Patent GB-561954-A (GB)
Title: Improvements in frequency discriminator circuits
IPC: H03D3/16
Filed: 1942-12-04
Abstract: 561,954. Automatic control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and CANTELO, H. R. Dec. 4, 1942, No. 17298. [Class 38 (iv)] [Also in Group XL] Frequency discriminators for use in the reception of FM carrier waves, or for stabilizing the frequency of a wave generator, comprise a pair of grid-controlled rectifiers with the input waves applied both to their anodes and grids, the relative phases of the excitations being determined by phasing circuits or piezo-electric crystals and varied according to any changes of the wave frequency ; a rectified output voltage corresponding to the frequency change being utilized as the received signal in the case of FM reception, or as a control voltage for correcting the frequency of a wave generator. As shown in Fig. 3, a wave source O is connected through frequency multipliers &c. AFM and heterodyne frequency changer FC to a valve V3, the anode circuit of which is coupled to a tuned circuit S2 with its opposite sides connected to the anodes of rectifiers V1, V2 having their respective load resistances R1, R2 connected between the rectifier cathodes end the mid-point of the circuit S2. The grids are connected to each other and to the junction of a series arrangement of a small.condenser C3 and piezo crystal PC connected across one half of the circuit S2. The crystal is series resonant at the mean operating frequency, at which frequency both grids are in quadrature with the anodes which are in phase opposition. When the frequency varies, the phase of the grid voltage moves towards that of one anode and away from the other, giving unbalanced rectified currents through the load resistances R1, R2. The rectifier cathodes may be connected to earth through resistances R1<SP>1</SP>, R2<SP>1</SP>, and to the grids of push-pull frequency-control valves RV1, RV2 through series resistances and shunt smoothing condensers C1<SP>11</SP>, C2<SP>11</SP>, so as to bring the frequency of the source O back to normal. The quadrature voltage for the grids may alternatively be taken from the anode of the valve V3 through a blocking condenser and a high series resistance, Fig. 2 (not shown). The condenser C3 may be replaced by a differential condenser with one electrode connected to the grids, and the other two to the respective anodes of the rectifiers V1, V2. The crystal PC may operate at its parallel-resonance frequency, or may be replaced by a series-resonant or a parallel-resonant circuit when operating as a demodulator of frequency-modulated waves. The resistances R1<SP>1</SP>, R2<SP>1</SP> may be omitted, and the AFC voltage taken from one only of the cathodes to a single control valve associated with the source O, the other cathode being earthed. In a modification shown in Fig. 5, the anodes of the rectifiers V1, V2 are jointly connected to one side of the tuned circuit S2 and the grids are separated by a crystal PC, each grid being connected to the junction of a condenser and resistance connected in series between earth and the respective sides of the tuned circuit S2. In a further modification, Fig. 6 (not shown) the anodes are connected to opposite sides of the tuned circuit, and the grids are connected to one anode by a crystal and to earth by a resistance shunted by a condenser. A variable condenser is connected between the grids and the other anode to neutralize the capacity of the crystal holder.