Abstract
A torsional-mode pole-type mechanical filter is studied, with the aim of further miniaturization and economy of channel translating equipment. New torsional-mode transducers have been developed. These are built of piezoelectric ceramics disks with a high coupling constant k1.j and of bars of a constant elastic modulus alloy. As the capacitance ratio of the transducer is about 9-1 l, the filter does not need inductors to compensate for the clamped capacitance of the transducers. The distances between the resonators are adequately determined, therefore, the couplers are constructed of only few types of single straight wire lines. Finite attenuation poles are produced by two oblique bridging wires. In the passband region, the attenuation ripples of the filter fulfill the desired specification (CCITT 1/20) over the temperature range of 5°C to 45°C. Delay distortion and minimum group delay time are nearly equal to that of the LC channel filter being used in Japan. The filter contains 7 torsional-mode resonators and 2 torsional- mode transducers. The filter volume is about 5.3 cm3 and its weight is 13 g. These values are about 1/10 those of the LC channel filter.
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