GB802006A - Improvements in or relating to crystal filters - Google Patents

Abstract

802,006. Filter networks. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. July 13, 1956 [July 18, 1955], No. 21731/56. Class 40 (8). A four terminal band-stop network is characterized in being the electrical equivalent of a lattice filter (Fig. 9), having in each of its four arms a piezo-electric resonator, each of the resonators having substantially the same characteristics and in each of one pair of arms an inductor the value of whose inductance is such as to resonate with the shunt capacity of the across the associated resonator at a frequency below the self-resonant frequency of the resonator. In the reactance diagram of Fig. 10, the solid line shows the reactance variation in the series arms and the dashed line the variation in the lattice arms. The reactance curve of series arms crosses the axis at points #0, #2, f5, #6, the location of the points #0 and f6 not being of great importance. The point f2 is the anti-resonant frequency of the crystal and may be adjusted by variation in the shunt capacity. A change in this capacity also changes the location of point f5 at which the inductance is in series resonance with the effective capacitive reactance of the crystal circuit. The points #1 and f3 at which the dashed curved crosses the axis are determined respectively by the internal series-resonance frequency of the crystal and the anti-resonance frequency determined in part by the value of the shunt capacitor. The attenuation curve is shown in Fig. 11. By adjusting the shunt capacitances of the crystals the points #2, #3 may be separated and an improvement in the rectangularity of the stop-band achieved. The circuit is realized in the manner shown in Fig. 8, using a ferrite-core hybrid transformer. U.S.A. Specification 2,212,840 is referred to.