Patent GB-542175-A (GB)
Title: Design of broad band repeaters
IPC: H03F1/50
Filed: 1940-06-28
Abstract: 542,175. Impedance networks; valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd. June 28, 1940, No. 11003. Convention date, Sept. 29, 1939. [Classes 40 (iii) and 40 (v)] A broad-band attenuation-equalizing amplifier with stabilized negative feedback, and designed to repress noise of internal origin, is coupled to the lines by matching-transformers T1, T2 which are shunted on their high-impedance sides by condensers C2, C4, these condensers being included in the feedback path and so dimensioned in conjunction with associated networks that the line attenuation is equalized in the working range, and impedance-matching is effected over at least the upper portion of that range. A resistive feedback impedance 5 causes the gain to be uniform in the working range. Singing due to phase-displacement in the feedback loop at frequencies far above the working range is prevented by designing these combinations in accordance with the principles set out in Specification 514,567. The invention utilizes mathematical theorems given in Specification 534,802. Part of the current fed back from valve 3, Fig. 1, is lost through the shunt effect of the internal capacity C3 of the valve, and part of the feedback voltage is similarly lost as a result of the internal capacity C1 of valve 1; for instance the grid of valve 1 receives only a fraction Z1/(Z1+ Z2) of the voltage across the feedback impedance 5, where Z1 is the input impedance of the valve and Z2 is the impedance between the transformer terminals, so that at very high frequencies Z1, Z2 comprise only the impedances of the capacities C1, C2. Accordingly C2 must be made as large (or Z2 as small) as is practicable. The condenser-shunted transformers acting as twoterminal impedances in the feedback loop are also designed to have a real impedance which is small and nearly constant in the working range, rises sharply just above that range, and falls at higher frequencies to a smaller and uniform value. The effect of this impedance on the gain is shown at I, Fig. 5, and the desired gain characteristic is shown at II. The difference III is supplied by the networks N1, N2, Fig. 1. Corresponding curves for the phase shift in the feedback loop are shown in Fig. 6. Thus the transformers supply the sharply selective parts of these gain and phase characteristics, so that, for the residue of the characteristics, relatively simple networks N1, N2, can be used within the amplifier. The output transformer coupling is similar to the input coupling, and in a coupling of this kind the internal gain characteristic is defined as taking account of the capacity C2 alone, while the volume performance characteristic takes account of the internal capacity Cl of the valve in addition. The resistance on the high-impedance side is the same in both cases within the working range and rises at the highest working pulsatance w, Fig. 9, to a value RM which is matched to the resistance of the line, but it exceeds the latter somewhat at lower frequencies, Fig. 8 (not shown). In the design of a transformer coupling having the optimum resistance characteristic, use is made of general theorem where R is the resistance of a two-terminal network which degenerates into an effective capacity C# at high frequencies. The left-hand side of this equation is referred to as the " resistance integral ", and in order that the volume performance may be high it is necessary that this integral should be derived as much as possible from frequencies within the working range, and that the resistance above that range should be small. The " resistance efficiency" is then said to be high. For this purpose the capacity C 00 , which in the present instance is (C 1 +C 2 ), should be as small as possible if high volume performance is to be obtained ; whereas it has been stated above that C 2  should be as large as possible if maximum feedback is to be obtained, while C 1  is fixed by the construction of the valve. To reconcile these requirements it is assumed that the optimum value of C 2  is that for which a slight change in it produces equal and opposite changes in the resistance integral and in a certain formula which specifies the available feedback. The optimum value for C 2  is deduced as 24 y C1/k, where k is the asymptotic slope, in decibels per octave, of the gain-characteristic in the complete feedback route while y is a small fraction dependent on the phase margin in the circuit. In certain refractors designed for co-axial cables the value of k is 18 and that of C,=1.2 C 1 . If the maximum resistance RM, Fig. 9, at the high-impedance terminals of the transformer ocurs at the upper edge # of the working range and is equal to 1/ # C1, the resistance may be assumed to drop to a value Ro which is equal to 0.4 RM at the lower end of the range, and C 2  is then chosen, on the basis of the area under the resistance characteristic, Fig. 9, to have a value usually lying between 1.5 C 1  and 2.0 C 1 ; such values, as compared with 1.2 C 1 , favour feedback at the expense of noise-suppression. In order that Z, and Z 2  may be completely determined, where Z, and Z 2  are respectively the highside impedances of the transformer coupling without and with the capacity C, in shunt, Fig. 7 (not shown), the reactive parts X, and X 2  have to be deduced. Within the working range X 1  and X 2  are taken to be of opposite sign but, like the resistances, of equal magnitude, while above that range both reactances and resistances will be unequal. The values of X, and X 2  within the working range are found by means of equation (7) of the Specification in terms of C, and the resistance. The resistance above the working range which is requisite in order that these values of X, and X 2  may hold within that range is then deduced from a theorem, equation (8) of the Specification, which gives a relation between resistance and reactance for the whole range of frequencies from O to #. The reactance above the working range is then deduced from the resistance above the working range, and from the formulµ obtained by the above processes the requisite elements of a physically realizable network can be computed. An approximative method of carrying out the above principles of design starts, in view of the over-riding importance of those values of the re-actance which obtain in the upper portion of the working range, on a single value of the reactance, namely that at the upper limit # of the working range, and the reactance is taken to be equal in magnitude to the resistance RM at that frequency. This value is consistent with a variety of shapes which the resistance characteristic may have above w , and some of these are given in Figs. 10 to 14 (not shown). The optimum characteristic is determined by trial and error in a case in which the resistance is uniform throughout the working range, Figs. 15, 16 (not shown), and a practical verification of the result as applied to a simple constant-k filter is given with reference to Figs. 17, 18 (not shown). The same method as applied to a resistance characteristic resembling that shown in Fig. 9 is illustrated in Figs. 19. 10 (not shown), the corresponding value of C 2  being approximately double that of C 1 . The choice of a resistance characteristic above # being to some extent arbitrary, a shape favourable to the feedback characteristic may be chosen. From the chosen characteristics an actual network is calculated by a procedure described with reference to a 3-megacycle amplifier illustrated in Figs. 21, 22 (not shown) and a structure of the form shown in Fig. 23 is deduced. The analysis is applied by way of example to a 2-Mc/sec. coaxial system, Figs. 24 to 29 (not shown), and to a 3-megacycle system for telephony and television, Figs. 30 to 35 (not shown). When tube noise is the main defect to be avoided, the invention is applied, Fig. 2 (not shown), to the production of a flat attenuation characteristic in a section comprising a valve, its output transformer coupling, and a line; but if resistance noise is the main defect, the section to be equalized may comprise in addition the input transformer coupling of the next valve.