Patent GB-310915-A (GB)
Title: Improvements in or relating to electric oscillation generators
IPC: H03B5/10
Filed: 1928-02-03
Abstract: 310,915. McLachlan, N. W. Feb. 3, 1928. Thermionic generators and modulating systems.-Relates to oscillation generators of the kind employing negative-resistance electronic valves, and comprises several circuit arrangements designed principally for generating both a carrier frequency and a modulating frequency whereby modulated oscillations are obtained. Fig. 1 shows one circuit arrangement, in which two or more oscillatory circuits 6, 7 tuned to different frequencies are connected in series in the plate circuit of a valve 1. A grid 5 is maintained at a voltage higher than that of the plate 4 so as to secure a negative-resistance characteristic for the plate circuit. Oscillatory currents are shunted from the batteries 8 by means of bye-pass condensers 10 and chokes 9. A further grid 3 may be connected to a point of lower voltage on the battery 8. Variable resistances may be connected in or across the circuits 6, 7 to adjust the amplitude of oscillation and the degree of modulation, and series resistances may be used for adjusting the voltages of the electrodes. A tuned circuit may be variably coupled to the lower frequency circuit 7 for adjusting the strength of its oscillations (Fig. 2, not shown). The low-frequency circuit 7 and the high-frequency circuit 6 may be connected in parallel in the plate circuit, with suitable chokes and condensers so placed as to direct the different. frequencies into the desired circuit (Fig. 3, not shown). Direct current from the battery 8 mav be fed to the plate 4 through a shunt path including chokes. Several valves may be connected in parallel; or valves with photo-electric cathodes, such as described in Specification 279,937, [Class 39 (i), Electric lamps, Arc &c.], may be used. In a modification shown in Fig. 4, the low-frequency circuit 7 is connected in the plate circuti of the valve 1<a>, whilst both the low-frequency circuit 7 and high-frequency circuit 6 are connected in series in the plate circuit of the valve 1, both valves operating as negative resistances. Frequencies may be stabilized by piezo-electric crystals connected across the oscillatory circuits. Further modulation may be applied to the oscillations by substituting A.C. sources for the batteries 8, by applying A.C. voltages to the grids, by influencing the electron flow by alternating magnetic or electrostatic fields, or by interrupting the activating light when photo-electric cathodes are used. In telegraphy, keying may be effected by altering the grid voltage, or by short-circuiting one of the circuits 6, 7. When the oscillator is used as a wave-meter, it may be adjusted so that the beat due to interference between the high-frequency wave and the wave to be measured is of the same frequency as the generated low-frequency wave. Any variation will then give a beat between the two low frequencies. Fig. 5 shows a modification for producing low-frequency interruptions. A choke 17 of high inductance and low selfcapacity is connected in the plate circuit of a negative-resistance valve 1, the various constants of the circuit (including the negative resistance of the valve itself) being so chosen that whilst the total effective damping is negative the frequency of oscillation (as given by a formula in the specification) is imaginary. A high frequency circuit may also be connected in the plate circuit, so that interrupted high-frequency oscillations are obtained. In a further modification, a circuit similar to that of Fig. 5 is used for producing low-frequency interruptions, but by increasing the capacity across the choke and the negative resistance of the valve, a real value of the frequency is obtained, whilst the damping is negative over only a portion of the working length of the characteristic.