A bulk micromachined tunable dual-mode microwave bandpass filter on coplanar waveguide

This paper proposes a bulk micromachined bandpass filter on 0.5mm-thick Pyrex glass, whose operation is based on the splitting of the odd and even degenerate mode in a dual-mode resonator, with the addition of a perturbation element. In such a way, a filter may require only half resonators of a traditional one, resulting in a more compact configuration over those ever reported. A 4%-bandwidth bandpass filter centered at 17GHz was designed. The square-ring-like coplanar waveguide (CPW) act as a dual-mode resonator, and an open-circuited stub attached to the inner corner of the square-ring acts as the perturbation element and dominates the filter bandwidth. The two bulk-micromachined switches for tuning are placed symmetrically along the diagonal line. Since the electrical length of CPW is extended after being loaded by capacitive switches, both the degenerate resonating frequencies and the filter mid-band frequency are shifted downward. Theoretical formulas are derived for various actuation states of all the switches. The filter performance is analysed also using finite-element fullwave tools. A frequency shift of 0.6GHz is seen which is close to 0.58GHz obtained from analytical solutions. Both the original and tuned state come with satisfying microwave performance, i.e. low passband insertion-loss ( < 0.5dB) and return-loss (< -15dB) and high stopband rejection. It is also shown that the simulated result is in good agreement with theoretical solutions. The bulk microfabrication is shown to be much simpler than surface micromachining conventionally used for RF MEMS switches. Since the bridge skeleton is Si, thermal mismatch between all-metal bridge and dielectric substrate is avoided, and reliability at elevated temperature is guaranteed or at least greatly improved. Besides, the p++ doped Si line patterned simultaneously with the microbridges, can also be used as a DC biasing resistor, which can eliminate the additional steps to fabricate the resistive alloy line. The tested results of the fabricated switch samples are also shown. The device may be a good candidate for novel Ku-band inter-scientific satellite communication or wireless networking. Copyright ? 2008 by ASME.EI