GB1095063A - Improvements in or relating to frequency selective circuits - Google Patents

Abstract

1,095,063. Detector circuits. MARCONI CO. Ltd. July 13, 1966 [Sept. 29, 1965]. No. 41457/65. Heading H3T. A frequency selective circuit adapted to respond selectively to short burst of a predetermined frequency comprises a series resonant circuit resonant at said frequency, means for deriving from the resonant circuit two components in quadrature, one substantially proportional to the square of the voltage at the junction points of the capacitative and inductive portions of the resonant circuit and the other substantially proportional to the current through it, and means for adding the said components to produce a resultant substantially proportional to the square of the amplitude of the envelope of the signal in the resonant circuit. As applied to a pay television system in which subscribers send short bursts at distinctive low frequencies to a central recording point, there is provided at this point a common input amplifier 2 of low output impedance, feeding in parallel a number of series-resonant selector circuits 3A, 4A &c. The tapping point on each resonant circuit feeds into an amplifier 10A &C of high input impedance, while the series current is fed into a corresponding amplifier 5A &c. of low input impedance. The outputs from each pair of amplifiers are squared in squaring circuits 11A, 12A which may comprise Hall effect devices, and added in additive circuits 13A: in order to obtain a resultant proportional to the amplitude of the envelope of the signals square-root circuits 14A &c. follow. Switches 8A &c. bring resistors 9A &c. into circuit to damp the tuned circuits following each signal burst and leave the arrangement ready to deal with the next subsequent burst.