Harmonic Distortion or Total Harmonic Distortion (THD) is the sum of all harmonics found in a system and translates to noise. Typically, it’s a component of resistance. Being so, as resistance increases so do the harmonics found in a high frequency switching device. Of particular interest is the third harmonic, which is noted to have the highest impact on high frequency (GHz) signals. RF-MEMS devices have an on-resistance in the low single ohm or less range, which result in third harmonics that are near zero.
Insertion Loss and Isolation Specifications -Undeterred by Tens of GigaHertz
Insertion loss, the power drop as a result of the inserted switch, is a key concern for wireless designers. Low insertion loss between the low-noise amplifier and the antenna most often results in higher receiver sensitivity and lower noise figures-mainly because the insertion loss lowers the amplitude of the signal.
Insertion loss specifications that one can expect for RF-MEMS switches are in the order of 0.3 dB over a very wide frequency range that can extend to 40 GHz. This compares to CMOS switches, where insertion losses are limited to around 0.5 dB, at best, and only around a center frequency of 1 GHz (where 0.1dB can be achieved with a well designed RF-MEMS switch). Another feature of RF-MEMS is the relative flatness of the insertion loss over the operating frequency. For example, one can expect a typical deviation in the order of just 0.15 dB over a frequency band of 9 GHz.
WiSpry points out that its RF-MEMS devices exhibit insertions loss in the tenths of a dB range over a very wide frequency range, up to as much as 30Ghz or higher. The company indicates that limitation tends to be the package itself rather than the MEMS device. More costly packages can even achieve lower insertion loss over wide frequency bandwidths.
Because of their higher and more stable off-resistance, RF-MEMS devices keep PIN and FET devices on the defensive in the Isolation Department. One can expect to see minimum isolation specs in the order of 20 to 30 dB for RF-MEMS switches that operate over a 6 GHz frequency range. Typical isolation specifications for an RF-MEMS switch run much higher, from the 50 to 70 dB range from DC to the maximum frequency. One can also expect the isolation to drop as the frequency approaches the high end frequency limits. However, over the majority of the band, the isolation for RF-MEMS is relatively flat. On the other hand, the flatness falls short for CMOS switches when compared to RF-MEMS. This is because a CMOS switch will quickly see a decrease in isolation at frequencies above 1 GHz, which makes them unsuitable for higher frequency RF applications.
CMOS (that is RF-CMOS) isolation specs are in general better than 50 dB at 1 GHz, but the insertion loss and bandwidth are still fairly poor when compared to RF-MEMS
A Few More Specifications to Consider
Off resistance for MEMS devices in general is greater than giga-ohms in RF-MEMS switches. This is because a RF-MEMS switch doesn’t use an actual physical connection between the two open contacts. Semiconductor devices, which are physically connected through a p-n junction in the off state, exhibit much higher leakage that is further dependant on the gate to source voltage, which can vary.
The off-capacitance is another specification to question vendors about. The capacitance between RF terminals for an ideal switch approaches zero. The capacitance of the open semiconductor switch is fairly high due to the dielectric materials and short distances across junctions. This limits the semiconductor RF isolation that can be achieved. RF-MEMS use physical gaps where the capacitance is extremely low leading to very high isolation.
Hot and Cold Reliability Specifications
FETs and PIN diodes, having been around for years, are known for their reliability. However, RF-MEMS devices are relatively new components without a long reliability history. Another point that gives designers doubts about RF-MEMS devices is their electromechanical nature. Relay type devices are noted for mechanical failures, such as worn metal contacts, pitting or hardening of the metal, which lowers conductivity. Adding fuel to these doubts is the very small size of the MEMS devices and the very small moving parts within them. Capping off the list of reliability concerns are questions about the ability of the small mechanical devices to tolerate variances in temperatures, humidity and shock.
To counter these doubts, MEMS manufacturers point to the maturation of the MEMS manufacturing technology, existing high volume highly reliable MEMS products such as Texas Instruments’ DLP (Digital Light Processing) optical MEMS technology as well as the development of standardized MEMS switch reliability tests.
When all limits of operation are held constant, reliability has become less and less of an issue in an RF-MEMS switch. The usable life span is typically reasonable and is simply a component of the mechanical nature of the device. With proper design techniques and appropriate application, an RF-MEMS switch can be used throughout the lifetime of any system.
To underscore the reliability of MEMS, WiSpry indicates that the materials used to build its MEMs switches make a big difference. Its switches are built on Silicon Oxide beams. As a result they do not suffer from any known work hardening effects, which would result in mechanical failure. Furthermore, the beams have very little mass and are air damped so makes them insensitive to shock or vibration.
Different Wireless Switches for Different Applications
Like RF-MEMS’ older counterparts, FETs and PIN diode switches, RF-MEMS switches have different electrical specifications, which require you to evaluate trade-offs for use in different applications. Where one RF-MEMS device may be well suited as an antenna switch or frequency band switch, it may not be appropriate for a transmit/receive switch.
Deciding on which RF-MEMS device is best for an application requires an understanding of basic switch specifications as well the performance specifications of the RF-MEMS switch over the frequency band or bands of interest. Subtleties related to the actual internal RF-MEMS design structure can affect the overall data transmission quality, the reliability of the final wireless product and the size of the target application system.
Regardless of the application, wireless system designers want an ideal RF switch, something that FETs and PIN diodes cannot come close to because of the non-linear nature of their I-V curves and their high parasitic capacitances. On the other hand, because an RF-MEMS switch is a miniaturized electro-mechanical switch, it comes much closer to the ideal. The I-V curves of the RF electromechanical switch closely approximate to that of an ideal switch, the straight line of a pure resistor. Furthermore, the MEMS characteristics do not depend on the RF signal level or on details of the control signals. Finally, the parasitic capacitance of MEMS devices are virtually non-existent.
There are several types of RF-MEMS switches that exploit both the mechanical and electrical behavior of silicon. The three basic types of RF-MEMS switch include the series contact cantilever switch, the capacitive shunt bridge switch and various hybrids. The types of RF-MEMS switch actuation mechanisms employed include electrostatic, magnetic actuation, piezovibration, thermal and a variety of hybrid actuation mechanisms that combine one, two or more actuation techniques.
Either series (in line with signal) or shunt (between signal and ground) switch elements can be used to implement signal switching. The series switch is used when the desired states are blocking the RF current with an open circuit or connecting the RF path. The shunt configuration is used when the desired states are connecting the RF path or blocking the RF voltage with a short circuit. Series and shunt elements can be used in combination to improve overall performance. Both capacitive and metallic contact switches can be used in both configurations although to date most capacitive switches have been used in shunt and most metallic switches have been in a series configuration.
Each of these switches offers benefits in certain type of applications. The series switch is most often used in applications that require lower frequencies to be passed. The series contact cantilever switch permits operation down to DC. On the other hand, the capacitance switch will act as an open to DC. Furthermore, because of the very low value of capacitance found in integrated circuits, the capacitance shunt is limited to high frequency applications.
Electrostatic switches, because they offer low power consumption but are limited in their switching speed, are often limited in wireless applications to antenna switches and frequency-band switches. On the other hand electromagnetic switches, because they offer higher switching speeds, are more often the choice for switching between the transmit and receive paths.
Different RF-MEMS Configuration Improve Specifications Further
Like PIN diodes and FET RF switches, different types of RF-MEMS switches can be used to optimize a RF-MEMS based switch for a specific specification such as isolation, insertion loss and linearity. Common combinations, utilize both capacitive shunt and series contact RF-MEMS switches to fit the exacting needs of specific applications, often without compromising another critical specification.
Series Contact Cantilever RF-MEMS Switch
Many RF-MEMS switches are based on the series contact switch relay arrangement. Such a device employs a cantilever suspended over an open trace. An electrostatic force is applied to the cantilever, which draws it down between the two contact points of the open trace, effectively short circuiting the trace and permitting current flow.
MEMS designers can trade-off isolation and actuation voltage in a series contact switch. In this type of switch, isolation and actuation voltage are functions of the distance between the contacts (RF-MEMS contact distances can vary anywhere between 1 and 3 microns ). Contact force is a function of the distance between the contacts. The further the distance between the contacts, the higher the actuation voltage needed. However, the further apart the contacts, in the off state, the better the isolation.
Figure: RF-MEMS Switch Designs Often Uses Gold Contacts on The Cantilever to Improve Reliability and Conduction Characteristics
Source: WiSpry, Inc.
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