Patent GB-518969-A (GB)
Title: Improvements in electrical tuned circuits
IPC: H03H7/01
Filed: 1938-09-05
Abstract: 518,969. Impedance networks. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and BRAILSFORD, J. D. Sept. 5, 1938, No. 25933. [Class 40 (iii)] In order that the selectivity of a radiofrequency filter, to which the aerial is coupled, may be independent of frequency, the resistance of the filter is made proportional to its inductance by the addition of a network whose effective resistance increases as the resonant frequency falls. The resistance thrown into the filter by the aerial may vary with frequency or be constant, and the additional network is designed accordingly. If the resistance of the aerial system is constant, this may take the place of the resistance in the additional network. The principles underlying the invention will first be explained. The network which must be added to a circuit having resistive inductance shunted by capacity (Fig. 1, not shown), in order that the selectivity may be constant, may have the form shown in Fig. 3a, the parts being so dimensioned that the resistance characteristic of the network coincides with the required characteristic at three points in the working range of frequencies. Alternatively a resistance R, Fig. 4a, of definite value, e.g., 100 ohms, may be so coupled to the terminals 1, 2 that the effective resistance between these terminals conforms to the desired characteristic. Other forms of twoterminal network suitable for the above purposes are described, Figs. 3b, 4b (not shown). Such a network may be added as at Z1, Z2, Fig. 5, to arms of a band-pass coupling ; and in order that the coupling reactance X may bear a constant ratio to the coil resistance, so that a constant band-width may be obtained, the coupling reactance is designed as a mixed coupling which remains approximately constant over the working range. The network Z2 may be omitted, and the coupling may be redesigned with unequal meshes, Fig. 6 (not shown). The circuit may be coupled to the aerial loosely, in order to minimize the effect of mis-matching of the aerial resistance, or the resistance of a feeder matched to a distant aerial may be coupled to the filter by an L-section, Fig. 7 (not shown) whose arms may have the forms illustrated in Figs. 8a, 8b, 8c (not shown). Fig. 9 shows a circuit embodying the invention and having a correction circuit Z in the first circuit of the filter, a mixed band-pass coupling X, and an aerial system throwing into the filter a constant resistance. Or the constant resistance (100 ohms) of a matched feeder AF, Fig. 10, may be coupled to the band-pass filter through a resistance-equalizing arm RE of the kind shown in Fig. 4a ; and the grid of the first valve may be connected to a tapping t if its dielectric loss is high or if its capacity is subject to variation. A filter having two coupling stages employing an equalizing network of the kind described with reference to Fig. 4b (not shown) is described with reference to Fig. 11 (not shown). The response curves at three different mid-band frequencies in the broadcast range are approximately coincident, Fig. 12 (not shown). If the aerial throws into the filter a resistance which varies with frequency, this resistance may constitute the whole of the correcting resistance of the correcting circuit shown in Fig. 4a or 4b. An increasing amount of resistance is thrown into the filter from the aerial system as the frequency is lowered by manual tuning, and the elements of the aerial coupling and correction network are so dimensioned that the resistance thrown in maintains the total effective resistance constant.