GB550554A - Improvements relating to frequency modulation - Google Patents

Abstract

550,554. Frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 25, 1941, No. 12464. Convention date, Sept. 26, 1940. [Class 40 (v)] In a frequency modulation system, the carrier is given a frequency shift in accordance with the asymmetry of the signal wave, thus allowing the frequency swing to lie symmetrically within the allotted channel. The carrier oscillator 10 is frequency modulated by a valve reactance 40 with the signal applied through a transformer 46 to its control grid. The main frequency of the oscillator 10 is maintained constant by causing its output to beat in the mixer valve 20 with a standard frequency controlled by a crystal 26, the beat frequency being fed to a discriminator 54 of the off-tune type, giving a correcting voltage across the rectifier load resistances 60, 61 which is smoothed by resistance 62 and condenser 64 and fed to the control grid of the reactance valve 40. The signal voltage is fed from transformer coil 49, or from the unsmoothed output of the discriminator 54, according to the position of the switch S, to amplifiers 70, 80 and peak rectifiers 90, 901 so as to create across resistances 89, 91 a voltage proportional to the asymmetry of the signal wave. This voltage is also fed to the control grid of the reactance valve 40 to effect the desired frequency shift. The amplifier valve 80 is given a low internal impedance by the feedback circuit 82, 84. At the receiver, the signal is de-modulated by a normal frequency discriminator and is then combined with a component derived from the de-modulated signal by amplifiers and rectifiers similar to 70, 80, 90, 901 at the transmitter, thus restoring the asymmetry of the original signal wave (Fig 3, not shown). Specifications 504,766 and 507,826, [Group XXXIX], are referred to.