| 摘要 |
931,780. Automatic frequency control systems; radio receiving circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. Oct. 26, 1959 [Oct. 29, 1958], No. 36145/59. Class 40 (5). In a superheterodyne television receiver receiving a vestigial sideband television signal the automatic gain control at low signal levels is arranged to de-tune the A.F.C. discriminator circuit or local oscillator so that the vision carrier is stabilized at a higher position on the Nyquist sloping flank of the I.F. pass-band. Amplified A.G.C. voltage from a gain control tube 2 may be applied to vary the capacitance of a back biased semi-conductor diode 18 connected across the discriminator tuned circuit in series with blocking capacitors 16, 17. The discriminator output is therefore varied and applied through a cathode follower 24 to vary the resistance of a diode 25 connected in series with capacitors 26, 27 in the local oscillator circuit, and tune it under both A.F.C. and A.G.C. control. To compensate the change in output of valve 7 when the Nyquist flank point is altered an off-tuned circuit 45, 46 may be connected across the cathode circuit and energized from the suppressor grid of the preceding tube 4, this grid being energized capacitively from the anode; and the tube 7 itself may also be a limiter. The tube 7 may be blocked during interference pulses by an input 48. A switch 49 may disconnect the Nyquist flank control. The diode 18 may alternatively be a voltage variable ferro-electric capacitor. In a modification the amplified A.G.C. voltage is applied directly to vary the resistance of diode 25 in the local oscillator circuit. Further, the A.G.C. voltage may be derived from the cathode circuit of tube 2, and applied to the cathode circuit of tube 24. The discriminator may alternatively be tuned to the sound I.F. |
