Electronic Design

MEMS RF Switch Operates To 26 GHz

For years, companies have been trying to make a fast, reliable microelectromechanical-systems (MEMS) switch that can switch quickly as well as work at the upper microwave frequencies. Many efforts over the past decade have resulted in products that didn’t meet the demanding specifications and reliability needs of test and communications applications. Now, TeraVicta Technologies has some MEMS switches that address the need for a tiny relay replacement. According to TeraVicta CEO Ray Burgess, the small signal switch market totaled $919 million in 2006 and will top $1 billion this year. Some analysts expect the market to grow to $1.4 billion by 2009. The market is roughly divided up equally between wireless handset switches, telecom infrastructure switching, military and aerospace, and automated test equipment (ATE) and instrumentation. Switching RF signals has always been a challenge. At low frequencies, relays work fine if speed isn’t a problem. Miniature electromechanical relays and reed switches have been available for years and have scaled rather well up to about 1 GHz, where their specs give out. Gallium-arsenide (GaAs) FETs and PIN diodes do a good job and are widely used to fill the need for very speedy transmit/receive switching in cell-phone handsets and other UHF/microwave products. But while they’re fast, they suffer from isolation problems and on-resistance limitations. In the ATE business, reed switches are still widely used in test heads in semiconductor testers. Yet the requirement to test ICs with ever higher speeds has ATE companies searching for a switch that works at higher frequencies. Everyone who uses such switches has generally agreed that a MEMS switch would be ideal if one could be made to provide the kind of performance required and at an affordable price. TeraVicta’s MEMS RF switch uses a patented mechanical design to achieve specifications and performance that meet the challenges of switching for test systems as well as microwave radio products. The switch has an unusual physical layout. It is in fact a "gold trampoline," a flat surface serving as one switch contact and supported by three arms that flex up and down. Called the High Force Disk Actuator (HFDA), the physical structure is made by sputtering gold and other metals on a ceramic substrate and then etching away areas to form the switch contacts. The switch contact arrangement is a single-pole single-throw normally open (SPST-NO). When an excitation voltage is applied to the structure, an electrostatic field is established to create attraction to another surface and cause the contacts to close. By connecting two of these devices together, the standard TeraVicta single-pole double-throw (SPDT) product is created. Excitation inputs to the two switch elements are separate, allowing the user to select the switching overlap or not as required. The excitation voltage is 68 V dc, which is needed to provide the fast switching as well as to balance the stout switch structure springiness required to ensure that the contacts don’t stick. That voltage is supplied by a charge pump dc-dc converter that accepts a 3-V switching input. This unique and patented physical structure solves the two basic problems encountered by other MEMS switch designs in the past—contact degradation and stiction. On top of that, it offers a switching life cycle better than almost any other mechanical switch. Most of the TeraVicta devices are reliable up to over 100 million switches. Some are reliable to 1 billion switch cycles. The basic TT712-68CSP’s switching speed is less than 100 µs and typically 70 µs with a contact on-resistance of about 0.15 Ω. Insertion loss is 0.15 dB, isolation when off is 35 dB, return loss is 20 dB, and the IP3 is greater than 65 dB. All of these specs are measured at 2.4 GHz. The operational frequency range is dc to 7 GHz. The power rating is 30 W peak and 15 W continuous. The TT2214 offers a double-pole double-throw (DPDT) configuration, and the TT414 is a single-pole quadruple-throw (SP4T) switch. Both have specs similar to those described above for the dc to 7-GHz range. A real breakthrough is the TT1244, which is a SPDT device that operates from dc to 26.5 GHz. All of the switches are housed in hermetically sealed chip-scale packages of varying sizes. The basic TT712-68CSP is 3.25 by 4.5 by 1.25 mm. Evaluation boards available for most of these devices include the driver ICs as well as connectors. With the technology firmly proven at this point, the company recently announced its product roadmaps. The baseline products in the dc to 7-GHz range address the instrumentation, ATE, and radio communications markets. Future multipole versions of the DPDT and SP4T are expected as well as custom versions to meet special needs. The high-bandwidth dc to 26-GHz product line will be extended to higher frequencies to target the military, aerospace and microwave, and high-speed serial test applications. A low-cost dc to 2.5-GHz product line will be available later, including custom products. These RF switches should find a real home in ATE test fixtures for ICs that typically use hundreds or even thousands of relays or reed switches. With the production of the faster serial data switch ICs and a wide range of wireless chips to test, ATE test heads have become difficult to implement with standard switching products. But with an upper limit to 7 GHz or even 26.5 GHz, the TeraVicta switches will surely make testing better and even possible on the more advanced ICs. And, their small size will be a welcome benefit in existing equipment. Consider the problem of loop testing in communications chip testing, which usually requires signal routing from the device-under-test (DUT) output back to DUT input bypassing the ATE in support of built-in self-test (BIST). The loopback path rarely has a bandwidth greater than 1 GHz. But with a MEMS switching arrangement, it can be extended to 7 GHz and beyond, making examination of third and fifth harmonics of high-speed pulses possible. Samples of the standard product are available now. Samples of the three new products are expected later in the second quarter. Production will come later in the year. Pricing varies greatly depending on volume, so contact the company directly. TeraVicta Technologies Inc. www.teravicta.com

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