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Low-loss NSPUDT saw filter with reflecting edge

Keywords:

S.A. Doberstein – Ph.D. (Eng.), leading research scientist, JSC «Omsk research institute of instrument engineering». E-mail: info@oniip.ru


This paper presents the low-loss Natural Single Phase Unidirectional Transducer (NSPUDT) surface acoustic wave (SAW) filter localizing an acoustic energy without reversing the directivity of NSPUDT. In the filter the usual interdigital transducers (IDTs) with a width of the electrodes and gaps of λ/4 (λ is the SAW wavelength at the center frequency of the filter) and the slope of ±25° off the X-axis on ST-Quartz are used. An asymmetric ST (X±25°) Quartz crystal orientations are used for the piezoelectric substrate where a SAW unidirectional radiation and receiving is possible by an usual IDT. In a filter design the inclined NSPUDTs are used and a SAW transmission between them is provided by a single reflection of SAW from a polished edge of a piezoelectric substrate. Testing the directivity of the usual IDTs along the ±25° directions off the X-axis on ST-Quartz was curried out for a verification of a natural unidirectivity in the proposed design. A theoretical analysis of the filter was provided with a computer simulation using an equivalent circuit model. As a first approximation the acoustic beam from an input NSPUDT fully passes in an output NSPUDT by a single reflection from the polished edge of the piezoelectric substrate. A good agreement between the simulated and measured data was obtained. The 54.1 MHz experimental sample of the filter had an insertion loss of 7.5 dB and very low amplitude ripple in a matched regime. The development of the low-loss NSPUDT SAW filters on the ST (X±25°) Quartz crystal orientations is essentially simplified with the use of an offered design. The sources of the filters losses have been analyzed and the ways of their reduction have been outlined.
References:

  1. Wright P.V. The Natural Single-Phase Unidirectional Transducer: A New Low-Loss SAW Transducer // Proc. IEEE Ultrasonics Symposium. 1985. P. 58–63.
  2. Lam C.S. and Gunes D. A Low-Loss SAW Filter Using Two Finger Per Wavelength Electrodes on the NSPUDT Orientation of Quartz // Proc. IEEE Ultrasonics Symposium. 1993. P. 185–188.
  3. Yamanouchi K., Takeuchi M., Odagawa H., Tanaka M. Low-Loss SAW Filter Using Thickness Difference Type of IDT on the NSPUDT Orientation Substrate // Proc. IEEE Frequency Control Symposium. 1995. P. 537–541.
  4. Takeuchi M., Odagawa H., Tanaka M., Yamanouchi K. SAW Transducer Configurations for Reversing the Directivity of NSPUDT Substrates // Proc. IEEE Ultrasonics Symposium. 1995. P. 17–22.
  5. Thorvaldsson T., Abbott B.P. Low loss SAW filters utilizing the natural single phase unidirectional transducer (NSPUDT) // Proc. IEEEUltrasonicsSymposium. 1990. P. 43–48.

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