GB462984A - Improvements in or relating to systems for carrier-frequency telephony - Google Patents

Abstract

462,984. Two - - way transmission. SIEMENS & HALSKE AKT.-GES. Sept. 18, 1935, No. 25923. Convention date, Sept. 19, 1934. [Class 40 (iv)] [Also in Group XL] In a two-way carrier system employing different frequencies for the two directions and filters for separating the frequencies at bridge connections between a two-way line and two one-way paths, the balance at the bridge is improved by arranging that the product of the impedances of the filters as seen from the bridge is equal to the square of the line impedance. In a particular case stated to avoid reflections, the impedance of one filter is double that of the line and that of the other is one half. As shown in Fig. 2, the filters B1, B2 are the normal filters in the two paths and are each associated with a dummy filter B<1>1, B<1>2 having the pass range of the normal filter in the other path. Reactances X associated with the filters are such that impedance of each combined normal and dummy filter as seen from the bridge is independent of frequency and equal to the line impedance. As shown, the filters of each combined pair are connected in parallel, but if #-section filters are used they are connected in series. By using suitable ratios for the bridge transformers, the impedances may be made double and half the line impedance respectively. The combined filters B1, B<1>1 or B2, B<1>2 may be replaced by the filters shown in Figs. 3 and 4. In the filter shown in Fig. 3, a T-section filter of impedance Z1 and a #-section filter of impedance Z2 are such that Z1.Z2=Zo<2> where Zo is independent of frequency. Each filter is connected in series with a resistance of value Zo and the filter-resistance combinations are connected across the path in parallel to give an input impedance Zo which is made equal to the line impedance. Fig. 4 is a corresponding arrangement with the filters in series.