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Circuit Packaging and Performance

Henrik — Thu, 03 May 2012 12:15:53 +0000

The devices were research demonstration vehicles tested using wafer probing techniques and were not packaged. Given the presence of MEM varactors, however, it is clear that packaging techniques and considerations similar to those employed in the case of phase shifters would be applicable. Figure 5.16 shows the measured performance [34]. The lumped-element filter exhibited a minimum insertion loss of 4.9 dB, a return loss of ~10 dB, and a tuning range of 4.2% over a voltage range of 65V. The coupled distributed resonator filter, on the other hand, exhibited a minimum insertion loss of 3.8 dB, an average return loss of ~15 dB, and a tuning range of 2.5% over a voltage range of 50V. In comparing the observed tuning range with the simulated one, Kim et al. [34] point out that the smaller measured tuning range results from the lower capacitance change of a partially deflected beam (i.e., larger deflection at the beam tip than at the anchor), whereas the simulated change assumed uniform gap shrinking.

Circuit Design and Implementation

Henrik — Thu, 03 May 2012 12:14:13 +0000

The filters were implemented in grounded coplanar waveguide (GCPW) media , and their design was carried out via a full-wave electromagnetic simulator [34]. For the lumped-element realization, the simulation entailed varying the inductive (magnetic) coupling between the spiral inductors of the LC resonators, which was achieved by optimizing the filter response as a function of the separation between the inductors. The halfwavelength resonator-coupled filter version can be designed using coupler synthesis tools and fine-tuned with the full-wave solver. In addition, in order to assess attainable tuning ranges, simulations were conducted while setting the varactor parallel plate gap to various distances: 6 mm, 5 mm, and 4 mm.

It is interesting to note that, in this simulation, the lumped-element version exhibited a downward shift of 6.4% in center frequency with a capacitor gap reduction of 2 mm, while the distributed resonator version exhibited a downward shift of 2.6% for a varactor gap reduction of 1 mm. Also shown in Figure 5.14 are RF chokes to permit the noninvasive application of the varactor control voltage.

Specifications and Topology

Henrik — Thu, 03 May 2012 12:11:59 +0000

Two filters intended for application in highly integrated transmitters and receivers utilized in millimeter-wave multiband communication systems were implemented: a lumped-element version with a 4.7% bandwidth, centered at 26.8 GHz, and a coupled-resonator version with a 8.5% bandwidth, centered at 30.6 GHz. The filter topologies are shown.

In the lumped-element version , the LC resonators were implemented as p-networks with series spiral inductors and shunt capacitors implemented with micromachined cantilever-type varactors. In the coupled distributed resonator version , the resonators were implemented by half-wavelength resonators terminated in varactor connected to ground. Thus, by varying the capacitance of the varactors, the effective electrical length of the resonators deviated from half-wavelength and tuning is affected.

Wave Micromachined Tunable Filter

Henrik — Thu, 03 May 2012 12:11:11 +0000

The motivations for developing tunable filters might be traced to three factors: cost, weight, and power dissipation. In modern satellite communications, for instance, where an enormous number of filters are employed, there is extreme interest in minimizing both manufacturing cost, weight, and power consumption. The potential of MEMS technology to permit the batch fabrication of tunable filters is rather appealing, as these might in principle obviate the need for expensive manual tuning in favor of electronic tuning during system integration and alignment, thus reducing cost.

On the other hand, the availability of inexpensive tunable filters opens up new opportunities, not only for novel systems architectures, but, by increasing filter functionality, thus reducing the number of individual filters needed, which reduces overall weight. Finally, since MEMS avails itself of electrostatic schemes for tuning, the power dissipation incurred in implementing tunability is negligible. In this section we study the cases of millimeter-wave micromachined tunable filters demonstrated by Kim et al.

RF MEMS Filters

Henrik — Thu, 03 May 2012 12:09:53 +0000

When it comes to filtering, MEMS fabrication technology has been exploited for two main purposes [32]: (1) the micromaching of structures (e.g., cavities) so small that their fabrication utilizing conventional machining techniques would be prohibitively expensive or virtually impossible due to tolerance limitations, and (2) the creation of micromechanical resonatorbased filters, typically utilizing polysilicon beam resonators.

As examples of the former, which is typically applied to millimeter-wave frequencies, the works of Brown and Rebeiz [33] and of Kim et al. [34] may be cited. As examples of the latter, which so far has only been applied to frequencies lower than 200 MHz, the works of Bannon, Clark, and Nguyen [35] and of Wang and Nguyen [36] may be cited. In what follows, we present two case studies: one being an example of a tunable micromachined filter, and the other an example of a micromechanical MEMS filter.