GB605715A - Detector for angular velocity modulated waves - Google Patents

Abstract

605,715. Frequency modulation circuits for cathode-ray tubes. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 18, 1945, No. 12611. Convention date, May 20, 1944. [Classes 40 (v) and 40 (vi)] [Also in Group XL (a)] A detector for angular velocity-modulated carrier waves comprises a demodulator valve of the beam type having an output electrode provided with at least two areas of different secondary emissivities, means for deriving a pair of rectified voltages of equal magnitude when the frequency of the receiver wave has a predetermined value, means for variably sweeping the electron beam across the two emission areas in response to inequalities in the derived voltages, and means for obtaining a signal voltage from the output electrode. A beamtype demodulator valve, Fig. 1, has deflection plates 1, 2 provided with flanges 7, 8, which co-operate with the cathode 1<1> to form diodes, a control grid 4, screen 5 and target 6. The target is of high secondary electron emissivity with an area 6<1> of low emissivity formed on the inner surface. A frequency discriminator, Fig. 2, comprises coupled in-tune circuits L1, C1, L2, C2 fed from a non-limiting I.F. amplifier V1. The output terminals of secondary circuit L2, C2 are connected to deflection plates 2, 3 of beam valve 1, the common point of the load resistances R1, R2 of diodes 1<1>, 7 and 11, 8 being earthed. The D.C. voltages appearing at the deflection plates 2, 3 have relative magnitudes varying with the frequency deviation, thus the primary electron beam is swept across target 6 in accordance with the frequency variations. The potential of target 6 varies with the number of secondary electrons emitted, and as the beam is swept across the low emission area 61 the signal voltage is developed across resistance 16. Target 6 may be earthed or connected to an H.T. supply of lower voltage than that applied to screen 5, which also acts as a collector for the secondary electrons. The LF. voltage is applied through condenser C3 to control grid 4, to provide an automatic bias voltage across resistance R3 to render the intensity of the electron beam independent of amplitude variations and to provide an A.V.C. voltage. In a modification, Fig. 3 (not shown), the beam valve has two separate diode anodes and a beam-forming plate between the deflection plates and the cathode. The diode, anodes are connected to the deflection plates through LF. filter networks and the control grid is mounted between the cathode and the beam-forming plate.