High Performance Stacked Crystal Filters for GPS and Wide Bandwidth Applications

Stacked Crystal Filters, (SCF), are composed of multi-layers of piezoelectric and metal layers. Normally these filters have a complex frequency spectrum containing near-in spurious responses. However, by having the SCF formed in a Solidly Mounted Resonator (SMR) format, the performance of the device is greatly improved by the finite bandwidth of the SMR reflector. The conventional SCF also has a bandwidth that is too small for some wireless applications. This paper will describe the design and performance issues associated with stacked crystal filters using bulk wave thin films with aluminum nitride as the piezoelectric. Filters without spurious responses, filters having wide bandwidths, and filters operating at frequencies up to 8.5 GHz will be described in addition to GPS filters. Stacked Crystal Filters, (SCF), are composed of multi-layers of piezoelectric and metal layers, as shown in Fig. 1b [1-3]. Normally these filters have a complex frequency response, shown in Figure 2, at approximate multiplies of the frequency corresponding to a half wavelength across the whole structure. The strong resonance is the one where there is a half wavelength across each of the piezoelectric resonators. The lower frequency response is at approximately half this frequency where there is a half wavelength across the whole structure. The upper frequency near-in spur occurs when there are three half-wavelengths across the entire structure.