GB353066A - Improvements in or relating to filtering circuits for use in electrical alternating-current or mechanical vibratory systems - Google Patents

Abstract

353,066. Acoustic wave filters. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.- (Assignees of Norton, E. L. ; 15, Summit Street, East Orange, New Jersey, U.S.A.) April 16, 1930, No. 12095. Convention date, April 16, 1929. [Class 13.] In a mechanical or electrical wave filter having one or more broad transmission bands, with nearly uniform attenuation throughout the bands, the output/input voltage ratio is proportional to #(1 + IXI<2n>) where IXI is the modulus of an imaginary odd rational function X of the frequency, and n is the number of impedance arms. Curves 12, 13, 14, Fig. 7 (in which the abscissa is the ratio of the actual frequency f to an arm-resonancefrequency fo) are the impedance characteristics of low-pass filters of this type, having three, five and seven arms respectively; while curve 15 is the characteristic of the corresponding 5-arm constant-k filter. The arms of the filter are of the inverse type, the series impedances and shunt admittances being all real multiples, such as A,XR, of the frequency-function X, Fig. 1, where R is the (resistive) iterative impedance at terminals BB<1>; but each arm has an individual value determined A<r>. The Specification shows how to evaluate the coefficients Ar, and how to design the arms either by treating the frequency at which IXI = <r>1 as a cut-off frequency, or by multiplying the impedances of the corresponding constant-k filter by factors which include the coefficients Ar. Figs. 2, 3, 4, 5 (not shown) indicate respectively low-pass, highpass, single-band-pass and 3-band-pass filters having a series nth branch (at the input end), the first branch being in series or shunt to the load resistance R according as n is odd or even. For the calculation of multi-band filters, however, a trial-anderror method is used: the type of section having been selected, a constant-k filter is constructed and its reactance-values are adjusted to give the required cut-offs. The reactance-values for a filter according to the invention are then deduced by use of the coefficients Ar. Fig. 8 (not shown) indicates a filter in which the nth branch is a shunt branch, the source of voltage 11 being connected in series with a high resistance if necessary. In Fig. 9 (not shown) the input and output terminals are interchanged with respect to the coefficients A,.