Electromechanical band-pass filter with low passband ripple - uses flexural resonators in cascade coupled by longitudinal vibrators quarter wavelength long using direct synthesis design method

摘要

The filter consists of cascaded elements, each consisting of two resonators undergoing flexural vibrations and coupled by a quarter wave resonator which undergoes longitudinal vibrations. The cut off frequency of the associated low pass prototype element leads to lower passband ripple and better band edge definition. The flexural vibrating rods are parallel and consist of elinvar. The filter is coupled electrically by piezoelectric ceramic plates as input and output transducers. The resonators of each element are dimensional so that one of the relative frequency limits is related by a mathematical formula to the frequency cut off of the corresp. low pass filter. The coupling of each stage is related by a mathematical formula to the upper or lower relative frequency of the stage. A 9 stage filter using the Chebyshev approximation with a 20% bandwidth at 50 KH2 and 0.04 dB passband ripple has been produced with the technique.