Miniature BAW Resonators and Filters Based on Single Crystals of Strong Piezoelectrics
III. Langasite Microminiature Resonators and Filters
Both upgraded technologies and necessary equipment allow growing in Russia big-sized crystals with low density inclusion and reproducible physical properties. Single crystal treatment that is used now has improved greatly wafer surface quality that enables fabricating crystal elements with inverse mesa-structure at 100 MHz on fundamental mode.
Langasite resonators have been fabricated in DW-type packages using rotated Y-cut rectangular crystal elements with length along X and Z'-axis. Hermetical sealing is made in vacuum. Regardless of crystal element length's orientation the temperature-frequency characteristics of such resonators looks like quadratic parabola with steepness coefficient (5.9-6.3)×10-8 °C-2. A temperature of extremum can be obtained at any temperature within the range of -30 °C to +100 °C. Figure 2 shows extremum temperature dependence on crystal element cut angle.
Q-factor of resonators with length along Z'-axis is 15-20% higher then that of resonators with length orientation along X-axis, while the latter have motional resistance R1 1.5 times lower and capacitance ratio C0/C1 - 20% lower. The key properties of these resonators are listed in Table 4.
Table 4| f0, kHz | R1, Ohm | Q, k | L1, mH | C0, pF | C0/C1 | Plate size, mm | |||
| 11800 | 10 | 30 | 20 | 40 | 5.5 | 3.7 | 110 | 1.0 | 4.5 |
| 13100 | 5 | 20 | 11 | 37 | 3.1 | 4.5 | 90 | 4.5 | 1.0 |
Fork langasite resonators with capacitance ratio less than 350, motional resistance 10-40 kOhm and steepness coefficient (30-32)×10-8 °C-2 have been designed for viscosity sensors.
Langasite monolithic filters
The use of langasite in filters with relative pass band
0.5-0.8% allows realizing them as 4-pole dual element
sets. 10.7 MHz filter composed of two UM-5 package
cans has been designed with group delay. Frequency
and group delay responses of the filter are illustrated at
Figure 3.
Fig. 3. Frequency and group delay responses of an LGS filter
Due to inverse mesa-structure technology langasite filters can be designed at a frequency range up to 100 MHz. While designing and fabricating a wafer with inverse mesa-structure the ratio between the thickness of buffer and operating zones has a great importance. A ratio of 2.9 to 3.1 is a critical one as additional spurious pass bands appear in frequency response when wafers with these parameters are realized. The characteristics of 91 MHz filter is shown at Figure 4.
Fig. 4. Frequency response of a 6-pole LGS filter
IV. Conclusion
It is possible to make the conclusion that microminiature resonators with lithium tantalate and langasite crystal elements have advantages over the analogous quartz crystal units as far as their motional resistance value R1, motional inductance L1 and capacitance ratio C0/C1 are concerned. They can be also used for designing new models of miniature piezoelectric filters and microoscillators. The cost of these resonators can be close to the cost of analogous quartz crystal units.
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