GB710666A - Improvements in or relating to amplitude modulated carrier wave transmitters - Google Patents

Abstract

710,666. Modulating systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 11, 1951 [Dec. 11, 1950], No. 29026/51. Class 40 (5) In an amplitude-modulated radio transmitter wherein negative feedback is used for the reduction of distortion, the modulated waves from the output stage are de-modulated and are mixed with the modulating signal reversed in phase, the resultant being fed back in reversed phase to re-modulate the transmitter. As shown in Fig. 1, the carrier source 1 is coupled through amplifier 2 and modulators 3, 4 to the transmitting aerial, and the modulating source 5 is coupled through an amplifier 6 to a transformer 7 whose secondary coil is: in the anode circuits of the modulating stages 3, 4. The output wave is fed to a de-modulator 9; and the detected signal amplified at 10, is mixed differentially at 11 with the modulating signal from 5 fed through a phase-adjuster 13 and amplifier 14. The adjuster 13 simulates as regards phase shift, the effect of the amplifier 6 and transformer 7 in the modulating channel. The output from the mixer, which comprises only distortion components, is amplified at 12 and applied to the grids of one or both of the valves 3, 4 to modulate the carrier in negative sense so as to balance out or reduce distortion in the radiated wave. In Fig. 3, the distortion components from amplifier 12 are fed to the grid of a triode valve 19 shunted across a resistor 20 which is connected in series with a low resistance 21 in the grid-cathode circuit of the final push-pull stage 22 of the transmitter. The valve 19 operates as a cathode-follower, and modulates the grid voltage of the valves 22. A further cathode-follower stage may be inserted between the amplifier 12 and the valve 19. For high radio frequencies, the valves 22 may be arranged' to operate as grounded-grid amplifiers. In another modification, Fig. 2 (not shown), the amplifier 12 is connected as a varying shunt resistance to the anode supply circuit of the h.f. stage 3. The voltage at the output of the mixer 11 may be used to measure the noise and disturbance level for monitoring purposes.