Luca Pelliccia

High-

To reduce the complexity and cost of future telecommunication systems, electrically tunable or reconfigurable filters based on radio frequency microelectromechanical systems (RF MEMS) and waveguide technology constitute a very promising solution, since they allow for very high Q and good tunability. Three concepts for tunable waveguide filters using ohmic RF MEMS switches are described in this paper, and compared in terms of performance and manufacturing complexity. Quality factors of the order of 1000 and frequency tunability of the order of 5% are shown both by High-Frequency Structure Simulator simulations and experimental results in K-band.

Q

This paper presents ultra-compact dual-mode dielectric-loaded waveguide bandpass filters with Nth-order pseudoelliptic filtering function and asymmetric positioning of N transmission zeros. A waveguide cavity is loaded with a very high-permittivity rod (εr>30). Two degenerate resonant TM modes are employed along with nonresonating modes. In contrast to previous works, that are limited to structures with a symmetric response, in the proposed solution both modes are directly coupled to the input and output interconnections. Nonresonating modes are exploited to produce a direct source-load coupling. In this way, a single cavity generates two transmission poles and two transmission zeros located either below or above the two poles, thus realising a basic building block for N-pole and N-zero filters with asymmetric response. The dimensions of a 3% FBW 4th-order filter with 4 zeros at 4.35 GHz are 40×40×12 mm3 (using εr=34). A preliminary prototype has been fabricated and measured showing an unloaded Q of 1750 and good agreement with HFSS® simulations.

Tunable Waveguide Filters Using Ohmic RF MEMS Switches

A new approach to bandwidth-reconfigurable planar filter is proposed. RF MEMS switches are used to modify the intra-resonator coupling in hairpin band pass filters so as to control the bandwidth while keeping constant the centre frequency. The design and full-wave simulation of a 2nd order hairpin-line filter is reported to demonstrate the feasibility of the proposed approach. The measurement of the device will be soon carried out.

Ultra-compact filters using TM dual-mode dielectric-loaded cavities with asymmetric transmission zeros

Innovative concepts are presented for both miniaturized high-selectivity and MEMS-based reconfigurable waveguide filters recently developed. The aim is to reduce size and weight of satellite telecommunication systems without compromising the high unloaded Q required in such applications. A first class of new miniaturized filters is based on ridges arbitrarily located and oriented within a waveguide. The ridges behave as quasi-TEM resonators and allow for pseudo-elliptic filter responses. A second class of very compact waveguide filters employs TM dual-mode cavities as building blocks to generate Nth order filters with N transmission zeros. Two new concepts have then been developed for high-Q bandpass filters with tunable central frequency or bandwidth using RF MEMS. Cantilever MEMS switches are suitably arranged in a rectangular waveguide so as to change either the TE101 mode resonant frequency or the coupling between E-plane filter resonators. Both approaches are shown to yield higher unloaded Q (∼1000) than previous approaches involving RF MEMS along with ridged or conductor-loaded cavities.

RF MEMS bandwidth-reconfigurable hairpin filters

This paper presents the design, manufacturing and testing of a new type of compact and low loss Ka-band filter in multilayer micromachined technology. It consists of a 4th order pseudo-elliptic filter realized by stacking six silicon layers for a reduced footprint. The filter is based on λ/2 TEM Si membrane resonators placed inside shielding cavities and short-circuited at both anchored ends. With respect to conventional cavities based on TE101 resonant mode, the footprint of the proposed resonators is reduced by more than 50% at the expenses of a reduced Q factor degradation (<;20%). The measurements of the 4th order filter at Ka-band show insertion loss better than 3dB and Q factor above 500 in Ka-band. The filter robustness to manufacturing and the assembly tolerances has been verified on eight samples, showing good reproducibility of the filter response. Environmental tests are on-going to demonstrate the space compatibility of the filter.

Recent progress in miniaturized and reconfigurable filters for advanced communications and space applications

This paper presents the modeling, manufacturing, and testing of a micro-electromechanical system (MEMS)-based LC tank resonator suitable for low phase-noise voltage-controlled oscillators (VCOs). The device is based on a variable MEMS varactor in series with an inductive coplanar waveguide line. Two additional parallel stubs controlled by two ohmic MEMS switches have been introduced in order to increase the resonator tunability. The device was fabricated using the FBK-irst MEMS process on high resistivity (HR) silicon substrate. Samples were manufactured with and without a 0-level quartz cap. The radio frequency characterization of the devices without 0-level cap has shown a continuous tuning range of 11.7% and a quality factor in the range of 33–38. The repeatability was also tested on four samples and the continuous tuning is 11.7 ± 2%. Experimental results on the device with a 0-level cap, show a frequency downshift of about 200 MHz and a degradation of the quality factor of about 20%. This is, most likely, due to the polymeric sealing ring as well as to a contamination of the ohmic contacts introduced by the capping procedure. A preliminary design of a MEMS-based VCO was performed using Advanced Design System and a hardwired prototype was fabricated on Surface Mount Technology on RO4350 laminate. The prototype was tested resulting in a resonance frequency of 5 GHz with a phase noise of −105 and −126 dBc at 100 KHz and 1 MHz, respectively, and a measured output power of −1 dBm.

Ka-band surface-mountable pseudo-elliptic filter in multilayer micromachined technology for on-board communication systems

A new concept for compact and low-loss Ka-band micromachined bandpass filters for next generation on-board communications is presented which combines micromachined cavities with multilayer technology on silicon. The resonator consists of a gold-plated micromachined shielding cavity with a longitudinal 20 μm thick gold-plated silicon membrane. The membrane is short-circuited at both anchored ends and it operates as a λ/2 TEM line resonator. The footprint of a single resonator is reduced by more than 50% with respect to conventional micromachined cavities using the TE101 resonant mode. The measurements of single transmission-type resonators are in very good agreement with the full-wave simulations, demonstrating a high filter robustness to process tolerance. The expected unloaded Q of the resonators for 500 μm thick layers is 600, and the measured value is above 500. Qs above 1000 are feasible with the available technology by simply increasing the number and the thickness of the silicon layers. A 4th order pseudo-elliptic filter at 30 GHz (FBW = 1.7%) has been designed and is under fabrication.

MEMS-based LC tank with extended tuning range for low phase-noise VCO

A stepped-impedance configuration for coaxial resonators is exploited to realise filters in the L-, S-, C-band exhibiting a wide spurious-free range. The proposed configuration is realised by optimizing the aspect ratio of a single rod resonator, by enlarging its top head, maximizing the ratio between the frequency of the first higher order mode and the frequency of the fundamental mode. This approach allows for designing a very compact-sized filter which shows high Q-factor performances and a wide spurious-free band, up to nearly 5 times the centre frequency. A 6th-order filter was designed and manufactured and a comparison between the full-wave simulation and the measurement of the filter is reported.

Micromachined filters in multilayer technology for on-board communication systems in Ka-band

This paper presents possible configurations of dual band filters based on transverse magnetic (TM) Dual mode cavities. The concept is to use TM dual-mode cavities in order to design compact dual band filters. High flexibility is obtained in terms of transmission zero (TZ) positioning. By using this approach a number of TZs up to the number of poles is attainable. Zeros can be placed in both the lower and upper stop-bands as well as between the two pass-bands to increase their insulation. The use of TM cavities allows a dramatic reduction of the filter length without significantly reducing the RF performances. Two three-cavity TM dual-band filters with six symmetric and asymmetric TZs have been designed and manufactured to validate the proposed geometry.

High Q -factor compact filters with wide-band spurious rejection

In this letter a class of very compact dual-band band-pass filters based on TM dual-mode cavities loaded with high permittivity dielectric is presented. Filters consist of cascaded doublets resulting in in-line structures with a high degree of design modularity where each doublet is designed separately. Each doublet provides two poles and two transmission zeros resulting in a filter with an equal number of poles and zeros. Zeros can be placed in both the lower and upper stop-bands as well as between the two pass-bands to increase their insulation. The feasibility of the proposed approach is demonstrated through the design and manufacturing of a 6th order dual-band filter with central frequencies at 4.25 and 4.55 GHz having six transmission zeros.