RF MEMS technology was initially developed as a replacement for GaAs HEMT switches and p-i-n diodes for low-loss switching networks and X-band to mm-wave phase shifters. However, we have found that its very low loss properties (high device Q), its simple microwave circuit model and zero power consumption, its high power (voltage/current) handling capabilities, and its very low distortion properties, all make it the ideal tuning device for reconfigurable filters, antennas and impedance matching networks. In fact, reconfigurable networks are currently being funded at the same level-if not higher-than RF MEMS phase shifters, and in our opinion, are much more challenging for high-Q designs.

Tuning in to RF MEMS

This paper presents a state-of-the-art RF microelectromechanical systems wide-band miniature tunable filter designed for 6.5-10-GHz frequency range. The differential filter, fabricated on a glass substrate using digital capacitor banks and microstrip lines, results in a tuning range of 44% with very fine resolution, and return loss better than 16 dB for the whole tuning range. The relative bandwidth of the filter is 5.1 /spl plusmn/ 0.4% over the tuning range and the size of the filter is 5 mm /spl times/ 4 mm. The insertion loss is 4.1 and 5.6 dB at 9.8 and 6.5 GHz, respectively, for a 1-k/spl Omega//sq fabricated bias line. The simulations show that, for a bias line with 10-k/spl Omega//sq resistance or more, the insertion loss improves to 3 dB at 9.8 GHz and 4 dB at 6.5 GHz. The measured IIP/sub 3/ level is > 45 dBm for /spl Delta/f > 500 kHz, and the filter can handle 250 mW of RF power for hot and cold switching.

A differential 4-bit 6.5-10-GHz RF MEMS tunable filter

This paper presents a state-of-the-art RF microelectromechanical systems (MEMS) wide-band tunable filter designed for the 12-18-GHz frequency range The coplanar-waveguide filter, fabricated on a glass substrate using loaded resonators with RF MEMS capacitive switches, results in a tuning range of 40% with very fine resolution, and return loss better than 10 dB for the whole tuning range The relative bandwidth of the filter is 57/spl plusmn/04% over the tuning range and the size of the filter is 8 mm/spl times/4 mm The insertion loss is 55 and 82 dB at 178 and 122 GHz, respectively, for a 2-k/spl Omega//sq bias line The loss improves to 45 and 68 dB at 178 and 122 GHz, respectively, if the bias line resistance is increased to 20 k/spl Omega//sq The measured IIP/sub 3/ level is >37 dBm for /spl Delta/f>200 kHz To our knowledge, this is the widest band planar tunable filter to date

A 12-18-GHz three-pole RF MEMS tunable filter

Low-loss tunable filters with three different fractional-bandwidth variations were designed and fabricated on epsivr = 2.2, 0.787 mm Duroid substrates for 850-1400-MHz applications. A detailed analysis for realizing predefined bandwidth characteristics is presented, and a design technique to take into account the source and load impedance loading is discussed. It is found that independent electric and magnetic coupling makes it possible to realize three different coupling coefficient variations with the same filter structure. The proposed topology is different from the comb-line design in that all three filters have identical electrical lengths, the same varactors, and the same filter Q values. Three different filters are built using Schottky varactor diodes with a tuning range from ~ 850 to ~ 1400 MHz. The constant fraction-bandwidth filter has a 1-dB bandwidth of 5.4%plusmn0.3% and an insertion loss of 2.88-1.04 dB. The decreasing fractional-bandwidth filter shows a 1-dB bandwidth decrease from 5.2% to 2.9% with an insertion loss of 2.89-1.93 dB (this is effectively a 40-45-MHz constant absolute-bandwidth filter). The increasing fractional-bandwidth filter shows a 1-dB bandwidth increase from 4.3% to 6.5% with an insertion loss of 3.47-1.18 dB. The measured Q of the filters are between 53-152 from ~ 850 to ~ 1400 MHz. The measured third-order intermodulation intercept point ranges from 11.3 to 20.1 dBm depending on the bias voltage. To our knowledge, these planar tunable filters represent state-of-the art insertion-loss performance at this frequency range.

Low-Loss Two-Pole Tunable Filters With Three Different Predefined Bandwidth Characteristics

This paper presents the development of tunable filters using ohmic contact microelectromechanical system switches. It is shown that this type of switch is very well suited for the fabrication of low-loss high tuning-range microwave filters. Two sets of tunable Ku-band microstrip filters and resonators have been fabricated, with measured tuning ranges of 20% and 44%, and unloaded quality factors better than 75 in all cases. The 2-bit 5.7% fractional bandwidth, tunable bandpass filters exhibit insertion losses lower than 3.2 dB in all states.

Low-loss 2-bit tunable bandpass filters using MEMS DC contact switches

This paper overviews the application of RF MEMS switches in tunable filters as well as circuit developments for bandpass filters covering 110 MHz to 2.8 GHz. RF MEMS have several desirable features, including small size, low power requirements, and low loss. The basic operation of Raytheon's RF MEMS capacitive membrane switch is described. An overview of the technique used to integrate the switch into a variable capacitor structure with sixteen capacitance states is provided. Variable capacitor structures are used to construct multipole lumped bandpass filter designs, each with sixteen states. Finally, measured data from two representative five- and six-pole bandpass filters are presented. Characterization data demonstrates that the insertion loss for the five-pole filter using on-chip inductors was between 6.6 and 7.3 dB, and between 3.7 and 4.2 dB for the six-pole filter using off-chip inductors. © 2001 John Wiley & Sons, Inc. Int J RF and Microwave CAE11: 276-284, 2001.

Jamie Yao

Papers

RF MEMS‐based tunable filters