I. A. Korzh, I. A. Tikhonov

The paper describes a design technique for temperature compensated narrowband SAW filters, an attempt is made for theoretical interpretation of the results. Based on the given theoretical relations required design elements for the thermostable narrowband SAW filter are calculated. For a known SAW filter consisting of an acoustic wavequide (a piezoelectric substrate with IDTs formed on its surface) and package for mounting the acoustic waveguide it is suggested to replace the bimetallatic plate on the substrate backside by a plane plate (metal or dielectric) with a hole in the center of the plate and holes along the plate edges. The plate is attached to the acoustic wavequide at the center hole by hard adhesive with a low linear thermal expansion coefficient and simultaneously to the package seat. On edges through holes, the plate is attached to the acoustic waveguide and simultaneously to the package seat by soft rubbery glue with a high linear thermal expansion coefficient. For IF sections with 65.128-65.540 MHz center frequency a narrowband coupled resonator SAW filter series with 40-76 kHz passband, 2-3 dB insertion loss, 28-40 dB rejection is developed using ST-cut quartz. The filters have high linearity, temperature stability and are mounted in DIP and SMD packages. It is shown that within the frequency range from -40 dB to +80 dB the best filter samples have a frequency drift of ± 2 kHz that is above the world level ± 6 kHz obtained for filters with the similar center frequency. Based on the 65 MHz narrowband SAW filters SAW microassemblies with IF amplifier are developed for HF receiver IF paths. To improve selectivity microassemblies use cascaded double-section and single-section coupled resonator SAW filters with a linear amplifier. The microassemblies provide 11 dB gain, 15-50 kHz passband, rejection > 70 dB, intermodulation selectivity 88 dBµV. Today 100 MHz thermostable filters and 450-500 MHz thermostable SAW resonators are under development.