GB546132A - Improvements relating to the generation of frequency modulated electrical oscillations - Google Patents

Abstract

546,132. Frequency modulation. COSSOR, Ltd., A. C., and BELL, D. A. Feb. 13, 1941, Nos. 1998 and 15519. [Class 40 (v)] A frequency modulated wave is produced by modulating the relative amplitudes of two waves of different frequency and combining these waves in a non-linear conductor. The two waves may themselves be produced as sum and difference frequencies derived from two original generators. In Fig. 1, one wave is generated in the tuned circuit 1 by its connection to the triode portion of a triode-hexode valve 3, this wave being modulated in amplitude by signal voltages 4 applied to a grid of the hexode. The other wave is produced in the valve 8 and is unmodulated, the two waves being mixed in the filter 9 which passes both waves to a limiter 10. The output filter 11 passes only a limited range of frequencies on either side of the mean frequency of the two waves, and eliminates the two original wave frequencies. The frequency of the wave at the output terminals 12 varies linearly with the modulating voltage 4 over a limited range. In Fig. 2, a wave from a source 20 is applied in phase quadrature to the inner grids of two hexode mixer valves 21, 23 and a wave of different frequency from a source 24 is similarly applied to the outer grids. The output waves are combined additively in circuit 33 and subtractively in circuit 30, and produce respectively a difference and a sum frequency in those circuits. The two frequencies are amplitude-modulated in opposite phase in modulators 59, 49 by a signal derived from 51<SP>1</SP>, and are combined in a filter 51 and limiter 52 to produce a frequency-modulated output, having a mean frequency equal to that of the source 20 and a frequency swing dependent on the frequency of the source 24. Waves of the mean frequency may be added to the output by a lead 54 from the source 20. In the first Provisional Specification switching means are described for applying the signal modulation in the same phase to the respective waves and for simultaneously shortcircuiting the limiter, so that the circuit may be used alternatively for transmitting an amplitudemodulated wave at the mean frequency of the two waves.