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In this section we study the design of an FBAR filter developed for application in integrated single-chip radios [29].

Specifications and Topology:

The filter was intended for applications in the 1.9-GHz PCS band and employed FBAR resonators in the ladder topology . In this T-cell, the filtering characteristics were realized by combining the impedance properties of FBAR resonators?namely, a low impedance, corresponding to the series resonance, near a high impedance, corresponding to the parallel resonance.

Circuit Design and Implementation:

By combining series and shunt resonators of slightly different frequencies [29]?particularly two identical series-branch resonators resonating at approximately the band center, and a shunt-branch resonator resonating at the lower edge of the passband?the passband is defined as follows. The lower skirt is set by the series-resonance of the FBAR in the shunt branch; the upper skirt is set by the parallel resonance of the resonators in the series branches; and the low-insertion loss passband is achieved by the cancellation of the parallel resonance of the shunt branch and the series resonance of the series branches.

The resonators themselves were of the solidly mounted type with an acoustic mirror of alternating layers of SiO2 and AlN, under piezoelectric AlN. While the topology introduced in Figure 5.12 realizes bandpass filters, the bandwidth may be limited enough to preclude meeting the specifications. To cover the desired bandwidth, external inductors in series with the FBAR resonators were employed. In this case, 1-nH inductors had to be utilized.

Circuit Packaging and Performance:

While the filter under discussion was in a prototypical stage of development, similar filters for the same application band can occupy an area of 0.63 ´ 1.25 mm [30] and are packaged in an ordinary SO8 assembly. Figure 5.13 shows the measured performance of the PCS FBAR filter.

Lessons Learned:

Reaching the full potential for filter miniaturization will require the integration of high-quality inductors together with the FBARs.

For example, the realization of an FBAR duplexer [31] for the PCS band required external inductors in the 1?4-nH range, which were implemented by meandering them around the packaged FBAR filters on a 5.6 ´ 11.9?mm printed circuit board.