GB873078A - Improvements in or relating to frequency control systems - Google Patents

Abstract

873,078. Automatic frequency and phase control systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. March 19, 1958 [March 22 1957], No. 8825/58. Class 40(5) A gate circuit permits signals to be passed to a phase detector retroactively controlling the frequency and phase of the signals (in conjunction with reference waves of the same frequency.) only when the signal and reference waves are within phase and amplitude relations detected by another phase detector which controls the gate circuit. As shown an intermediate frequency (I.F.) is applied through a gated amplifier V 1 , to a phase discriminator 21 with its other and reference input applied from a terminal 24 to derive, at 22, an automatic frequency or phase control voltage. The I.F. after phase shift of 90 degrees in circuit 2 is also applied with the reference oscillation to a phase sensitive rectifier circuit 9, 10, 11 whose output, in series with a negative bias voltage applied at 25 and a positive bias voltage derived by rectifying part of the I.F. input in circuit 20, 14, 15, is applied to open the gate valve V 1 when the inputs to the phase sensitive rectifier 10, 11 are in a predetermined phase or anti-phase relation for some time, i.e. the input I.F. and reference wave approximately in the phase quadrature relation around which the D.C. output at 22 is variable in magnitude and polarity for AFC control. Due to rectifier circuit 14, 20, 15 the A.F.C. control ceases if the I.F. is of insufficient amplitude. The switch 23 may reverse the phase of the reference oscillation, including its input to discriminator 21, so that the image of the controlled frequency producing the same I.F. frequency with a stable heterodyning source may instead be controlled. In a modification (Fig. 2) the valve V 1 feeds either end of the tuned circuit 8 through a phase reversing switch 24, and its output applied through a 90 degrees phase shift RC network 9, 10 to one end of a rectifier circuit 12, 11 to the other end of which an amplified reference wave is applied to produce, when the input I.F. and reference wave are in a predetermined 90 degrees phase lead or by relation, a positive bias on a normally blocked valve V 3 whose output opens a normally blocked rectifier gate circuit 5, 6 between the valve V 1 acting also as a cathode follower and the phase discriminator 21, to which the reference wave is also applied from the cathode follower valve V 2 . The gate circuit 5, 6 normally self blocking by rectifying the input from valve V 1 .