GB573406A - Improvements in and relating to frequency modulated radio transmission - Google Patents

Abstract

573,406. Frequency modulation; electro-optical light valves. PHILIPS LAMPS, Ltd. July 12, 1943, No. 11313. Convention date, July 11, 1942. [Classes 40 (iii) and 40 (v)] In a process of frequency modulation, an electro-optical device, such as a Kerr cell, is controlled by the signal, or preferably by an integrated version of the signal, to modify the intensity of light passing to a photo-cell, thus producing in the cell circuit a voltage which is a sinusoidal function of the signal or integrated signal. Two such sinusoidal functions in quadrature are combined multiplicatively with carrier oscillations also in quadrature, their resultant being a carrier modulated in phase proportionally to the control signal, and therefore modulated in frequency proportionally to the original signal, if that signal is integrated before it is used for controlling the Kerr cell. As shown in Fig. 1, light from a source S passes through two complementary Nicol prisms PI, P2 and an interposed Kerr cell to which.control voltages V related to the signal are applied, the emergent light passing to a photo-cell F from the circuit of which an output voltage is derived. This output is caused to be a sinusoidal function of the signal by (a) making V proportional to the square root of the signal by the use of valve circuits with suitable response'curves, or (b) making V a combination of a constant voltage and a relatively small voltage representing the signal, or (c) making the length of the path l through the cell proportional to the signal, for example by providing the cell with reflecting walls A and directing light into the cell by a rotatable mirror E under the control of the applied voltage so as to vary the number of to and fro passages before emerging, as shown in Fig. 4. Fig. 5 shows the system as a whole. The carrier wave SP is divided into two quadrature phases by shifters PS1, PS2, and these are modulated in balanced modulators M1, M2 by two voltages derived from the apparatus PM of Fig. 5 and proportional respectively to the sine and the cosine of the applied signal. The modulation products are combined at T. Fig. 7 shows an alternative system using three pairs of Niool prisms, and two pairs of interposed Kerr cells. Light from the source S is divided into two beams, both passing through three Nicols and two Kerr cells, and being reoombined at the photo-cell F. A carrier wave is supplied in quadrature to the Kerr cells K1<1>, K1<11>, and the applied signal V controls the Kerr cells K2 <1>, K2<11>, an output circuit O tuned to the carrier frequency being provided in the photo-cell circuit.