Hybrid transformer

1,115,216. Hybrid transformer circuits. STANDARD TELEPHONES & CABLES Ltd. 28 Feb., 1967, No. 9360/67. Heading H4R. The balancing network of a two-to-four wire hybrid transformer circuit is automatically adjusted in accordance with error signals which are derived from signals in a narrow frequency band extracted from the send pair and the receiving pair. Fig. 2 shows one embodiment in which the balancing network for the hybrid circuit includes variable resistors R 1 and R 2 which are adjusted in accordance with error signals derived by filtering from the incoming signals on line 3 frequencies of, e.g., 800 c.p.s., by filter F1. The output of filter F1 is applied to a phase shifter 6 in order to obtain signals in quadrature which are applied to multipliers 7 and 9, to the other inputs of which are applied the signals selected from the outgoing line 4 by the filter F2, which is tuned to the same frequency as filter F1 The output of the multipliers is integrated in integrators 8 and 10 and the resulting signals adjust the values of the respective resistors R1 and R2, which may be field effect transistors, Fig. 3 (not shown). The integrator output signal are representative of the echo signal fed from the receive line to the send line via the unbalanced hybrid circuit. Signals from the subscriber's line 5 have no effect on the integrator outputs since they do not reach the input of filter F1 due to the presence of amplifier A1 while components of the subscriber's line signal which pass through filter F2 are uncorrelated with the signals from filter F1 and do not therefore produce an output from the integrators. In a modified system, Fig. 4 (not shown), the input of filter F1 is connected across the balance network since no signal from the subs line 5 reaches this point. In an alternative arrangement only one error signal is derived, and the resistors R1 and R2 are adjusted simultaneously, Fig. 5 (not shown).