Saw resonator filters for communications systems
The results of researches of designs of low-loss resonator SAW filters, intended for usage in input cascades of different communications systems are presented. The filters for a wide spectrum of passbands 0,5 … 10 % are practically realized.
Now filters on surface acoustic waves (SAW) are one of the basic devices executing frequency selection in modern communications systems. They provide high parameter stability of systems and have small sizes and effective enough cost. Use of new designs and materials considerably allows to expand area of their application, that is caused by an opportunity of a decrease of the insertion attenuation level up to 1 … 6 dB in such filters.
II. Основная часть
In this work the designs of SAW filters which principle of operation is based on partial reflection of the wave from weight - electrical heterogeneities on a surface plate were investigated. By development of computational methods a modified Р-matrix and main specifications of the COM – theory  obtained by an experimental way were used. The software is executed on the MathCAD base.
Longitudinally - coupled resonator filters (LCRF). The small level of an insertion loss was achieved due to use bilateral reception and additional reflection gratings on edges of topology . The cascading method (2 … 4 cascades) was applied to maintenance of high out-of-band selectivity (up to 60 dB). Filters on frequency band 100 … 800 MHz with passband width 0,5 … 7 % including SAW filters on 6 … 60 television ethereal channels are practically realized (fig. 1а - amplitude-frequency characteristic of the 29 channel filter). Depending on bandwidth and required selectivity the level of an insertion attenuation made 0,8 … 5,0 dB.
SAW filters exploiting U-shaped multistrip couplers (U-MSCF). The directivity of radiation in this design is provided due to spatial overlapping of the lateral transducers and 3 dB-MSCs . The filters on 4 and 5 (fig. 1в) television channels with passband accordingly 10 % and 8,5 % are realized on the given type of a design. The level of an insertion attenuation did not exceed 4,5 dB.
SAW resonator filters exploiting reversive multistrip couplers (RMSCF) represents the two-channel design containing input and output transducers and two RMSC arranged on the topology edges . Advantage of a design is the weak sensitivity of a filter response to the transducer position. Except this the two-channel inclusion allows to exclude influence of volumetric waves that is important for obtaining high selectivity of filters in out-of-band area of the characteristic. Filters on frequency range 40 … 400 MHz with passband 1 … 3 % and level of an insertion attenuation 2,0 … 5,0 dB are practically realized, fig. 1с.
The design methods of decrease of the insertion attenuation level of SAW filters are investigated. That has allowed use them in input cascades of modern communications systems of different purpose.
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