Patent GB-706758-A (GB)
Title: Improvements in television receivers
Inventor: SPRACKLEN JOHN GEORGE
IPC: H04N5/08
Filed: 1952-08-08
Abstract: 706,758. Television receivers. ZENITH RADIO CORPORATION. Aug. 8, 1952, No. 20070/52. Class 40 (3). [Also in Groups XXXV and XL(c)] In a television receiver, a single beam-deflection tube is used to separate the synchronizing signals, to operate as a phase detector for effecting synchronization of the local scanning oscillations, and to provide automatic gain control of the received video signals. The receiving circuit, Fig. 1, comprises high-frequency stages 2, 3, an IF stage 4, detector 5, video amplifiers 7, 8 and image reproducer 6. Scanning current at line frequency is supplied to the coil 40 from a source 39. A transformer 52, 41 has its primary coil 52 connected in the scanning circuit and its secondary coil centre-tapped to earth and shunted by a condenser 47. The oppositely-phased secondary terminals are connected through series condensers and condenser-shunted resistors to a pair of deflector plates 29, 30 controlling an electron beam issuing to the left from a cathode 13 and collected by overlapping anodes 27, 28. As the beam swings, negative-going square pulses are derived while the beam is on the anode 27, which are differentiated by the condenser 54 and resistor 55 and are supplied as short pulses to trigger the scan generator 39. The incoming composite video signal is applied through a potential divider 33, 34 to a deflector electrode 24 opposite a positively biased electrode 23. A plane sheet of electrons issuing to the right from the long cathode 13, Fig. 2, is swept across a collector electrode 16 in such a way that the sync signals bring the beam to slots 17 which allow the electrons to pass to three separate collector strips 19, 20, 21. The strip 19 passes the sync pulses to an integrator 36 the output of which synchronizes the frame scan source 37. The strips 20, 21 positively biased by a source 51, are driven alternately more and less positive by their connection to the line frequency transformer 41. If these alternations are in phase with the sync signals, the strips 20, 21 assume equal D.C. voltages ; but out-of-phase conditions cause one of the strips to predominate in its D.C. voltage and thereby creates a corresponding voltage across the deflector plates 29, 30 of the left-hand electrode-system. The width of the pulses at the anode 27 is thus varied, and the corresponding variation of the timing of the differentiated pulse applied to the line scan source 39 brings that source into phase with the received sync pulses. In order to provide for automatic gain control of the radio receiver, an additional slot 18 and collector plate 22 are provided in the tube 12, the plate 22 being connected through a condenser 56 to a point in the line scan circuit 39 at which positive voltage is present at the moment of flyback. Any excess amplification in the receiver causes a proportionate movement of the electron sheet across the slot 18, and a proportionate increase of electron current to the plate 22. The negative voltage produced across the resistor 57 is integrated at 59, 60 and passed to the RF and IF amplifiers 2 ... 4 to reduce their gain. Extreme deflection of the electron sheet brings it to a narrower portion of the slot 18 giving a reduced current to the plate 22 under strong signal conditions when the receiver is first switched on.