GB1077539A - Circuit arrangement for the phase and frequency comparison of two signal voltages - Google Patents

Abstract

1,077,539. Colour television; phase and frequency discriminators; automatic phase control systems. STANDARD TELEPHONES & CABLES Ltd. Jan. 22, 1965 [Jan. 29, 1964; March 13, 1964], No. 2891/65. Headings H3A and H4F. One of two voltages to be compared in frequency and phase is split into two phase shifted versions, each of which is added to the other voltage to produce two phase shifted sum (beat frequency) voltages which are applied respectively to the bases of a pair of transistors 14, 15, with their emitter/collector paths in series. The transistors conduct only during peaks of one polarity of the beat frequency signals applied thereto and from the junction of the transistors a frequency or phasedependent error correcting D.C. voltage is derived after smoothing in a network 10-13, and used for automatic frequency and subsequent phase control of one voltage with respect to the other. The output of the transistor 15 tends to increase the D.C. voltage and that of the other, 14, to decrease it, and the input to transistor 15 or 14 which first produces a peak value of the beat (i.e. difference frequency) if such exists is dependent on whether one frequency is greater or smaller than the other. Thus the storage capacitor 10, with an output circuit of large time constant, produces a maximum or a minimum D.C. output voltage dependent on the sense of the frequency error and amplitude variations of the error correcting signal are avoided or reduced. As shown, networks 28, 27 and 25, 26 produce two phase shifted versions of between 45 degrees and 135 degrees phase difference, of one input A which may be the colour burst signal of a received colour television signal, the other input B being a signal frequency from the local chrominance subcarrier oscillator to be phase controlled. One transistor 15 may be self biased by R-C network 7, 9 and the other 14 by a fixed bias from potentiometer 32, 31, so that if failure of the colour burst arises only transistor 15 is conductive and a positive voltage on capacitor 10 is produced, which, amplified in a normally blocked transistor 35, controls a colour killer circuit. Similarly, in the case of a very noisy television signal, the low pass filter 28, 27 transmits a greater signal than the high pass filter 25, 26, and the colour killer will again be operated. Alternatively, the transistors 15, 14 may be of opposite conductivity types, with their collectors connected together and to capacitor 10, or each of the two frequency inputs may be phase split before summation. The capacitor 10 may be replaced by other storage circuits, e.g. a bi-stable circuit.