Patent GB-250539-A (GB)
Title: Radio transmitting apparatus
IPC: H03C1/20
Filed: 1926-02-11
Abstract: 250,539. Standard Telephones & Cables, Ltd., (Assignees of Farrington, J. F.). April 7, 1925, [Convention date]. Thermionic generators and modulating systems and apparatus; transmitting systems and apparatus. - In a wireless transmitter in which the frequency of oscillations derived from a thermionic oscillator is stepped up to a high value by means of harmonic-producers one of the latter functions also as a modulator, means are provided for preventing variations in the load from affecting the frequency, and a phase-regulating network is associated with the oscillator in order to bring the plate current of the latter into phase with the grid voltage. The apparatus is protected by an electrostatic shield and is suspended on springs to prevent disturbance of the circuit constants which determine the frequency. Thermionic oscillator. In the oscillator 1 the fundamental frequency (a harmonic of which is ultimately utilized) is determined by the circuit 2, the filament being connected to the middle and the grid and plate to the ends of the inductance in this circuit. Plate voltage is supplied by the generator 3 through choke 4, a shunt 4<1> being provided for stray high frequency currents. In order that the plate current mav be in phase with the grid potential a phase-shifting device is employed, comprising an adjustable resistance 28 and condenser 29. Generating harmonics. The output of the oscillator 1 is amplified at 7 and applied to the harmonic producing valve 8 in such strength that the strongly negatively biased grid of the latter valve becomes strongly positive during part of each cycle, the output of the valve 8 being consequently rich in harmonics. The coupling 15, 16, is tuned to the octave of the fundamental frequency, and the frequency is again doubled by a second harmonic-producing valve 17, the circuit 19, 20, being tuned to the double octave of the original frequency. The output of the valve 17 is modulated and applied through an amplifier 26 to the transmitting aerial 27. Intervalve couplings. The oscillator 1 is coupled to the amplifier 7 through a non- inductively wound potentiometer 6, and the remaining valves are coupled by circuits comprising inductances and condensers as shown. Preventing variation of frequency with load. In order that the frequency of the oscillator 1 may be unaffected by variations in the load, the amplifier 7, the grid of which is always negative, is interposed between the oscillator 1 and harmonic generator 8, to prevent back action of the latter on the former. Further, the circuits 15, 16, and 19, 20, are tuned to harmonics of the fundamental and so act as short circuits to currents of the latter frequency. Grid biasing arrangements. The source of current 31 is associated with potentiometers 44, 34, 36, 39, and filters 45, 32, 37, 40, for supplying separately adjustable grid bias to each valve, condensers 46, 35, 38, 42, being provided as short circuits for stray high frequency current. Keying and modulating. For telegraphic purposes the key 48 may be employed to control the circuit of the coil 47, which affects the frequency of the oscillator. For telephony, the microphone 21 controls the plate current of the modulator 22 which is arranged on the constant current system in conjunction with the harmonic generating valve 17: thus the current through the low frequency choke 23 is substantially constant, a high frequency choke 18 being also provided.