GB846999A - Improvements in or relating to electro-mechanical high frequency filters - Google Patents

Abstract

846,999. Electro-mechanical filters. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Nov. 10, 1958 [March 3, 1958], No. 6714/58. Class 40(8). Spurious frequencies in an electro-mechanical filter comprising a plurality of mechanical resonators linked by couplers are eliminated or substantially reduced by providing at least one resonator having a length of less than one half wavelength and having different cross-sections at different points along the direction of propagation of the mechanically vibrating energy. The filter shown is supported between end clamps 1 and 2 and comprises cylindrical resonators 6, 7, 8, 9, 10 and 11 of equal diameters, each n#/2 long at the desired operating frequency, where n is any integer (including unity), linked by cylindrical couplers 15 each m#/4 long, where m is any odd integer (including unity), but whose diameters may vary. The drive and pick-up devices are similiar transducers each comprising a magneto-strictive member 3 and a dust iron shell 4 containing a coil 5. Three non-uniform resonators 12, 13 and 14, each in the form of a dumb-bell, are provided in the centre of the filter and are linked by two couplers 15 of smaller diameter than the other couplers. Alternatively each non-uniform resonator may be in the form of an inverse dumb-bell, or barrel-shaped, or approximately hour-glass shaped (Figs. 2-4, not shown). The filter may alternatively comprise a plurality of discs vibrating in the flexural mode and coupled by a wire or wires fixed to the edges or centres of the discs, the direction of vibration being perpendicular to the flat surfaces of the discs. The non- uniform resonator is then a disc whose thickness varies either continuously or in one or more steps between the centre and the periphery.