Guan-Leng Tan

RF MEMS phase shifters: design and applications

Recent results obtained with MEMS phase shifters demonstrate that their performance is much better than GaAs phase shifters using either standard (switched-line, reflect-line) or distributed designs. The reliability of MEMS phase shifters is worse than of single switches since they employ 8-16 MEMS switches and do not tolerate a failure in any of the switches. On the other hand, a large phased array will still function properly, albeit with a slightly decreased efficiency and higher sidelobes if 3-4% of the phase shifters fail. Currently, the failure mechanisms of MEMS switches are being investigated and will greatly benefit the reliability of MEMS phase shifters. Also, the hermetic packaging of MEMS phase shifters is not straightforward, due to their relatively large size. It is for these reasons that the authors believe that MEMS phase shifters will be mostly used in satellite and defense applications in the next five years.

Low-loss 2- and 4-bit TTD MEMS phase shifters based on SP4T switches

2- and 4-bit microelectromechanical system (MEMS) X- to K/sub u/-band true-time-delay phase shifters with a very low insertion loss are described. The phase shifters are fabricated on 200-/spl mu/m GaAs substrates and the low loss is achieved using MEMS SP4T switches, which reduce the number of switches in the signal path by half when compared to conventional designs with SP2T switches. Measurements indicate an insertion loss of -0.6/spl plusmn/0.3 and -1.2/spl plusmn/0.5 dB at 10 GHz for the 2- and 4-bit designs, respectively. The measured losses agreed very well with Momentum simulations and are the lowest reported to date. The 2-bit phase shifter performed well from dc-18 GHz, with -0.8/spl plusmn/0.3-dB insertion loss at 18 GHz and a return loss of <-10.5 dB over dc-18 GHz.

High-isolation W-band MEMS switches

This paper presents the design, fabrication and measurement of single, T-match and /spl pi/-match W-band high-isolation MEMS shunt switches on silicon substrates. The single and T-match design result in -20 dB isolation over the 80-110 GHz range with an insertion loss of 0.25/spl plusmn/0.1 dB. The /spl pi/-match design results in a reflection coefficient lower than -20 dB up to 100 GHz, and an isolation of -30 to -40 dB from 75 to 110 GHz (limited by leakage through the substrate). The associated insertion loss Is 0.4/spl plusmn/0.1 dB at 90 GHz. To our knowledge, this is the first demonstration of high-performance MEMS switches at W-band frequencies.

A DC-contact MEMS shunt switch

This paper presents the design, fabrication, and performance of a metal-to-metal contact micro-electro-mechanical (MEMS) shunt switch. The switch is composed of a fixed-fixed metal beam with two pull-down electrodes and a central DC-contact area. The switch is placed in an in-line configuration in a coplanar waveguide transmission line. This topology results in a compact DC-contact shunt switch and high isolation at 0.1-18 GHz. The isolation at MM-wave frequencies is limited by the inductance to ground and is -20 dB at 18 GHz. The application areas are in wireless communications and high-isolation switching networks for satellite systems.

A 2-bit miniature X-band MEMS phase shifter

The design and performance of a compact low-loss X-band true-time-delay (TTD) MEMS phase shifter fabricated on 8-mil GaAs substrate is described. A semi-lumped approach using microstrip transmission lines and metal-insulator-metal (MIM) capacitors is employed for the delay lines in order to both reduce circuit size as well as avoid the high insertion loss found in typical miniaturized designs. The 2-bit phase shifter achieved an average insertion loss of -0.70 dB at 9.45 GHz, and an associated phase accuracy of /spl plusmn/1.3/spl deg/. It occupies an area of only 5 mm/sup 2/, which is 44% the area of the smallest known X-band MEMS phase shifter . The phase shifter operates over 6-14 GHz with a return loss of better than -14 dB.

A very-low-loss 2-bit X-band RF MEMS phase shifter

A novel low-loss phase shifter, based on RF MEMS series switches and a single-pole four-throw (SP4T) switch design, is presented. The phase shifter is fabricated on a 200 /spl mu/m-thick GaAs substrate, and occupies less than 12 mm/sup 2/ of space. The measured average insertion loss is -0.55 dB, with a reflection loss of less than -17 dB over the 8-12 GHz frequency range. The 2-bit phase shifter performs well up to 18 GHz with an average loss of only -0.85 dB and a near-perfect linear phase shift from DC-18 GHz. This is the lowest loss MMIC-type phase shifter to-date at 8-18 GHz.

A 4-bit miniature X-band MEMS phase shifter using switched-LC networks

A compact 4-bit true-time-delay MEMS phase shifter fabricated on 8-mil GaAs substrate is described. Miniaturization of the phase shifter is achieved using phase shift networks based on semi-lumped LC circuits and a compact SP2T switch configuration. Trade-off in size and loss is performed to achieve a phase shifter with an area of 7 mm/sup 2/ and an average insertion loss of -1.47 dB at 9.4 GHz. The phase shifter has a measured return loss of < -10.5 dB over 8-12 GHz and an RMS phase error of 2/spl deg/ at 9.4 GHz.

Design of X-Band MEMS Microstrip Shunt Switches

This paper presents the design and performance of X-band MEMS switches built in microstrip technology. The switches result in an insertion loss of less than 0.1 dB and a small isolation bandwidth, less than 10%, and are limited by the radial stubs band-widths. The isolation value is also not dependent on the down-state capacitance of the switch. The isolation bandwidth (less than ¿20 dB isolation) is improved to 8-13 GHz with the use of a ¿-network and two MEMS switches. The up-state insertion loss of the ¿ switch is less than 0.25 dB. The paper demonstrates that the performance of microstrip switch circuits without via-holes is dominated by the shorting (radial) stubs, and careful design must be done to result in an acceptable bandwidth of operation.

Microwave Absorptive MEMS Switches

This paper describes the design and performance of a 30 GHz CPW SPST absorptive MEMS switch based on silicon surface micromachining technology. The MEMS switch is implemented using capacitive shunt bridges with fixed-fixed beams. A return loss of better than 15 dB and an insertion loss of 0.8-1.0 dB is achieved in the up-state position. The return loss is better than 20 dB and the isolation is 25-30 dB at 30 GHz in the down-state position. Potential application areas include switch matrix communication systems.