GB546323A - Frequency-modulation television signal receiver - Google Patents

Abstract

546,323. Television receivers; frequency modulation. HAZELTINE CORPORATION. July 16, 1941, No. 8979. Convention date, Aug. 2, 1940. [Classes 40 (iii) and 40 (v)] In a receiver of television signals frequencymodulated on a carrierwave, a detector device is used adapted to possess the frequency characteristics shown in Fig. 2 and means is provided to adjust the zero of the curve to the frequency corresponding to " black." This reduces interferences of spurious signals with synchronizing and stabilizing circuits and minimizes accidental white spots in the dark portions of the picture As shown in Fig. 1, the detector 16 comprises two diodes 28, 29 with load resistances 31, 32 and byepass condensers 33, 34 in series supplied with the signal through a transformer 25, 26. The secondary 26 is tuned by a condenser 27 to the mean frequency of the signal, the primary 25 is coupled by a condenser 30 to a tapping 38 on the secondary, and this tapping is connected to the junctions of the resistances 31, 32 and condensers 33, 34. This arrangement feeds sum and difference of primary and secondary voltages to the diodes in such a way that when the tapping is at the centre of the secondary, the characteristic is curve A of Fig. 2. By adjusting the tapping along the secondary, curve B is obtained such that the zero point f 3 corresponds to the frequency representing " black." In a modification, Fig. 3, the secondary of the coupling transformer comprises two separate sections 40, 41 tuned respectively above and below the frequency band and both coupled by mutual inductance to primary 25. The zero point of the characteristic curve is adjusted by varying the coupling between primary 25 and one section of the secondary. Instead of adjusting the coupling transformer to determine the frequency at which the characteristic curve crosses the zero line, the oscillator-modulator 13, Fig. 1, and the intermediate-frequency selectors may be adjusted to produce the frequency corresponding to "black" at the zero of the curve. The circuit may include stabilizing means including a detector-coupled to a sharplytuned circuit resonant in the immediate vicinity of the intermediate frequency corresponding to the tips of the synchronizing signals and utilizing the output of this detector to control the frequency of the oscillator section in a manner analogous to automatic frequency control of conventional amplitude-modulation receivers.