Patent GB-692182-A (GB)
Title: Improvements in or relating to ultra-high-frequency circuits
IPC: H01J21/06
Filed: 1948-04-06
Abstract: 692.182. Thermionic cathode tubes. PHILIPS ELECTRICAL, Ltd. April 6, 1948 [April 9, 1947], No. 9660/48. Class 39(i) [Also in Group XL(c)] A circuit for amplifying or generating ultra-highfrequency electrical oscillations includes a discharge tube wherein oscillations are set up between the cathode and control grid and wherein the control grid is connected to a secondary-emission electrode through a capacitive impedance which is so small at the frequency of the oscillations that the control grid and secondaryemission electrode have substantially the same oscillatory component of potential, and wherein the output circuit is connected between the anode and the secondary-emission electrode or the control grid. The control grid potential is slightly different from and preferably negative with respect to the cathode potential, the anode potential is positive with respect to the cathode potential and the potential of the secondary-emission electrode is positive with respect to the cathode potential but considerably less than the anode potential. The capacitive impedance is preferably located within the tube to minimize the self-inductance of the wiring and the control grid and secondary-emission electrode are preferably connected, so far as ultra-high-frequency oscillations are concerned, to a point of constant potential, preferably earth. The capacity between the control grid and secondary-emission electrode is large in comparison with all other inter-electrode capacities in the tube. Electric discharge tubes with bilaterally emitting or cylindrical cathodes and cylindrical (not necessarily circular cylindrical) grids in the neighbourhood of the emitting layers on the cathodes are particularly suitable. An amplifier circuit is shown in Fig. 1, the voltage on the control grid 3 being zero, on the cathode 2 being + IV, on the anode 5 being +400V and on the secondary-emission electrode 4 being + 150V. Other voltages may be employed but the anode voltage must be approximately twice the voltage on the secondaryemission electrode. A capacity 8 of about 50 pF. is connected between the control grid 3 and the secondary-emission electrode 4. The input circuit 6 is connected between the cathode 2 and the control grid 3. The output circuit 7 is connected between the secondary-emission electrode 4 and the anode 5. For a wave length of 40 cms. an energy-amplification factor of 50 is obtained over a band-width of 20 Mc./s., and for a wave length of 100 cms. an energy-amplification factor of 225 is obtained over a band-width of 3.5 Mc./s., and a factor of 100 over a band-width of 12 Mc./s. Fig. 2 shows an electric discharge tube 1 for use in a circuit according to the invention wherein the cathode 2 is surrounded by a control grid 3 provided at one end with a screening cap 20 and the the other end with a flange 15 shaped as an annular metal disc which acts as a screen between the cthode and the other elements and as a conductive path between the pins 11 and the grid 3 and also as one of the plates of a condenser between the control grid 3 and the secondary-emission electrode 4. The dielectric of the condenser is an annular disc 16 of mica and the other condenser plate is a metal ring 17 connected to the secondary-emission electrode 4. The anode 5 comprises a plurality of, e.g. four, rods and the secondary-emission electrode 4 may consist of rods which are preferably profiled and arranged on a circle. The rods of the secondary-emission electrode 4 are provided with cooling fins 18 connected to pins 12, preferably by means of a disc (not shown) in a manner similar to that adopted for the control grid 3 and the pins 11. Disc-seals may be used instead of pins, at least for the control grid and the secondary-emission electrode. The pins are connected to electric resonators in the form of coaxial tubes 13 and 14. Fig. 3 illustrates a method of providing capacity between the control grid and the secondary-emission electrode by using the cooling fins 18 as condenser plates. The control grid 3 may consist of a wire helically wound on supports 21 and the anode rods 5 are arranged in the electron shadow of conductors 22 connected to the control grid. Two rods forming the secondary-emission electrode 4 are each provided with a cooling fin 18, on either side of which mica plates 24 are provided to constitute the dielectric of the condenser. The other condenser plates are metal plates 23 connected to the control grid, for example by the flange 15 shown in Fig. 2 and serving as a screen. The circuit described may also be used for generating oscillations, positive feedback between output and input circuits being provided.