Patent GB-647702-A (GB)
Title: Improvements in or relating to ultra high frequency apparatus having modulation control means
IPC: H01J25/10
Filed: 1944-03-13
Abstract: 647,702. Modulating systems for velocitymodulated tubes. SPERRY CORPORATION. March 13, 1944, No. 4688. Convention date, March 15, 1943. [Class 40 (v)] [Also in Group XL (a)] Modulation of electron oscillators or amplifiers of the velocity - modulation type is rendered linear by detecting the output signal and comparing it with the input signal, any difference due to distortion being fed back into the signal input circuit in such phase as to reduce the distortion. The modulation may be in amplitude or frequency or in a combination of both. As shown in Fig. 1, for amplitude modulation, the signal passes from terminals 11 through an amplifier 12 and transformer 13 to a control grid 14 of an amplifier 15 of the velocity-modulation type disclosed in Specification 531,251. Carrier input is fed to the resonator 16 by coaxial line 22<SP>1</SP>, and output is taken from resonator 17 by the line 23. A second output line 24 feeds a detector 25, the signal output being fed to the grid 27 of a double triode 28. The other grid 34 is fed with the original signal through an amplifier 32 and potentiometer 33, in equal amplitude and in opposite phase to that fed to the grid 27. The differential output voltage appearing across the anode load 38 is fed into the amplifier 12 in series with the original signal, its effect being to oppose and reduce the distortion appearing in the carrier output. The amplifier 15 may be replaced by an oscillator of the back-coupled type, or by oscillators of the kind described in Specifications 528,041 and 535,767, and the line feeding the detector 25 may pick up its energy from a radiating aerial connected to the line 23. Frequency modulation may be effected by applying the signal to a hollow cylindrical electrode 20, Fig. 5, surrounding the beam in the drift space, thus varying the phase of the back-coupling energy and causing a change of frequency. Any change of amplitude caused by the electrode 20 may be counteracted by applying the signal also to the grid 14. In Fig. 7, the retroactive voltage is taken from a collector electrode 52 and tapped resistance 57 and is applied in series between the signal input 11, 13 and the signal amplifier 12. The signal amplifier 12 is isolated from the high negative voltages on the grid 14 and cathode 21 by a network 58. Fig. 8 shows the invention applied to an oscillator having auxiliary electron beams and arranged in the manner described in Specification 647,700, to produce amplitude or frequency modulation or a combination. The oscillator comprises back-coupled cylindrical resonators 66, 67 traversed by three parallel electron streams from cathodes 72, 76, 77, which are held at increasingly negative voltages by sources 73, 78, 79. These driving-voltages are such that the electron grouping is in opposite phase in the two right-hand beams which therefore contribute no energy to the output resonator. The signal from a source 60 is applied to two potentiometers 62, 63, earthed at their mid-points. A tapping 65 applies one phase of the signal to control grid 74 of one stream, and a tapping 64 applies the opposite phase of the signal to control grids 81, 82 of the other two streams. For pure amplitude modulation the tap 64 is set so that the signal voltage on grids 81, 82 is half that on grid 74. For pure frequency modulation, grid 74 is disconnected by switch 80 from the signal source, and tap 64 is set to give full signal voltage on grids 81, 82. For maintaining linearity of modulation, output is taken to the detector 25 and the signal thereby produced is balanced against the original signal in the double triode 28, the differential output from which is fed to the grid 74. In Fig. 9 (not shown), a collector electrode is provided for the electron stream from the'cathode 72, and a load resistance 37 and balancing circuit of the kind shown in Fig. 7 replace the balancing circuit shown in Fig. 8.