Patent GB-602464-A (GB)
Title: Electric space discharge circuits
IPC: H03C1/32
Filed: 1945-03-19
Abstract: 602,464. Inductive output tubes ; valve modulation circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 19, 1945, No. 6863. Convention date, March 27, 1940. [Class 40 (v)] [Also in Group XL (a)] An electron discharge device of the inductive output type comprises at least one electrode for accelerating the electron stream past a single tank circuit a source of modulating potential being applied to the positively biassed accelerating electrode to vary simultaneously the transit time qf all electrons in the stream and thereby modulate the angular velocity of the currents induced in the tankcircuit. The device may be used as an oscillator for frequency modulation or as an amplifier with or without regeneration for amplitude and phase modulation respectively. As applied to a F.M. oscillator, Fig. 1, the electron gun comprises an indirectly heated cathode 2, grid 4, accelerator electrodes 5, 5<1> collector electrode 6 and secondary emission collector 7. The electron beam is focused by magnetic lenses consisting of gaps cd, ef between iron sleeves 14, 15, 16 surrounding the glass envelope 1 and serially arranged with iron strips 17 and a yoke 18 in a magnetic circuit energized by a coil 19. The tank circuit consists of a doughnut shaped copper cavity resonator 12 symmetrically arranged around the glass envelope 1. The output coupling loop 21 thereof is connected to lecher wires 25 tuned at 26 and back coupled, over a line 23 adjustable by trombone sliders 24, to the tuned input circuit 9 to sustain oscillations. Modulation potentials are applied from source 30 to vary the normal positive bias on one or as shown, both accelerator electrodes 5, 5<1> which are directly connected to each other. The transit time of the electrons which move with substantially constant velocity throughout the length of the stream is correspondingly affected and proportionately varies the phase of the feed-back energy which causes a change in frequency. The phase shift required in the feed-back circuit to produce a given frequency change is kept to a minimum by making the Q of the input circuit 9 and the output coupling loop 21, and the feed-back path as low as possible. The coupling loop 21 may be replaced by any other coupling element(s) suitable for controlling the effective Q of the resonator which may be in the form of a concentric line. Compensator circuits may be necessary where large frequency deviations are required to make the frequency change proportional to the modulating potential and to keep the A.M. present to a minimum. For phase modulation, Fig. 2, the feed-back circuit of Fig. 1 is dispensed with and the H.F. source 32 to be modulated is applied across the grid and cathode of the valve to modulate the density of the electron stream. As in Fig. 1 the modulating potentials from source 30 are applied to the accelerator electrodes 5, 5<1> to vary the normal positive bias thereon. These potential variations affect the transit time of the electrons in the stream and thus change the phase of the input voltage on grid 4 relative to the induced voltage in the resonator 12. The input voltage phase modulated in accord. ance with the signal voltage from source 30 is taken from coupling loop 21. The Q of resonator 12 is made as low as possible to reduce the amplitude modulation present or a limiter stage may be used to remove the A.M. For amplitude modulation, Fig. 3 (not shown), the circuit of Fig. 2 is provided with a regenerative feed-back path from the coupling loop 21 to the input circuit 2, 4. As before, the modulating voltages applied to the accelerator electrodes vary the transit time of the electrons in the stream. This varies the phase between the grid voltage and that induced in the resonator which in turn causes a corresponding change in the regeneration resulting in A.M. in the output voltage. In order that the coincidental phase modulation present may be a minimum, a high Q is chosen for the resonator. In a modification of the F.M. oscillator of Fig. 1, the accelerator electrode 5<1> is dispersed with and the accelerator electrode 5, Fig. 4 (not shown), connected to the cavity resonator 12. In such a case if the output is taken off by means of a coaxial line the outer conductor of which is grounded, the resonator is isolated from ground by a condenser of low R.F. impedance and the modulating potentials on the accelerator electrode isolated from the resonator by R.F. chokes or resistors.