Low-noise arrangement with a running time roehre, in particular drift field roehre

摘要

780,811. Travelling-wave tubes. ENGLISH ELECTRIC VALVE CO., Ltd. July 25, 1955 [Nov. 18, 1954], No. 33394/54. Class 39(1). In a travelling wave tube there is interposed between the gun 2, 3 and wave retarding, helix 7 means such as a cavity resonator 13 or prehelix at a point of maximum noise current to extract velocity and convection current noise and to feed it via suitable amplitude and phase adjusting means 17 to give it equal amplitude and opposite phase to the noise signals in the output of the helix so that when mixed therewith noise can be eliminated from the output over a limited range of frequencies. In the form shown the gun 2, 3 is followed by a " drift tube " 4, noise extracting cavity resonator 13 disposed at a noise current maximum, velocity jump electrodes 5, 6 and helix 7 having input and output waveguides 11, 12. The extracted noise is fed to the output by coaxial line 14 and waveguide section 16, a delay line such as a length of cable and suitable phase and amplitude adjusting means (all represented by 17) being provided to give the extracted noise signal the right amplitude and phase for mixing with the main output signal. Waveguide 16 forms part of or is associated with output waveguide 12. The extracted noise signal and main amplified signal can be mixed without change of frequency or their frequencies can first be shifted by a common local oscillator and the resulting intermediate frequencies then mixed. The conditions are so adjusted that there is substantially no feedback of energy along line 14 and it is stated that the tendency to self oscillation can be further reduced by making the helix dispersive. If the cavity 13 is not quite in the best position to pick up noise, the potentials of the velocity jump electrodes and drift tube can be adjusted to improve noise pick up. The invention may be combined with known travelling wave tubes in which the helix starts at a noise current minimum to minimise noise in the output or if this condition is not fulfilled the voltage of a drift tube preceding the helix can be adjusted to minimize noise in the output.