GB1268811A - Improvements in or relating to microwave devices - Google Patents

Abstract

1,268,811. Insulating materials; filter assemblies; printed circuits; microwave filters. GENERAL ELECTRIC CO. Ltd. 26 June, 1970 [22 April, 1969; 11 Dec., 1969], Nos. 20465/69 and 60579/69. Headings H1A, HlB and HlW. A narrow band, high Q filter, Fig. 1, designed to operate at a frequency of 4 GHz, comprises five resonator discs 1 formed of a specified ceramic material, each disc being adapted to resonate in the TE 011 mode. The resonator discs are supported in a copper outer casing 2, by means of a tube 3, cylindrical spacers 4 and rings 5, all formed of a low loss, low permittivity dielectric material, the tube 3 being closed at both ends by copper caps 6. The resonator discs 1 have central holes 7 into which are inserted rods 8 of the same material as the discs, and tuning screws 9 are used to bear upon the rods 8 to adjust the resonant frequency of the discs as required. Two 50 ohm type-N connectors (10, Fig. 2, not shown) are used, the signal input and output being via copper coupling strips (11, 12) supported by rings. The ceramic of which the discs are made consists essentially of one or more compounds of the general formula ABO 3 , where A is barium, strontium or calcium and B is zirconium, or both zirconium and titanium in atomic proportions of at least 80% zirconium and not more than 20% titanium, the composition of the ceramic material being so chosen that it does not include significant amounts of both barium and titanium and the material will have, at microwave frequencies, permittivities in the range of 25 to 75, a substantially constant temperature coefficient of permittivity, and a loss tangent not exceeding 0À005 at 20‹ C. The microwave losses of some of these materials, especially the barium strontium zirconates, can be reduced by the addition of a small proportion of niobium pentoxide and/or tantalum pentoxide. The ceramic material may, alternatively, be used as the substrate (16, Figs. 4, 5, not shown) on which is formed a filter circuit in microstripline. A metal coating is formed on the opposite side of the substrate to the circuit and both this coating and the circuit are formed of chromium covered with gold, by evaporating the chromium and the gold on to the face of the substrate, increasing the gold layer by electroplating, part of the coating being then removed by photo-etching to leave the desired circuit. The ceramic material is produced by preparing an intimate mixture of suitable powdered starting materials in the required relative proportions, pressing the mixture, and heating the pressed compacts to effect reaction and sintering. To ensure accurate reproduction of the properties of the ceramic material two compositions may be prepared, these compositions departing from the preferred composition in opposite directions, the powders of the compositions being then mixed to give the required properties.