Feedback oscillator with plural forward transmission paths

1,005,222. Transistor oscillators. NIPPON ELECTRIC CO. Ltd. May 25, 1962 [May 29, 1961], No. 20202/62. Heading H3T. An alternating-current generator comprises two oscillatory paths connected in parallel between input and output circuits and a feed-back loop containing a non-linear element connected between the input and output circuits, the arrangement being such that in the event of failure of one oscillatory path, the alteration in output level operates upon the non-linear element to alter the feed-back in such a direction as to tend to restore the output to its previous value. The oscillatory paths may be constituted by amplifiers Á 1 , Á 2 , as shown in Fig. 3, in which the feed-back path provides overall positive feed-back at a frequency determined by a frequency-sensitive circuit in the feed-back path thereby to sustain oscillations. Alternatively positive feed-back may be provided within each forward path, as shown in Fig. 6, comprising two oscillators, connected in parallel and synchronized by a circuit S, overall negative feedback being provided by the feed-back path including the non-linear element l. The feed-back path in the embodiment of Fig. 3 may be modified, Figs. 4(a)-(f) (not shown) to incorporate both positive and negative feed-back paths connected in parallel. One possible such arrangement employing transistor amplifiers is shown in Fig. 10, the non-linear element comprising two Zener diodes connected back-to-back connected in a negative feed-back path and the frequency-determining circuit F connected in a positive feed-back circuit. The inputs and outputs to the amplifiers are coupled through hybrid circuits H1, H2. Alternative non-linear elements which may be used are directly or indirectly heated thermistors or ballast tubes. Means similar to those described in Specification 977,946 may be provided to warn of the failure of one path by comparing the outputs of the two oscillatory paths and providing an indication if they differ substantially from equality, Figs. 11 and 12 (not shown).