GB592628A - Improvements in or relating to modulation detector circuits for angle-modulated oscillations - Google Patents

Abstract

592,628. Valve receiving circuits ; frequency and phase modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 27, 1945, No. 2164. Convention date, Jan. 27, 1944. [Class 40 (v)] A detector circuit for angle-modulated oscillations comprises an electron discharge tube having at least a control electrode, anode and cathode arranged to operate as a limiter, and having a reactive impedance in its space current path, so arranged that voltage variations are developed across the impedance corresponding to the angle-modulations and rectifier in shunt with the impedance which produces a rectified version of the variations across its load impedance. As shown, applied to a F.M.-A.M. receiver, F.M. waves received at 1 and A.M. waves received at 2 are selectively fed by switches 4, 5 through a R.F. amplifier 3 and frequency changer 6 to the I.F. amplifier 7, the output of which includes a circuit 8 tuned to the mean I.F. for the F.M. waves, e.g. 4.3 Mc., in series with a circuit 9 tuned to the I.F., e.g. 455 kc., for A.M. reception. The circuits 8, 9 are coupled to correspondingly tuned circuits 22, 23 connected in series with a resistor 24 in the grid circuit of the double-diode-triode 10. For A.M. reception, switches 19 and 28 are closed. Switch 28 short-circuits resistor 24 and applies a suitable bias to the grid 12. Switch 19 applies the full H.T. to the anode 13 so that the triode section of the tube 10'functions as a class A amplifier, the inductance 16 in the cathode lead acting as the output load and producing degeneration so that the gain is unity. The diode anodes 14, 15 rectify the signal, the demodulated voltages appearing across the diode load resistors 32, 34 being applied respectively to the audio-frequency amplifier and to the A.V.C. line 37 which acts on the stages 3, 6, 7. A single diode, however, may be used for both these functions. For F.M. reception, switch 28 is opened to disconnect the grid biassing source and insert the series resistance 24 in the grid circuit, and switch 19 is opened to insert a resistance 17 in the anode lead whereby the triode saturates at low input voltages. The plate current is thus substantially constant so that the voltage across the inductor 16 in the cathode lead follows the frequency variations of the input. The limiter resistance-capacity network 24, 27 prevents undesired amplitude variations in the F.M. input from appearing across the inductor. 16 which thus functions as a discriminator. The voltage variations across inductor 16 are rectified by the diode 14 as for A.M. and passed to the A.F. stage. The A.V.C. circuit 37 is rendered inoperative automatically since input amplitude variations are not repeated across the cathode load 16. The inductor 16 may be replaced by suitable network, e.g. combinations of series and parallel resonant circuits, if a linear impedance-frequency response therefor is required.