GB394697A - Improvements in or relating to wave filters - Google Patents

Abstract

394,697. Impedance networks. SIEMENS & HALSKE AKT.-GES., Siemensstadt, Berlin. Oct. 3, 1931, No. 27537. Convention date, Oct. 4, 1930. Addition to 364,561. [Class 40 (iii).] The parent Specification describes filters comprising oppositely - connected component filters having different band-widths but the same geometric mean of cut-off frequencies. The present Specification extends the theory to filters with wide transmission bands, and shows how to dimension them with respect to steepness and symmetry of the attenuation curves and flatness of the characteristic-impedance curve. In the simplest case the filter is composed of two components, one of which comprises the, branches C1L1, C4L4, and the other C2L2, C3L3, where the lower-frequency arm L4C4 of one component is grouped with the highestfrequency arm L2C2 of the other, so that the components are oppositely connected. The frequencies of the components are chosen by giving pairs of equal and opposite values to the quantity k where w = # exp (¢ kb), or # #(1 + kb), or # # (1 - kb), or some similar function which = when x = o, # being the frequency of an arm, # a constant ("transmission-band centre") for the whole filter, and b another constant ("relative band-width"). The steepness of the attenuation curve is regulated by choice of the ks. The degree of symmetry of the attenuation characteristic is regulated (e.g. for matching a line having an asymmetrical characteristic) by a parameter which depends not on the #s but on the inductance-ratios and capacity-ratios of the two branches having the highest and lowest frequencies ; thus with the first formula given above, gamma<4> = L1C4/L4C1. Fig. 1 relates to this case and shows attenuation (nepers) against frequency plotted as #/#. Curves 1, 2 are symmetrical, with gamma equal to 1, but curve 3 is asymmetrical, with y equal to 1À034. Curve 2, for which k = 0À56, gives a steeper cut-off than curve 1, for which k = 0À5. When there are more than two component filters there is still a single y determined by inductance-ratios and/or capacity ratios of the two extreme arms, but there are (m - 1) different parameters k1, k2, .. , corresponding to the various resonances, when there. are m component filters or 2m arms. The characteristic impedance is rendered uniform within the transmission band by adding a component filter, such as COLO, C5L5, Fig. 2, having impedance proportional to and wave range the same as those of the extreme component, C1L1, C1L4, but is oppositely connected. The flatness of the impedance curve then depends, in the manner shown in Fig. 3, upon a further parameter n, which is a function of k, y, (#4 = #1) and L1/L0. Several quantitative examples are given in the Specification.