Frequency-difference detector system

765,856. Frequency discriminators. HAZELTINE CORPORATION. Dec. 21, 1953 [Dec. 31, 1952], No. 35444/53. Class 40(5) Inputs in quadrature phase are applied at 26, 27 from an oscillator to mixers or phase detectors 30a, 30b to which inputs in phase are also applied from a reference frequency source through a limiter or gate 61. The resulting difference frequencies in phase quadradure pass through filters 31a, 31b of pass-band e.g. 100-7500 c/s having a lower frequency cut-off and after one is shifted in phase by 90 degrees, by a differentiating network 34, they are in phase or anti-phase depending on the sence of the frequency difference, and on being applied, e.g. to different grids of a signal multiplier tube 34, an output is derived across an integrating circuit 60 in sign and magnitude representing the frequency difference provided a certain frequency difference, determined by the lower cut-off frequency of the filters, is exceeded. Since the signals are multiplied noise effect errors are averaged out and the system is insensitive to noise. The system may be applied to decreasing pullin time in automatic phase control of a colour television phase reference oscillator. Specification 765,855 [Group XXXV] is referred to.