on Quartz Substrate,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 2, pp. 335–342, Feb. 2007
  38. ↑ K. Van Caekenberghe and T. Vaha-Heikkila: "An Analog RF MEMS Slotline True-Time-Delay Phase Shifter," IEEE Trans. Microw. Theory Tech., vol. 56, no. 9, pp. 2151-2159, Sep. 2008

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