Patent GB-485959-A (GB)
Title: System for amplitude modulation employing impedance inversion for high-frequency electric signalling
IPC: H03C7/02
Filed: 1937-06-01
Abstract: 485,959. Modulating systems ; impedance networks. PHILCO RADIO & TELEVISION CORPORATION. June 1, 1937, No. 16217. Convention date, June 10, 1936. [Classes 40 (iii) and 40 (v)] A carrier wave source S, Fig. 2, is connected to a line feeding an aerial or other load (represented by R) through an impedance-inverter N1, the surge impedance of which matches the impedance of the line ; and modulation is effected by a signal-varied resistance RM also connected to the line through an impedance-inverter. The inverters may comprise transmission lines of effective electrical length equal to a quarter wave-length, or an odd multiple thereof; or they may comprise lumped impedances L, C1, C2 arranged as shown in Fig. 4 or in other networks such that the impedance Z measured at one end is inversely proportional to the resistance R connected across the other end. The Specification gives formulae for the impedance at frequencies differing by a small amount from the carrier frequency. Fig. 1 is a diagram of a radio transmitter using quarter-wave inverters 3, 10. The wave generator 4 is connected through frequency - multipliers and amplifiers 5, 6, 7 to a tank circuit 8, which is coupled to the quarter wave-length inverter 10. At its other end 13, this inverter is connected to a line 11 feeding a quarter-wave aerial 12 projecting axially through a flat earthed conductor 14. The point 13 is connected to the second inverter 3, which is terminated by a symmetrical pair of valves 2 acting as dissipative resistances of a value dependent on the signal voltage applied to their grids from a signal amplifier V4, which may itself be fed from an amplifier of the type described in Specification 485,990. The high resistance of the valves 2 appears as a low effective resistance at 13 comparable with that of the line 11 with which it is in parallel. The low effective resistance of the source comprising the tank circuit 8 is effective at the point 13 as if it were a source of high internal resistance and therefore supplying a constant current under varying load. As the load varies due to the variable shunting effect of the modulating impedance on the aerial feed line, the aerial current varies in the rhythm of the signal. The aerial current responds immediately to the modulating signal, however steep-fronted the signal may be. The system is thus particularly applicable to television modulation, in which the signal comprises very high frequencies. The modulating-valves 2 may have their anodes connected to the lines 3 and their cathodes earthed, as shown in Fig. 1, or the valves may be connected separately across the line and in reverse direction, the cathodes being connected to their separate earthed batteries through carrier-frequency chokes, Fig. 8 (not shown). The capacity of the valves 2 may be compensated by shortening the lines, by adding parallel inductance, or by extending the lines by a short-circuited section about one-eighth of a wave-length long. Several pairs of modulating-valves 2d, 2e, 2f may be connected through quarterwave lines to points on the carrier feed line which are separated by a quarter wave-length, as shown in Fig. 9. In this case separate signals, Em may be applied to the respective grid pairs, and in a television system the video signal and the synchronizing signal may be applied to the respective valves 2d, 2e. The feed line 34 is connected to the mid-point of a quarter-wave line which feeds a half-wave aerial 12d. When lumped impedances are used in the impedance-inverters as shown in Fig. 10, one of the inverters N1 may comprise a resonant output circuit for the wave generator or amplifier S. The other impedance-inverter N2 is connected between the load impedance R and the modulating-valves 2. In Fig. 11, the signal is applied in push-pull relation to the grids of four pairs of valves 2c each terminating an impedance-inverter I, II, III, IV. Two of these are connected as shunts across the terminals a, b and c, d of the aerial feed-line 11c, and the other two in series with the line between the terminals a, d and b, c.