Patent GB-540245-A (GB)
Title: Improvements in modulation systems for radio and like transmitters
IPC: H03F1/02
Filed: 1940-04-03
Abstract: 540,245. Wireless transmitting systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and PRICE, T. H. April 3, 1940, Nos. 6051 and 12752. [Class 40 (v)] Relates to amplitude modulation circuits, for radio transmitters in which the load is fed by two output stages excited in different phases, and during negative modulation cycles the phases are kept constant and the amplitudes reduced ; whereas positive modulation reduces the phase difference and augments the combined amplitude. In the circuit shown in Fig. 2, the carrier is applied to the grid of a modulating valve MA which is series modulated by a valve M connected to a modulating source SM. The output of the valve MA is applied across the horizontal diagonal of a bridge p, q, r, s giving in-phase voltages to the grids of amplifiers A 1 , A 2  from which the load R is fed in parallel. Carrier in quadrature phase is applied across the vertical diagonal of the bridge and energizes the grids of valves A 1 , A 2  in push-pull relation. A further valve A 3  has the carrier applied to its grid in series with a fraction of the modulating-voltage, and carrier is also applied, to its anode in the same phase as the grid. The valve A 3  is inoperative during negative modulation, so that the relative phases on the grids of valves A 1 , A 2  remain unaltered ; but during positive modulation, the operation of the valve A, reduces the quadrature excitation and brings the excitation of the grids of valves A 1 , A 2  more nearly into phase agreement so that the resultant load excitation increases. In a modification shown in Fig. 4a, the modulated input wave V, is applied across the horizontal diagonal of the bridge p, q, r, s, and the ends of the vertical diagonal are connected to the grids of output valves A 1 , A 2  which feed the load RA jointly, preferably through impedance inverters N 1 , N 2 . Energy is fed back from the load through inverter N 3  to a transformer T 2  connected across the vertical diagonal of the bridge. The feed-back voltage V 2  is in quadrature with the voltage V 1 , and gives a phase shift in opposite directions to the grid excitation of the valves A 1 , A 2 . - These valves are arranged to saturate at the mean value of the carrier, with the result that positive modulation causes a reduction in the ratio of V 2  to V 1 , bringing the excitation of the grids of valves A 1 , A 2  more nearly into phase agreement, and giving an increased load current. Further control of the Voltage V 2  is afforded by a pair of balanced amplifiers X, Y fed from the input V 1 , and the load RA respectively by potentiometers P 1 , P,. Their common output circuit Z is connected to the grid of a valve A 3  feeding the transformer T 2 . In a further modification (Fig. 5, not shown) a circuit similar to Fig. 2 is employed, except that the bridge p, q, r, s is placed on the input side of a pair of valves which are anode-modulated by a series modulating valve. Each valves feeds a separate amplifier and these are jointly connected to the load. Rectifying voltmeters may be coupled to the final amplifier circuits to indicate the voltages in these circuits, and a further similar voltmeter may be connected between them. The phase angle may be deduced from the voltmeter readings. Specification 537,076 is referred to.