Patent GB-628570-A (GB)
Title: Improvements in or relating to electric high frequency oscillation generators
IPC: H03B5/02
Filed: 1946-08-16
Abstract: 628,570. Velocity-modulation tubes. STANDARD TELEPHONES & CABLES, Ltd., and STARR, A. T. Aug. 16, 1946, No. 24497. [Class 39 (i)] [Also in Groups XL (b) and XL (c)] In a stabilized oscillator using an amplifier of the velocity modulation type, the input and output resonators are coupled through a stabilizing bridge which is the short-wave counterpart of the bridge stabilizing arrangement described in Specification 510,379, and comprises four transmission lines joined and arranged in such a way that the lines are conjugate in pairs and to which are coupled a frequency determining resonator and a thermally sensitive impedance for controlling the feedback. Fig. 1 shows a travelling wave amplifier 1 with its input and output resonators 4 and 5 coupled through a frequency and amplitude stabilizing bridge comprising a " Magic Tee " junction, arm 10 of which leads to a cavity resonator, and arm 9 to frequency and amplitude controlling thermistor elements 29 and 30 (Fig. 3). The thermistors are connected between the wall of the guide and rods 23, 24 secured to the side walls of the guide. Thermistor 30 is connected at an even or odd number of quarter-wavelengths from the junction of the guides, depending on whether the thermistor has a negative or positive characteristic. Thermistor 29 is controlled by currents from microphone 31 and is placed an integral number of one-eighth wavelengths from the junction of the guides to produce a varying reactance at the junction which frequencymodulates the oscillator. Any consequent amplitude-modulation will be counter-acted by the action of thermistor 30. Details of the coupling between the waveguides and the amplifier are shown in Fig. 2. Each end of the helical line 6 within the valve terminates on a metal cylinder 17, 18 one quarter-wavelength long, which face corresponding flanges of the resonators 4 and 5 and of the waveguides. Preferably the resonators and guides enclose one turn of the helix to produce the necessary coupling. The envelope of the valve is enclosed in a metal tube 21 around which the focusing coil 22 is wound. In an alternative arrangement (Fig. 6) the input and output resonators 36, 38 of a velocity-modulated valve are coupled by coaxial lines through a coaxial line network 42 to which are coupled a thermistor 48 and a frequency-determining resonator 45 to form a bridge. A second thermistor may be arranged in arm 47 to enable frequency-modulation of the oscillator in a similar manner to that described with reference to Fig. 1. The network, which is shown in detail in Fig. 7, comprises a coaxial line arranged in a closed ring with the arms radiating therefrom. These arms are arranged in pairs (i.e. 40, 41 and 44, 47) such that the difference between the two paths round the ring from one to the other member of a pair is an odd number of half-wavelengths. Thus the waves arrive in antiphase and will cancel if the interposed arms are balanced. Preferably the sections between adjacent arms should be an odd number of quarter-wavelengths. In the embodiment described the circumferential distances between members of a pair are a half and a whole wavelength and between adjacent members a quarter wavelength. Either type of line and bridge may be used with either type of amplifier. In addition the central conductor of the coaxial ring shown in Fig. 7 may be omitted and the arrangement operated in conjunction with waveguides.