Patent GB-679767-A (GB)
Title: Improvements in or relating to circuits for automatic frequency-control
IPC: H03C3/09
Filed: 1950-03-13
Abstract: 679,767. Phase measurement. PHILIPS ELECTRICAL INDUSTRIES, Ltd. March 13, 1950 [March 16, 1949], No. 6235/50. Class 37. [Also in Group XXXV] A high frequency oscillation is synchronized with a harmonic of a low frequency oscillation, one of which oscillations is the frequency of an oscillator to be controlled in frequency, by a phase comparator comprising a cathoderay tube having a collector electrode struck intermittently by the beam as it scans a slotted mask electrode under the control of the low frequency oscillation, the beam also being controlled in intensity by the high frequency oscillation, and a control voltage being derived which, fed over an integrating circuit to a control valve, controls the oscillator to be controlled. To produce odd harmonics, the beam may be suppressed once per cycle of the traverse of the mask 8 (Fig. 1), by a quadrature voltage applied to an intensity control electrode 4 or to further electrodes 6 deflecting the beam at one side, off the mask slot 7. The time constant of the integrating (control) circuit 15 16 is made less than one quarter of the period of the maximum frequency difference, e.g. 100 c/s., which may be corrected, a variation of the oscillator frequency otherwise resulting. Further the control range of the reactance valve 17 or cut-off frequency of the control circuit 15, 16 is made less than one half the frequency of the pulses to avoid interlocking at an unwanted harmonic. The beam alternatively may be circularly rotated over a number of slots feeding a corresponding number of H.F. oscillators to be controlled in frequency Alternatively, Fig. 3 (not shown, the H.F. oscillator 13 may deflect the beam along a wedge-shaped slot in the mask to vary the pulse duration with any frequency drift, or may be connected to a cavity resonator velocity modulating the beam so that maximum variation of the beam intensity occurs at the position of the slot. In Fig. 5, the low frequency is derived from a crystal oscillator 34 and applied to C.R.T. deflection electrodes 39 through two coupled tuned circuits 37, 38,'the first of which also applies a phase quadrature operating voltage to the collector electrode 51 reaching a maximum at instants when the beam passes through the slot 52 in the mask electrode, and limited by a biased diode 49. The H.F. oscillator to be controlled at, say, the 200th harmonic of the crystal oscillator 34, comprises a grid tuned oscillator 55 whose frequency is applied to a modulator electrode 61 of the tube. The output of the collector electrode is applied through a choke 62 to a circuit 65 controlling the bias of a further valve 56 in series with the anode resistor 58 of the oscillator 55, resulting variation of the oscillator anode voltage correcting its frequency. The oscillator output is limited by biased rectifiers 57. To avoid low-frequency voltages being applied to the deflection electrodes so large that the beam strikes them, a harmonic of the low frequency may be applied, and the harmonic and fundamental frequencies superimposed (Fig. 7, not shown), applied to an electrode normally suppressing the beam so that once only per cycle of the low frequency a pulse of current passes. The harmonic and superimposed voltages may be taken from an oscillator (Fig. 6, not shown), having serially connected anode circuits tuned to the fundamental and harmonic.