GB522939A - Improvements in or relating to discriminator circuits for use in automatic frequency control systems - Google Patents

Abstract

522,939. Automatic frequency control Systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and RUST, N. M. Sept. 20; 1938, No. 27448. [Also in Group XL] In a phase type discriminator circuit, for use in A.F.C. systems of the kind shown in Fig. 2 comprising a resonant circuit AC the voltage across which, in the " in-tune " condition, is in quadrature with a voltage between the mid-point B of the circuit AC and the mid-point E of the load impedance of the rectifiers D, a voltage swing over the range of operation of at least 180 degrees is obtained between the paths AB and CD by coupling the resonant circuit AB and the corresponding tuned circuit of the receiver by at least one further tuned circuit and providing a feed back path between the discriminator and the tuned receiver circuit. There are thus obtained two frequencies corresponding to points 7, 8 on the control voltage curve, Fig. 1, besides the " intune" point 3 at which the voltages are in quadrature and the range of the automatic control is thus predetermined. In Fig. 5, circuit 4, 5, 6 is interposed between the I.F. stage 1, 2, 3 and the discriminator 7, 9 10 and this latter is coupled to the I.F. stage by a tuned circuit 13 and valve 12. In Fig. 6 a band-pass circuit 3 is used and coils 15, 16 tuned above and below the I.F. are provided in the cathode leg of the I.F. valve 2 to the grid of which the discriminator is coupled capacitatively by a condenser 18. In a similar circuit, Fig. 7 (not shown), the discriminator is connected directly to the cathode of the valve 2. Improved characteristic, of the control voltage curve is obtained by applying reaction to one or other of the tuned resonant circuits, Figs. 16 and 17 (not shown).