Patent GB-517516-A (GB)
Title: Improvements in or relating to electric wave transmission networks
IPC: H03H17/02
Filed: 1938-06-28
Abstract: 517,516. Impedance networks ; valve amplifying circuits ; cathode-ray tubes. BLUMLEIN, A. D., EALLMANN, H. E., and PERCIVAL, W. S. June 28, 1938, Nos. 19096/38, and 4939/39. [Classes 39 (i), 40 (iii) and 40 (v)] In a network which can be designed to have any prescribed attenuation characteristic and/or phase characteristic (preferably a constant time lag), the signal is divided into components which are subjected to prearranged attenuation and phase delay in a corresponding number of parallel paths and are eventually, superposed. The invention is specially suitable for shaping, in the neighbourhood of the cut-off frequencies, the characteristics of a filter used for cutting out one side-band from a band of television frequencies which have been transposed by modulation for the purpose of avoiding lowfrequency disturbances. In the system described in the first Provisional Specification, signals pass from input terminals 13, E1, Fig. 5, to output terminals 22, E2 through a delay network 12 which is tapped at intervals through high resistances, 16a, 16b ..., and the elements of current passing through the resistances are fed, in superposition to a terminal 22 either directly through line 18, if they have to be of positive sign, or with phase reversed through line 20 and a phase-reverser 21, if they have to be of negative sign. The dimensioning of the elements may be based upon a curve 23, Fig. 6, in which there is plotted, against time t, the output current I (t) produced when a fixed D.C. voltage (Fig. 2 not shown) is suddenly applied to the input terminals of a hypothetical network whose attenuation characteristic a (#) and phase characteristic b (#) are prescribed, Fig. 1 (not shown). The Specification gives a Fourier integral representing I (t) in terms of the functions a (#) and b (#), and states that if the network is so designed that an output current of the form I (t) is produced in the above circumstances, it will have the required characteristics a (#) and b (#). When a steady voltage is suddenly applied at terminals 13, Fig. 5, it travels along the network 12 at a finite speed and on reaching the tapping 15a sets up a small current through resistance 16a and terminal 22 ; the rise and persistence of this element of current are represented by curve 24, Fig. 6. When the voltage wave subsequently reaches tapping 15b, Fig. 5, it sets up through resistance 16b an element of current 25, Fig. 6, which is added to the element 24 in the output circuit, and by the additions of further current elements 26, 27 ..., suitably delayed, the total current 23 is built up, additive elements passing through the lead 18, Fig. 5, while elements to be subtracted pass through the phase-reverser 21. Alternatively the design may be based upon the current dI(t)/dt, Fig. 7 (not shown), which would be produced, by an impulsive input e.m.f. In a modification, Fig. 8 (not shown), a balanced delay network is used instead of the network 12, and thermionic amplifiers are used instead of the resistance paths 16a, 16b..., the phase reverser 21 is omitted, and the valve grids are connected through potentiometers to one or the other side of the delay network, according to the algebraic sign of the relevant current element; the leads 18, 20 are replaced by a further delay network. In the arrangement described in the second Provisional Specification, a prescribed transmission characteristic is built up by combining the current elements drawn from pairs of tappings, 13, 13<1>, 16, 16<1> . . . , Fig. 2, on a delay network 10, or a cable, the interval between successive tappings corresponding to a constant time delay #. By a suitable choice of the members of a pair, the transfer admittance relating the input 11 and the resultant output between the pair and earth can be given the form exp(-j#)T). 2A# cos r##, where # is the pulsatance, r is an integer, Ar is an adjustable constant; T is the mean of the delay times of the members of the pair of tappings, which are equidistant from the centre 17, so that T is the same for all pairs ; and T determines, saving a constant, the overall delay time between input and output. Thus the output from the rth pair is proportional to Ar and to the cosine of a multiple of the frequency. By combining the outputs of different pairs of tappings a Fourier cosine series can be built up and thus any transfer admittance characteristic which is an even function of frequency can be produced. By using the differences between the transfer admittances of the members of a pair of tappings instead of their sum, a term proportional to sin r## can be produced, and by means of a series of such terms a transfer characteristic which is an odd function of frequency can be built up. Fig. 4a shows an ideal even-function characteristic a in full line together with an approximation b in dotted line derived from three pairs of tappings ; the ordinates show the output response to an input of given amplitude, while the abscissae are proportional to the phase delay ## and therefore to the frequency. A characteristic determined by the sum of even and odd functions may be produced by the addition of functions of the kind described above, or by combining unequal fractions of the outputs from the members of each pair of tappings. In further modifications, Figs. 5, 6, 7 (not shown) the output is drawn from a tapping (which may be at the input end) on a delay network which is short-circuited or opencircuited at the far end, so that reflections take place ; or the line 10 may be reflectionlessly terminated at 36, Fig. 8, while the characteristic impedance has discontinuities at uniformly spaced points 37, so that reflections occur and are added together at the input terminals 32, 32<1>. Filters of the kind described, having a large number of pass and stop bands, such as B1. B2, B3 ..., Fig. 4a, may be combined with known filters which cut out all but one of the pass bands B1, B3 ..., so that the sharp cut-off given by the present filters may be utilized. The Specification describes other combinations which form a single-stop-band filter, a low-pass filter (Figs. 9a..9d, not shown) and a wide-band filter with a sharp lowfrequency cut-off (Figs. 10a..10d, not shown). Cathode-ray tubes. Elements or components of current may be added and subtracted, as required by the invention, by means of a cathode-ray tube having multiple pairs of plates, as described in Specification 525,145, the beam being deflected on to a target anode formed by a uniform strip of resistance material, one end being connected to positive potential and the other to an output terminal (Fig. 11 of Provisional Specification 4939/39, not shown). Phase-shifting networks. A network of the kind described in Specification 473,028, comprises resistance, inductance and capacity R, 6, C, Fig. 3a, associated with output terminals 28, 28<1>, and with an input valve 18, equal resistances 22, 23, and a blocking condenser 27. The time delay varies with frequency as shown in Fig. 3b for various values of Q, equal to R-<1>#(L/C). When a series of sine terms is developed as described with reference to Fig. 2, the terms are subject to a phasedisplacement of “ R which produces appreciable differences of time delay at the lower frequencies ; these are neutralized by means of a delay network of the kind described.