GB676840A - Improvements in or relating to microwave apparatus - Google Patents

Abstract

676,840. Valve modulating circuits. WESTERN ELECTRIC CO., Inc. June 30, 1950 [July 1, 1949], No. 16391/50. Class 40 (v). In an ultra-high-frequency apparatus having a valve with a grid septum forming a common boundary between the cathode-grid and anodegrid resonant cavities, a co-axial quarter-wavelength transformer is employed to match the anode-grid cavity to the output circuit. The carrier to be modulated is fed over wave-guide 57, Fig. 2, to the cathode-grid cavity 49, while the modulating signal is fed over network 65 to the cathode 40. The capacity between the cathode and the shell 44 in contact with the spring fingers 52 effectively shortcircuits the modulating source at carrier frequencies, the modulating frequency being prevented from passing down guide 57 due to its dimensions. The resonant cavity 50 formed by the plane grid 38, anode 39 and the walls of the cylindrical block 73, is tuned by displacing the block for coarse tuning and cylinder 74 for fine tuning. The length of the bore of cylinder 74 is chosen to be a quarter-wavelength at the selected side-band frequency to match the low impedance presented by the anode-grid cavity to the higher impedance of the output guide 70. To prevent energy at the side-band frequencies from passing into guide 57, adjustable screws 68 are provided while a filter 93 discriminates against unwanted frequencies in the output. Coarse tuning is effected by displacing the block 73 by means of tools passing through an insulating disc 82, Fig. 3, into screwed holes 76, the cylinder 74 being of such a length that the quarter-wave transformation is unaffected. Fine tuning is effected by means of a screw-driver 94 of a dielectric constant near unity which engages a rod 98 screwed into a disc 97 attached to the cylinder 74. Rotation of the screw-driver causes the cylinder to rise and fall, a pin 100 preventing the cylinder from revolving. A screwed stud 84 allows matching of the guide 70 and cavity 30. The band-width of the anode resonant cavity may be increased by slidable resistive elements 81 displaced by screws 80. The H.T. supply to the anode is fed through a quarter-wave stub 86 and wire 88, the stub being shorted by a small disc capacitor 89 to the walls of the cavity 30. A tuning screw 58, Fig. 2, improves the match between the guide 57 and cathode cavity 49. The anode current may be stabilized by returning lead 23 through a cathode resistor to a point negative with respect to earth. In a modification, Fig. 4 (not shown), a quarter-wave line is used to isolate the modulating source and a quarter-wave stub in the anode feed is terminated by a quarter-wave disc.