Patent GB-664387-A (GB)
IPC: H03C7/02
Abstract: 664,387. Modulating systems. RADIO CORPORATION OF AMERICA. Dec. 29, 1949 [Dec. 30, 1948], No. 33240/49. Class 40 (viii). [Also in Group XL (c)] Depth of modulation in a radio transmitter is adjusted by feeding carrier from a source 1, Fig. 4, to a power-divider 30 of controllable ratio, one path 33 leading direct to one input 36 of a decoupling bridge 35, the other path 34 leading through a modulator 38 and a phaseadjuster 43 to a second bridge input 37, and the aerial 36 or load circuit being connected at a third bridge terminal 66. The source 1 is connected through quarter-wave lines 2, 3 of surge impedance Zc to impedances Z1, Z2 which are controllable in opposite senses by an adjuster 8 in such a way that Z1, Z2=Zc<SP>2</SP>. The resistances presented by lines 33, 34 are also equal to Zc. The impedances Z1, Z2 may comprise quarterwave lines 22, Fig. 2, closed at the outer end by a variable capacity 21. A stub 23 at the outer end is tuned to provide parallel resonance with the capacity 21 when at its minimum. This provides low impedance at the input end of the line 22. The stub 25 is adjusted to resonate with the high inductive reactance at the input end when the capacity 21 is at a maximum. If the impedance Z1 is of the form shown in Fig. 2, and the impedance Z2 is similar but with a quarter-wave line connected in front, the two capacities 21 can be varied in identical fashion. Thus by manual adjustment of the impedances Z1, Z2 the division of carrier power between the leads 33, 34 may be varied. The lead 34 terminates in a similar power-divider 31, in which the fraction passing to the output lead 40 is cyclically varied by a motor 39 driving the link 8a controlling the impedances 6a, 7a. The output lead 40 feeds modulated carrier to a second similar power-divider 32, also controlled by the motor 39, which accentuates the amplitude modulation, but balances out undesired phase modulation effected in the stage 31. With this object, the reactances 6b, 7b are made equal in value but opposite in sign to the reactances 6a, 7a. The modulated carrier in output lead 41 is fed to a phase-adjusting bridge 43 (shown in detail in Fig. 3), in order to ensure that carrier energy reaches the terminals 36, 37 of the de-coupling bridge 35 in the same phase. The bridge 35 is of the kind shown in Figs. 1 and 4 of Specification 20927/49 (as open to inspection under Sect. 91), having three quarter-wave arms 62, 63, 64 and one three-quarter wave arm 65, with inputs applied at terminals 36, 37 and with output terminals 66, 67 connected to an aerial 36 and a dummy load 68 of equal impedance. The arrangement is such that while the two inputs are decoupled relative to each other, the inputs add in the aerial and subtract in the dummy load. If the aerial is connected to the terminal 67 in place of the dummy load, the pure carrier may, by suitable adjustment of the power divider control 8, be balanced out in the aerial, and only the side-bands radiated. Fig. 3 shows the phase-adjusting bridge 43 with the several arms drawn as two-wire lines. Three of the arms 45, 46, 48 are quarter-wave arms, and the fourth arm 47 is crossed over so that in effect it is a three-quarter-wave arm. Input is applied at the terminal 51 and output taken at 52, the other corners 49, 50 being connected to lines 57, 59, the lengths of which are simultaneously adjusted by moving the shorting bars 58, 60 but one of them always being a quarterwave longer than the other. Owing to reflection effects at the bars 58, 60, the phase of the carrier emerging at 52 is dependent on the length of the lines 57, 59. These lines may terminate in an open circuit instead of a shortcircuit; or they may be terminated by adjustable lumped reactance elements 53, 54, Fig. 4. The modulating system may be used in connection with the landing of aircraft. U.S.A. Specification 2,506,132 is referred to.