GB589303A - Improvements in or relating to electron discharge tube reactances - Google Patents

Abstract

589,303. frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 1, 1944, No. 23993. Convention date, Dec. 1, 1943. [Class 40 (v)] [Also in Group XXXV] In a thermionic reactance wherein a phase shifting network, comprising a condenser in series with an impedance, is connected across the anode cathode of a thermionic valve and the control grid is connected to the junction point of the condenser and impedance, the impedance consists of a circuit which presents a substantially pure resistance over a wide frequency range so that the device simulates a substantially pure reactance throughout that range. Fig. 2 shows a reactance valve 40 adapted for angular velocity modulation, of the output of an oscillator the tank-circuit of which, Fig. 1 (not shown) is connected across the points X-X. The anode and control grid of valve 40 are coupled by a condenser C1 the reactance of which is large compared with the resistive impedance connected across the grid and cathode at points A, B. This impedance comprises a circuit 41 tuned to the carrier or mean frequency of the oscillator and a circuit 43 coupled thereto and also tuned to this frequency. Since the circuits 41, 43 are resonant at the midfrequency of operation the impedance between A and B is nearly purely resistive and since the characteristic of the tuned circuit is broadened by circuit 43 and resistance 47 the resistive impedance across A, B is substantially constant over a wide range. 42 and 45 are coupling condensers presenting substantially zero R.F. impedance. To cover a wider modulation range, the resistive impedance across points A, B, Fig. 3, may comprise a known MM type network consisting of a shunt branch 52-54, a mid-half series MM transducer 55-58 and a MM shunt branch 59-64 as shown on page 311 of the Bell System Technical Journal for April 1931. The input reactance of the tube 40 is taken into considera.- tion in determining the network constants, so that the network presents a substantially constant resistive impedance at all frequencies. Figs. 2A, 3A (not shown) disclose modifications of the arrangements of Figs. 2, 3 respectively wherein the modulating signals are applied to the suppresser (Fig. 2A) or screen grid (Fig. 3A) rather than to the control grid. The invention is applicable to other uses, e.g. automatic frequency control.