GB597950A - Frequency modulation of radio-frequency oscillations - Google Patents

Abstract

597,950. Frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 25, 1945, No. 10448. Convention date, April 25, 1944. [Class 40 (v)] A frequency-modulated radio generating system comprises two valves, each associated with a parallel circuit tuned to a separate frequency, which are made to oscillate at a common frequency by a connection including a wave-filter, the pass-band of which is centred on a frequency between the frequencies to which the parallel circuits are tuned. In one embodiment, Fig. 1, the anode circuits L1, C1 and L2, C2 of the oscillating valves V1, V2 respectively are tuned to frequencies separated by an amount at least equal to the total frequency deviation required. The valves V1, V2 are made to operate at the same frequency by a common connection between their grids and cathodes through coupling condensers C and a two-electrode crystal X, the frequency of which is midway between the frequencies to which the anode circuits are tuned. If the oscillatory circuits and valves V1, V2 are balanced, the arrangement operates at the crystal frequency when no modulating voltage is present, but the application of a modulating potential, which may be an audio-frequency signal or a keying potential, causes a variation in the gains of valves V1 and V2, thus providing a frequency-modulated output. This output may be amplified by V3, V4 or used directly. In a modification, Fig. 2, an intermediate point on each of the coils L1, L2 of the tuned anode circuits is cormected to the earthed cathodes of valves V1, V2 through coupling condensers BP. The grids of valves V1, V2 are connected together through condensers C and through a three-electrode crystal X and condensers 38, 40 to adjustable tappings on coils L1, L2. Neutralizing condensers N1, N2 are connected between the crystal electrode D and tapping points on L1 and L2 respectively. In another modification (Fig. 3, not shown), the grids of the oscillating valves are connected to the capacitive branches of the tuned anode circuits through two band-pass filters, a common terminal of which is connected to the grids. The filters may have similar characteristics, or may have characteristics overlapping in the carrier frequency region. Reference is made to Specification 595,421 to indicate the manner in which keying potentials are applied.