GB642560A - Frequency modulation of electric oscillation generators - Google Patents

Abstract

642,560. Frequency modulation. STAN DARD TELEPHONES & CABLES, Ltd. April 2, 1948, No. 9307. Convention date, May 24, 1947. [Class 40 (v)] A crystal-controlled oscillator is modulated in frequency by applying the signal voltage to the grid of a modulator tube to vary its gridcathode capacity, which is connected across the crystal of the oscillator; and means are provided to neutralize the shunt capacity of the crystal and its holder in order to increase the frequency deviation obtainable. In Fig. 1 the oscillator tube 10 has the crystal 16 connected in its grid circuit, and the pentode modulator tube 18 has its grid-cathode circuit connected across the crystal. The grid-cathode capacity, and the frequency of the oscillations generated, are varied by applying the signal voltage from a source 19 to the grid of the tube 18, which has its anode circuit tuned to the crystal frequency. The shunt capacity across the crystal 16 is neutralized by a corresponding inductance coil 20. The modulating signals may be distorted by a network 33 favouring the higher signal frequencies, in order to obtain linear modulation. In Fig. 3, the tuned anode circuit of the modulator tube 18 is centretapped and a voltage of opposite phase to the anode voltage is fed to the grid through a condenser 34. The grid.cathode impedance of the valve 18 is then a negative capacity which is variable by the modulating signal 19, and at its mean value neutralizes the positive shunting capacity of the crystal 16.