GB1174070A - Improvements in Modulator Circuitry - Google Patents

Abstract

1,174,070. Amplitude modulation. C.I.T.- COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS. 22 May, 1968 [24 May, 1967], No. 24409/68. Heading H3R. [Also in Division G4] Circuitry providing an amplitude modulated radio frequency carrier comprises a generator of ratio frequency energy, a bridge circuit with two inputs and at least one output from which the amplitude modulated carrier is obtained, and a numerically controlled phase-shifting means to apply the said energy to the two inputs of the bridge circuit, simultaneously varying in complementary relation the phases of the two portions of the energy applied to the two inputs. R.F. energy from source 5, Fig. 2, is applied to a bridge having arms 1, 2, 3, 4 in which arms 1, 2, 3 comprise lines which are a quarter of a wavelength long at the radio frequency and arm 4 is three-quarters of a wavelength long; if the R.F. energy is fed in at the centre M of line 6 connecting corners A, B of the bridge and corners C, D are terminated in the characteristic impedance of the lines, then no output is obtained at corner C and a maximum output at corner D. Varying outputs are obtained at the points C, D by varying the connection points E1-E8 of generator 5 along line 6; this is performed by switch 10 so as to stepwise modulate the outputs at points C, D, giving two outputs mutually phased 90 degrees apart. Stepping of switch 10 is controlled by control element 11 connected to output terminals n11-8 of a logic circuit 12, the input terminals ml-8 of which are connected to the output terminals of cascaded binary counter stages comprising bi-stable multivibrators 14-17 driven from timing signal generator 13. The outputs from points C, D of the bridge are connected to switch contacts p 1 , q 1 respectively and by half wavelengths of line 19, 22 respectively to switch contacts P 2 , q 2 , and the output terminals S 1 , S 2 are connected to switches 20, 23 respectively, whereby a reversal of phase can be introduced. The switches are operated by means of control elements 21, 24 respectively, connected to terminals n 9 -n 12 of logic circuit 12, the corresponding input terminals m 9 -m 12 being connected to the output terminals of binary counter stages comprising bi-stable multivibrators 18, 18<SP>1</SP>. The Specification also describes diode switching arrangements for the switches 10, 20, 23 (Figs. 6, 7, 8, not shown), and a network comprising AND and OR gates, suitable for the logic circuit 12 (Fig. 9, not shown). The tapped portion of the line 6 may be an effectively quarter-wavelength long zig-zag metal strip 65, Fig. 10, sandwiched between insulating sheets separating it from two metal plates, switching being performed by means of diodes 30 connected to the points E 1 -E 8 in the metal strip. The arrangement may be used to feed two transmitter aerials with a set of sidebands to provide an omnidirectional VOR radio beacon or may be used to produce 90 c/s. and 150 c/s. modulated sideband signals in an I.L.S. system.