GB626595A - Improvements in or relating to frequency modulation systems - Google Patents

Abstract

626,595. Automatic frequency control systems. STANDARD TELEPHONES & CABLES, Ltd., and GARFIELD, W. L. Aug. 15, 1947, No. 22688. [Class 38 (iv)] [Also in Group XL (c)] Frequency or phase modulation is effected by applying the signal directly or indirectly to the grids of a free-running multivibrator and deriving the modulated carrier wave from the output; and the carrier is maintained at a desired mean frequency by feeding back the output voltage of a frequency discriminator to the control grids of the multivibrator. Fig. 3 shows one form of the system, and Fig. 4 the multivibrator circuit. The signal originating at 19 is applied as a variable bias to the multivibrator 21, the pulse output of which is mixed at 22 with the output of an oscillator 23. The upper side-band, resulting from the mixing of the oscillations from 23 with a harmonic from the multivibrator 21, is selected by a filter 24, multiplied in frequency at 25, again selected by a filter 26, and passed through an amplifier 27 to the radio transmitter. The mean frequency of the multivibrator is maintained steady by feeding back the D.C. output of a discriminator 29 connected to the output of the last filter 26. The multivibrator circuit, Fig. 4, includes condensers 32, 33 for manually adjusting the pulse frequency, and a slider on a resistor 37 in the screen grid circuits for equalizing the mutual conductance of the valves 30, 31. The signal is applied at terminals 48, 49 through a blocking condenser 51 and highfrequency choke 52 to the valve control grids. The frequency-correcting voltage is applied to the grids at terminals 45, 46 through a highfrequency choke 44, and a resistance or impedance 43 for preventing the signal from reaching the discriminator circuits. The pulse output is taken from one of the valve anodes.