Patent GB-539562-A (GB)
Title: Electric wave transmission systems
IPC: H04L25/03
Filed: 1940-05-03
Abstract: 539,562. Impedance networks; valve circuits. STANDARD TELEPHONES & CABLES, Ltd May 3, 1940, No. 7975. Convention date, May 5, 1939. [Classes 40 (iii) and 40 (v)] A four-terminal equalizing network whose transmission characteristics can be adjusted to any desired form comprises elements adapted to produce a series of different delays and attenuations which are varied until an indicial response (i.e. an output p.d. produced by the sudden application of unit input p.d.) of desired form is obtained. The invention is based on principles described, more particularly with reference to a wide-band filter, in Specification 517,516. The present invention is particularly adapted for methods employing experimental adjustment as opposed to calculation. Square signals of suitable frequency are applied at the input end of the network and the indicial response is observed with an oscillograph. The network is then adjusted to give the desired response. The network may comprise a number of channels connected in parallel, Fig. 2 (not shown) ; the delays imposed by these channels are in arithmetical progression with a small constant difference, and the signals passing through them are attenuated or amplified in adjustable ratios ; the output of each channel is added to or subtracted from the outputs of the remaining channels'as may be necessary to produce the desired response. In another embodiment, the output terminals of a valve are shunted by an impedance comprising the input impedance of a retardation line reflectionlessly terminated at its distal end. Any alteration in one of the impedance elements will produce-reflections which affect the shape of the indicial response of the amplifier, which can thus be modified as desired. In this embodiment the ordinates of the indicial response can be varied by only a small percentage of their mean value, but a greater percentage variation can be obtained, Fig. 9, by applying to a mixer M the undistorted signals b, Fig. 10, through a channel B in the opposite sense to the modified signals a, which pass through an equalizer A of the kind described above. In the difference c between a and b, the modifications produced by channel A constitute a large percentage of the mean strength of the signals. In another embodiment, Fig. 11, the input circuit AB and output circuit connected to terminals CD of the network are the diagonals of a wheatstone bridge, one of whose arms comprises the input impedance of a reflectionlessly terminated retardation line. The amplitude of the indicial response depends on the unbalance of the bridge, and its shape on the design of the retardation line L. In a further embodiment, distortion in a transmission system may be corrected by inserting a device having the requisite indicial response in the feedback path of an amplifier. In another embodiment, Fig. 3, a dipole Z having the requisite indicial impedance is inserted in series with the filament of a high-resistance valve V whose output terminals are shunted by a resistance Ra. According to the Specification as open to inspection under Sect. 91, a ladder network may be used (Fig. 3, (Cancelled) not shown) in which the shunt arms comprise amplifiers giving adjustable gain while the series arms are provided by tappings on a distortionless transmission line. This subject-matter does not appear in the Specification as accepted.