Patent GB-195782-A (GB)
Title: Improvements in wave filters
IPC: H03H7/01
Filed: 1922-01-12
Abstract: 195,782. Western Electric Co., Ltd., (Weatern Electric Co., Inc.). Jan. 12, 1922. Filter circuits.-Relates to filter circuits consisting of a number of sections each having impedance in series in the line and impedance in shunt, and so arranged as to pass currents of certain ranges of frequency without attenuation whilst practically suppressing currents outside the said frequency ranges, as in Specification 142,115; and consists in substituting, for one or more sections of the filter, sections having different values of shunt and series impedance so calculated that the cubstitution does not interfere with the critical frequencies of the filter. The substituted sections may be designed to give a sharp cut-off at one or other side of the freetransmission range. The addition of a modified half-section at the end of a wavefilter may be arranged to give a comparatively uniform impedance to the filter over the free transmission range. In order to reduce reflection losses when a filter is interposed between two lines cr apparatus of different impedance, a " tapered " filter is used, that is to say a filter built up of a number of progressively-varying sections, the shunt and series impedances of the sections being so calculated as to give a gradual transition of the impedance of the filter from that of the input line to that of the output line. A filter such as is shown in Fig. 2 (in which each series impedance has a value sz, where s lies between 0 and 1, and each shunt impedance has a value (1-8<@>)/(48)z1+z2/s)has certain properties in common with the filter having series impedances z1, and shunt impedances z2. It is shown by analysis in the Specification that both filters have the same critical frequencies, and the same impedance measured across the line at the mid-points of the series elements. The modified filter has, however, a sharper cut-off at one of the critical frequencies and a more gradual cut-off at the other side. Intermediate effects may be produced by combining sections of the z1, z2 filter with sections of the modified filter, the junction of the sections being always at the mid-point of the series impedances. Fig. 9 shows a filter with series impedances z1 and shunt impedances z2 furminating at a mid-point A, B of a series impedance. Half a modified section is added to the left of the points A, B, the section being divided at the middle of the shunt impedance, so that the terminal shunt impedance element has twice the value of the shunt impedance element of a complete modified section. The addition of this half-section does not alter the critical frequencies, and it also leaves the impedance of the filter a pure-resistance over the free range. But by giving a suitable value to the constant s, the resistance of the filter may be rendered substantially constant over the free range. An example is given in the Specification showing the addition of terminal half-sections, and the substitution of modified whole sections, in wave-filters employed to separate low and high frequencies at the receiving-station of a telephone system. Fig. 12 shows a " tapered " wave filter employed for the purpose stated above, Fig. 13 being a diagram showing an intermediate stage in its development. Referring to Fig. 13, the first section to the left of G has series impedance z, and shunt impedance z2, and the remaining sections GH, HI, &c. are modified sections for which 8=@ &c. The impedances measured across the line at the points G, H, I, &c. will be uniform for this filter. If all the impedance elements to the right of G are multiplied by t, the total impedance at that point will be multiplied by the same factor. The " tapered " filter is therefore obtained by multiplying all the impedance elements to the right of G by t, all to the right of H by t', and so on. This restores all the series impedance, elements to the value z., and gives the (n+1)th shunt impedance element the value “(t<2n-1>)z@+t<2n> z2. By choosing t so that t<n>+1 is equal to the ratio of the output line impedance to the input line impedance, and using n sections, the impedance of the filter changes progressively from section to section and so reduces the reflection which would otherwise occur at the junctions. The tapered filters may be adapted for cases in which the output line impedance, is less than that of the input line. The Specification also describes a form of filter derived from the normal z1, z2 type by increasing the shunt impedance elements z2 in the ratio h to 1, and substituting for the series impedance elements z1 two impedances in parallel of the values z1/h and 4h z2/b2-1 These filters have cf the values z1/h and 4h z2/b<2-1> These filters have the same critical frequencies as the normal filter, and have the same impedance, as measured at the middle of the shunt impedance elements. When therefore sections of this type of filter are employed, the division points are always at the middle of the shunt impedances.