GB900693A - Improvements relating to frequency modulated oscillators - Google Patents

Abstract

900,693. Frequency-modulated oscillators. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 22, 1958 [Dec. 20, 1958; Oct. 14, 1958], Nos. 39584/57 and 32756/58. Class 40 (5). An F.M. oscillator comprises two valves 1, 2 (Fig. 1 of drawing accompanying the first Provisional Specification connected to a frequency-determining resonant circuit 6, 7, each valve being capable of sustaining oscillation in the external circuit when the other valve is not conducting, a phase-shift circuit 4, 5 being connected in the feed-back connection between the external circuit and the control grid of valve 2 only, the respective sustained oscillation being of different frequency due to this phase difference in the feedback signals. The output is taken at 8, and a common cathode resistor 3 maintains the total current drawn by the valves constant. The mean potential at the grid of valve 2 will be varied positively or negatively with respect to earth by the modulating signal 1 applied through choke 11, valve 2 carrying all the current if the modulating signal is sufficiently positive and valve 1 carrying all the current if it is sufficiently negative, so that the oscillator frequency has definite upper and lower limits corresponding to which valve carries all the cathode current. The valves may be replaced by transistors, the phase-shift circuit by different combinations of resistances, inductors and capacitances and the tuned circuit by a crystal magnetostriction resonator. A phase-shift network may also be provided between the grid of valve 1 and the tuned circuit 6, 7 and the modulating signal fed in push-pull to both grids or alternatively, the grids may be connected directly together and the phase shift network (or networks) inserted between the tuned circuit 6, 7 and one (or each) of the cathodes. In another embodiment (Fig. 3, not shown) the anodes of the valves are connected to opposite ends of the tuned circuit instead of the cathodes being connected to a common point on the tuned circuit and the anodes are crossed coupled to the grids via phase advance and phase lag networks giving equal phase-shift, to give the regenerative connection between anodes and grids. In a further embodiment (Fig. 2 of the drawings accompanying the second Provisional Specification) pairs of valves 4a, 4b and 5a, 5b are used, the feed-back connection of the controlgrids of valves 4a, 5a, including phase advance networks 11, 13 and 12, 14 and those of 4b, 5b phase-retard networks 15, 17 and 16, 18. The modulating voltage applied at 23, 24 selectively biases either 4a and 5a or 4b and 5b, thereby dividing the total current between 4a, 5a or 4b, 5b and as this division is varied the frequency of oscillation is also varied.