GB675439A - Improvements in or relating to phase modulators for electric carrier waves - Google Patents

Abstract

675,439. Phase modulation. STANDARD TELEPHONES & CABLES, Ltd. July 12, 1950, No. 17455/50. Class 40 (v). A phase modulator, of the kind in which phase-shifted versions of the carrier wave are amplitude-modulated in opposed senses by the signal and then re-combined, employs a pair of principal modulator valves 1, 2 and a pair of auxiliary modulator valves 22, 23 with the object of securing linear relation between carrier phase-shift and signal voltage. The carrier is applied at terminal 6 to the grids of both pairs of valves, the lead to valve 1 including a phaseretarding coil 7 and the lead to valve 2 a phaseadvancing condenser 8. The leads to valves 22, 23 include coupling capacities 28, 29 large enough to avoid substantial phase shift. The modulating signal is applied through a transformer 15 in push-pull to the grids of both valve pairs. The valves 1, 2 are so biased that they operate as linear amplitude modulators and their combined anode output is fed to one section of a tapped primary coil 3, the secondary of which is connected to output terminals 16, 17. The other section of the coil 3 is connected to the anodes of both auxiliary valves 22, 23, the grids of which are biased to near cut-off so that the valves are non-effective except when substantial positive signal voltages are applied from the transformer 15. Thus for signal peaks of one polarity, the phase-shifted wave at the output terminals 16, 17 resulting from the combined output from the valves 1, 2 is given a supplementary phase shift by the output from valve 22, and the valve 23 is ineffective. For peaks of the other polarity, the supplementary phase shift is provided by the valve 23. Bias adjustments by the cathode resistors 20, 21, 25, 27 enable modulation distortion to be substantially eliminated. The triode valves shown are preferably replaced by pentodes.