Patent GB-621850-A (GB)
Title: Radio frequency telegraph and like communication systems
IPC: H04L27/16
Filed: 1945-11-01
Abstract: 621,850. Frequency - modulation; automatic tuning correction. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Nov. 1, 1945, No. 29067. Convention date, Nov. 2, 1944. [Class 40 (v)] In a receiver particularly for a telegraph system in which marks and spaces are distinguished by energy of different frequencies, means are provided for disabling the automatic tuning control-motor in the absence of a received signal of adequate strength. The receiving aerial feeds a first mixer stage 10 which supplies a first intermediate frequency to a second mixer stage 14 having a local oscillator tunable by a condenser 16 operated by a two-phase. motor M. Unit 14 also includes an A.G.C. diode 22 feeding a circuit which includes a switch S for varying the time constant. The output from 14, in the form of two low intermediate frequencies corresponding to the mark and space frequencies, is fed through a limiter 30 to circuits M, S tuned respectively to the mark and space intermediate frequencies. Circuits M and S are oppositely connected to diodes A, B having differentially connected load resistors 36, 38, so as to produce at point 40 or 401, negative and positive potentials corresponding respectively to marks and spacers, according to which of the frequencies is used for marks and which for spaces. A reversing switch S1 is provided so that, whichever method of transmission is used, a negative potential corresponding to marks is fed through filter FC to cut off a keying valve 50. This allows the grids of valve 60 to go positive and pass the output from a tone generator. Similarly, a positive voltage at point 40, due to a space, opens keyer valve 50 and cuts off valves 60 to key off the tone. For automatic tuning correction, the mark and space potentials are fed through resistance 72 to condenser 74, the voltage across which is fed through switch S2 to a motor control unit MCU to motor M to tune the local oscillator so as to maintain the mean output of the second mixer stage 14 at the mean of the mark and space frequencies. A valve 80, coupled as shown, ensures that a zero voltage appears at point 90 so long as the mark and space potentials have their normal values and are equal positive and negative. Thus a negative marking potential cuts off valve 80 and the positive potential at the tapping point on resistor 82 compensates to give zero voltage at 90. Similarly a positive space potential causes valve 80 to draw current and the negative potential at the tapping point on 82 compensates at 90. If, however, the mean frequency at the output at 14 is not the mean of the resonant frequencies of circuits M, S, the mark and space potentials at 40 will not be symmetrical with respect to zero, and a positive or negative potential will appear at 90 which is used to correct the mis-tuning. A modified arrangement of valve 80 is also described (Fig. 4, not shown). The switch S2, which is ganged with S, ensures that the A.F.C. potential is applied in the correct sense according to the method of transmission. A connection is made from limiter unit 30 to motor control unit MCU, so that in the absence of an adequate signal, the motor control is inhibited. This prevents the motor tuning the circuit right away from the operating frequency. The limiter unit 30 is shown in Fig. 2 and includes an input amplifier 100 coupled by a band-pass filter 102 to a first limiter 110, which feeds through amplifier 116 and band-pass filter 115 to a second limiter 120 and output amplifier 130. Limiter 110 comprises two triodes having a common cathode resistance R10. Thus on negative half-cycles of the input applied to grid 109, the left-hand triode cuts off to limit the negative half-cycle of the output. On the positive half-cycle the current through the lefthand triode and resistance RIO causes the righthand triode to cut off and limit the positive halfcycle. Limiter 120 is identical. The motor control unit MCU comprises a pair of balanced valves (Fig. 3, not shown) fed from an A.C. source and controlled by the error voltage from point 90 to give an A.C. output which corresponds in amplitude and phase to the magnitude of the error voltage. U.S.A. Specifications 2,276,565, 2,057,640, 2,070,418 and 1,874;866 are referred to.