GB925157A - Circuit arrangement for effecting automatic frequency readjustment in television receivers - Google Patents

Abstract

925,157. Radio receiving circuits; automatic frequency control systems; frequency discriminators. STANDARD TELEPHONES & CABLES Ltd. May 26, 1961 [June 29, 1960], No. 19121/61. Class 40 (5). In a television receiver of the vestigial sideband type provided with automatic frequency control, the video carrier being normally automatically adjusted to fall half way down the sloping side of the I.F. passband characteristic, this carrier is automatically shifted to lie higher up the response curve when the signal to noise ratio becomes poor so as to obtain the optimum performance possible in these circumstances. Across the secondary of the frequency discriminator there is connected via blocking capacitors 2, 3, Fig. 1, a voltage variable reactance comprising a point contact diode 1, the reactance of which is varied by a voltage derived from a source 6 over resistors 4, 5. A television receiver comprises preliminary stages 2, Fig. 2, I.F. stages 3 and video detector and video frequency stages 4 feeding picture tube 11. The local oscillator 8 is automatically controlled in frequency by discriminator 7, and automatic gain control is applied to stages 2, 3 from an automatic gain control voltage generator 5. According to the invention, the automatic gain control voltage for stage 2 is also applied via resistor 9 and delay diode 10 to a buffer stage 6, the output of which controls the frequency to which the discriminator 7, which may be as in Fig. 1, is tuned. Delay may be produced by the control voltage from source 5 changing sign (Fig. 3, not shown), or if the control voltage is always of the same sign a delay bias may be applied to the diode 10. Alternatively, diode 10 may be omitted and delay produced by suitable biasing of the buffer stage 6.