GB533929A - Tapered band width dead-end filters - Google Patents

Abstract

533,929. Impedance networks. HAZELTINE CORPORATION. Sept. 11, 1939, No. 25601. Convention date, Oct. 7, 1938. [Classes 40 (iii)] and 40 (v)] In a coupling circuit of the kind disclosed in Specifications 524,547, 524,548 and 524,549, in which the shunt reactance between a pair of terminals is complemented by means of a dead-end filter so that the resultant impedance is kept at a uniform maximum value over a wide frequency band, the filter comprises several portions of which the bandwidths increase progressively towards the dead-end, where a resistance termination matches the image impedance over the useful pass-band. Thus the filter portion at the dead-end has a pass-band much wider than the useful pass-band of the whole filter. In Fig. 1, the low-pass filter is of constant k configuration, but the values of the elements vary as shown so that the band-widths of successive halfsections towards the dead-end increase by a factor a. The terminating half-section away from the dead-end comprises only the mid-shunt capacitance aCu, and the resultant impedance Z is built up across the total capacitance Cn (1+a). In Fig. 3 (not shown), the tapering of bandwidth is applied to whole sections instead of to half-sections. These arrangements represent tapering according to an exponential law, but better results are generally obtained when the bandwidth varies inversely as the square root of the number of the section (or half-section) counting from the dead-end, although a cube-root or even a fourth-root law may be used if a wider band is desired at the expense of uniformity. The band-pass filter, Fig. 6, which comprises two full sections terminated by a constant k midshunt arm L3, C3, the tapering being preferably according to the square-root law, presents uniform impedance across the capacitance C3 and also uniform transfer impedance between the pairs of terminals 1, 2 and 3, 4. An equivalent of the latter arrangement is shown in Fig. 8 as a coupling circuit between two thermionic valves 10, 11, the condensers Ca, C6 being formed wholly or partly by capacitances inherent in the valves. The inductances La, Lc are disposed to have zero coupling between them, and the two inductances Lb<SP>1</SP>, Lb<SP>11</SP> in series are dimensioned so that the coupling coefficients kl, k<SP>11</SP> are equal. The dead-end Cc, Lc may be utilized as a coupling to an auxiliary circuit. Fig. 9 shows a simplified arrangement which is equivalent to that of Fig. 8.