Patent GB-727368-A (GB)
Title: Improvements in or relating to circuits for frequency-modulating stabilized high-frequency oscillators
IPC: H03C3/09
Filed: 1953-05-22
Abstract: 727,368. Automatic frequency control. PHILIPS ELECTRICAL INDUSTRIES, Ltd. May 22, 1953 [May 27, 1952], No. 14491/53. Class 38 (4). [Also in Group XL (c)] The mean frequency of a carrier oscillator is stabilized by an A.F.C. system which compares the oscillator phase with that of a chosen harmonic of a reference pulse generator in a mixer, which is cut off in the intervals between pulses, the mixer providing a pulse output which is integrated and used as an A.F.C. voltage for the carrier oscillator. The invention consists in effecting frequency modulation of the stabilized carrier oscillator by applying the modulating voltage both for position modulation of the reference pulses and for direct frequency modulation of the stabilized carrier. As shown in Fig. 1, the reference pulse generator 3 is locked with a crystal-controlled reference source 2 and is connected to a phase-comparing mixer 4 fed also from the carrier oscillator 1. The mixer output pulses are supplied through an integrator 5 and low-pass filter 6 to a tube reactance 7 associated with the oscillator 1 to synchronize that oscillator with an harmonic (e.g. the 40th) of the pulse frequency. The modulating source 8 is coupled through an amplifier 9, transformer 10, amplifier 12, and transformer 13 to add its voltage Vm, Fig. 3, to the voltage Vs of the source 2, the effect being to shift the position of pulses i 0  which occur whenever the rising slope of the combined curve Vs +Vm crosses a base line a. The provision of an integrating circuit 11 ensures that the modulation of the pulses and the associated harmonics should be of the nature of a frequency modulation rather than a phase modulation. The integrated output of the phase comparer 4 now has a component at modulation frequency as well as the D.C. component required for stabilizing the mean frequency of the oscillator 1, and this modulation component if able to pass the filter 5 would result in the desired modulation of the source 1. The filter 5 would require however, to be designed to pass the modulation frequencies, and this is not compatible with its requirements for A.F.C. purposes. The invention therefor provides means for feeding the modulating voltage by transformers 10, 14 to the input lead of the reactance tube 7 with the result that the networks 5, 6 have only to deal with the A.F.C. voltage. If the modulation of the oscillator 1 is to have the characteristics of phase modulation, the integration stage 11 is omitted, and a differentiating stage is inserted in the feed to the transformer 14. In a modified circuit shown in Fig. 2, a cathode-ray tube CR functions as the pulse producer, pulse modulator 3, and mixer 4 of Figure 1, in conjunction with a modulation source 33 and a crystal reference source 15. As in Fig. 1, the modulating source modulates the pulse position through the transformer 29, and modulates the frequency of the carrier oscillator 25 through the transformer 37 and reactance tube 28. If it is desired to operate on a carrier frequency between consecutive harmonies of the pulse frequency, the circuit of Fig. 2a is substituted for the portion of Fig. 2 to the right of the dotted line 35. An interpolation oscillator 37 mixes its output with that of the main oscillator 36 in a mixer 38, and one of the resultant side frequencies is fed back to the grid of the tube CR. In a further modification, in which the circuit of Fig. 2b is substituted for the integrator 26 of Fig. 2, the interpolation oscillator 43 may be controlled in frequency conjointly with an amplifier 41, both of which feed a mixer 42 whose output passes to the filter 27. Specifications 663,619, [Group XL (c)], and 692,003 are referred to.