Patent GB-679760-A (GB)
Title: Improvements in electronic signal translating devices
IPC: H01J25/42
Filed: 1950-05-05
Abstract: 679,760. Electron transit time apparatus. ZENITH RADIO CORPORATION. May 5, 1950 [May 10, 1949], No. 11179/50. Class 39 (i). A signal translating device of the travelling wave amplifier type suitable for use in the range 100-1,000 megacycles comprises an elongated cathode parallel to the axis of a low-velocity transmission line, electrons from which interact twice with longitudinal portions of the line; these portions may be opposite sides of a helix of rectangular section, the first portion controlling the electron stream according to its instantaneous potential, the same electrons interacting with the second portion, which is longitudinally displaced with respect to the first, to induce a signal which everywhere reinforces the signal wave travelling along the line. In the embodiment of Fig. 2A, the helical transmission line 32 is maintained at a direct potential near that of cathode 20. The potential of an accelerating electrode 35 is adjusted so that the transit angle along the electron trajectory 36, measured between the points of interaction with control portion 33 and receptor portion 34, is 180 degrees at the signal frequency. A magnetic field perpendicular to the plane of the paper is of such intensity as to curve the electron trajectory so that the two points of interaction with the helix are displaced axially by -wavelength of the travelling signal wave. For such a wave moving in the forward direction from input terminal 37 to output terminal 38 the phase difference between the controlling signal at control portion 33 and the induced signal at receptor portion 34 is 360 degrees, made up of 180 degrees due to the electron transit angle, 90 degrees due to the axial displacement, and 90 degrees due to space charge coupling between the electrons and the receptor portion 34. For a signal wave travelling along helix 32 in the opposite direction, the phase difference due to both transit angle and space charge coupling remain unchanged, but the 90 degrees phase shift caused by the magnetic field is reversed in direction; thus a direction-sensitive attenuation characteristic is imparted to transmission line 32, and a stable exponential amplitude increase of a signal wave travelling in the forward direction may be obtained. In Fig. 7A, of which Fig. 7B is an end view, the electron trajectory is not curved by a magnetic field, an equivalent effect being obtained by winding the transmission line 75 as an oblique helix such that transversely aligned points on control portion 33 and receptor portion 34 represent an effective displacement of -wavelength of the travelling signal wave. Improver operation may be obtained by the addition of focusing electrodes 42, 43 and collector electrodes 44, 45, the latter having a potential intermediate between that of the former and the accelerating electrode 35. The embodiment of Fig. 6A has a double-sided cathode 20 between two helical transmission lines 52, 53, of which adjacent ends are connected by an impedance-matching coil 66, forming a two-stage cascade-connected amplifier. The input and output terminals 67, 68 are thus at the same end; with an electrostatic shield 69 between them. By applying different potentials to corresponding electrodes of the two halves of the device, or by utilizing different predetermined electrode spacings, different attenuation characteristics can be imparted to each half, allowing of broad band amplification. In Fig. 8 (not shown), an additional repeller electrode reflects the electrons completely, the longer transit period allowing of operation at lower frequencies. In the arrangement of Figs. 9A (not shown), and 9B, the cathode 20 is disposed inside the helical line 75; accelerating electrode 35 and collector electrodes 44, 45 being as in previous embodiments. A virtual cathode is set up adjacent control portion 33, which now functions as a negative transconductante control grid; electrons passed by this control portion are collected by an external electrode 85; while rejected electrons are redirected towards accelerator 35 and thence towards receptor portion 34, where they induce a re-inforcing signal. This again is an arrangement dispensing with a magnetic field, and since the phase conditions are the reverse of those obtaining in the arrangement of Figs. 7A arid 7B, the angle of obliquity of the transmission line windings 75 is in the opposite direction. In a further embodiment, Fig. 10 (not shown) the control and receptor portions are formed by adjacent, instead of opposite, sides of the transmission line. Fig. 12 (not shown) is a practical form of the device, some exemplary dimensions and operational data being given.