GB609496A - Improvements relating to locked-in oscillator circuits - Google Patents

Abstract

609,496. Valve-generating circuits; frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Nov. 14, 1945, No. 30479. Convention date, Nov. 14, 1944. [Classes 40 (v) and 40 (vi)] A synchronized valve oscillator of the kind described in Specification 562,993 is arranged to " lock in " over a wider range of frequency by associating a suitable network with the tank circuit of the oscillator. The system may be used in a FM receiver for reducing the intermediate frequency to a fractional value while retaining its frequency modulation. In Fig. 2, a pentagrid valve 1 has its tuned anode circuit 5 back-coupled to the middle grid 6 so as to produce oscillations at an integral fraction F/n of the control frequency F applied to the first grid 2. Across the circuit 5 is connected a network comprising a condenser 30 in series with a circuit 31-33 parallel-tuned to the frequency F/n. The network has an effective capacity which varies with frequency in the manner shown in Fig. 4b, and has the effect of varying the tuning of the tank circuit 5 to keep it at an integral fraction of the control frequency F. The network may take other forms, Figs. 6a.... 6d (not shown), but the effective capacity should follow the law indicated by Fig. 4b. The output from the tank circuit 5 may in the case of a FM receiver be fed through an isolating valve to a discriminator, Fig. 8 (not shown), or as shown in Fig. 7, the added network may itself constitute the discriminator, and comprise a variable series condenser 12, a resistance 12<1> and the parallel-tuned circuit 31<1> ... 33<1>, the opposite ends of the latter being connected to the anodes of a pair of diode rectifiers 17, 18. The diode cathodes are connected respectively to the audio circuit and to earth, and the junction of the load resistances 19, 20 is connected to the anode of diode 18. The capacity and resistance of the diode 18 connect the low-potential side of the circuit 31<1>... 33<1> to earth. Specification 489,094 and U.S.A. Specification 2,341,240 also are referred to.