Patent GB-243321-A (GB)
Title: Improvements in or relating to mechanical and electro-mechanical wave filtering systems
IPC: H03H9/50
Filed: 1925-10-05
Abstract: 243,321. Western Electric Co., Ltd., (Assignees of Norton, C. L.). Nov. 24, 1924, [Convention date]. Filter circuits. - Electrical and mechanical wave filters, particularly for use between unequal input and output impedances, are so designed as to render unnecessary the use of transformers, transformer combinations of the type shown in Figs. 1 and 2 being replaced by networks of the type shown in Figs. 1<a> and 2a. The same principles are applied to the design of the electrical and mechanical parts of a loudspeaking telephone and also for reducing the number of parts employed in known filters. The Specification refers to theoretical discussions in Specification 142,115 and elsewhere. Electric filter circuits. When the impedance connected across the input terminals of a wavefiltex is not equal to that connected across the ouput terminals, reflection takes place unless a transformer or its equivalent having a suitable transformation ratio be inserted. For instance, if a filter of the form shown in Fig. 3 be designed for equal end impedances R<1>, and if a similar filter be required for an increased output impedance R<2>, transformers having suitable step-by-step ratios #1, #2, may be introduced at AA<1>, BB<1>, in which case all impedances between these two points must be increased in the ratio #1<SP>2>: 1, and all impedances to the right of BB<1> in the ratio #1<SP>2> #2<SP>2>: 1. In order, however, to avoid the introduction of transformers, use is made, according to the present invention, of certain equivalences : thus an ideal or perfect transformer T, with transformation ratio # in combination with shunt. and impedance ZA, Fig. 1, is equivalent to a star wherein the impedances have the values shown in Fig 1<a>, while a similar transformer in combination with series impedance ZB, Fig. 2, is equivalent to the mesh indicated in Fig. 2<a>. In Figs. 1<a> and 2<a> the impedances are of the same nature as the impedances ZA, ZB, respectively, the factor # being a real number; but in each case one of the substitute impedances has a negative value. The negative electrical impedances are realised in practice by reducing the value of a similar neighbouring impedance in the filter : thus if ZA be an indicative impedance, the quantity #(1-#)ZA must be deducted from an inductive impedance (not shown) connected in series to the terminal 3, Fig. 1<a>. These principles are applied to a filter of the form shown in Fig. 3 and designed for equal end-impedances, to adapt it for use with unequal end impedances, the resultant filter being shown in Fig. 4. The network L1, C1, L2 is replaced by a network L6, C, and in part L7, these two networks being (according to a further theorem) equivalent to one another. The inductance L3 together with an ideal transformer at AA<1> is replaced, as in Figs. 2 and 2<a>, by inductances Ls, L9 and a, fictitious negative inductance which is realised in practice by reducing the value of L7. The impedances C2, L4 become C7, L10. The condenser C3 together with an ideal transformer at BB<1> is replaced, as in Figs. 1 and 1<a>, by condensers Cs, C9, and a fictitious negative condenser, the magnitudes being so calculated that this fictitious condenser together with the condenser C4 is equivalent to a conductive connection so that these two condensers disappear. The parts L5, C5, become L11, C10' and the output impedance R, has the new value R2. Formulae are given in the Specification whereby the quantities concerned can be calculated. The theorems illustrated in Figs. 1, 1<a>, and 2a, can also be applied, by taking limiting cases, to reduce the number of impedances in a network. Thus the mid-series terminated filter shown in Fig. 5 is first supposed to have transformers with transformation ratios #<D>, #<F>, inserted at DD<1>, FF<1>, the impedances to the right of DD<1> and FF<1> being scaled up or down accordingly. Next, the substitutions illustrated in Figs. 1 and 1<a>, are made; these give a network similar in form to, but different in dimensions from that of Fig. 5; the inductance, corresponding to L12 can however be made zero, provided a certain equation connecting #<D>, #<F>, and L12/L be satisfied. Hence in the new network no inductance is required in the situation of L12. By choosing a suitable further relation between #1 and #2 this simplified network can be adapted to the cases of (1) a filter connecting equal endimpedances : (2) a filter in which one of the fictitious transformers at D, D<1> or F, F<1> is omitted : or (3) the simplified equivalent of a uniformly tapered chain of similar sections. Similarly the theorem of Figs. 2 and 2<a>, may be applied to eliminate one impedance element in a mid-shuntterminated filter.