GB661082A - Improvements in and relating to automatic frequency control - Google Patents

Abstract

661,082. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd. March 4, 1947 [March 4, 1946], No. 6152/47. Class 38 (iv). The mean frequency of a reflex oscillator is controlled by applying an A.C. voltage to the reflector electrode and from the resulting frequency modulation deriving a D.C. frequency correcting voltage which is fed back to the reflector electrode. The output of a highfrequency source 14 (Fig. 1), which may be an adjacent transmitter or a receiving aerial is applied to a mixer 13 together with the output of a reflex oscillator 1 which may be the local oscillator of a receiver and the resulting output is applied to an amplifier 15 and a detector 16 which in turn feeds a frequency control circuit 17. An A.C. voltage is fed from a source 22 through a capacitor 26 to the reflector electrode of the oscillator and as a result of the selectivity characteristic 30 (Fig. 2), of the amplifier the resulting frequency modulation indicated at 31, 32 and 33 produces an output from the detector of double frequency 34 when the oscillator has a mean frequency f<SP>0</SP> and an output of the same frequency but of one phase 35 when the mean frequency drifts to f<SP>1</SP> and of the opposite phase 36 when the mean frequency drifts to f<SP>2</SP>. This output is applied in parallel to the grids of valves 18, 19 and the D.C. voltage which builds up across the cathode capacitors 20, 21 is larger for one capacitor than the other according to which grid voltage is in phase with the corresponding anode voltage, and the difference voltage which is fed to the reflector from the cathode of valve 19 therefore depends upon the direction of frequency error. When the output of the source 14 is pulsed the output of the A.C. source 22 may be correspondingly pulsed. One of the valves 18, 19 may be omitted or may be used as the detector 16. If the frequency correcting voltage is taken from the cathode of valve 18 the oscillator will be stabilized at the frequency corresponding to the other sideband produced at the mixer output. In an alternative arrangement (Fig. 8 (not shown), the oscillator is stabilized in a similar manner but with reference to a cavity resonator tuned to the oscillator frequency and operating as an acceptor or rejector circuit. The A.C. modulating voltage which in this case is derived from a 250 kc. R.C. oscillator and the output from the detector are applied respectively to two control grids of a valve and the frequency correcting voltage appears at the anode. The valve may be arranged such that the oscillator voltage has a similar or a much greater control of the anode current and the correcting voltage is derived from the peak and mean values respectively of the anode current. In the final embodiment (Fig. 12), the oscillator is frequency modulated by an 800 cycle modulator 56 and is stabilized with reference to a resonator 52 the detected output of which representing the frequency error is applied to one grid of valve 97 while the other grid receives a voltage from the modulator. The correcting voltage is obtained from the anode and passed to an amplifier 94. The frequency of the oscillator is also controlled by the grid voltage of a blocking oscillator 96 which would normally vary as indicated up to A, Fig. 18. However, when the voltage indicated at A is reached the oscillator is at the correct frequency and the frequency correcting voltage which is applied to the grid of a gas-filled valve 95 causes it to fire. This discharges a capacitor 112 and lowers the potential of the upper terminal of a resistor 122 to that of the negative terminal of a battery 111, as indicated by the dotted line 136. This slowly increases the negative voltage at the lower terminal of resistor 122 but the potential of the upper terminal rapidly rises allowing the potential of the lower terminal to rise causing the gas filled valve to fire again. The potential of the lower terminal accordingly varies as shown by the full line 137. If the frequency of the oscillator decreases the correcting voltage increases and the gas-filled valve fires at an earlier point on its anode voltage recovery curve with the result that the potential of the lower terminal of resistor 122 tends to become more negative, as indicted between C and D, Fig. 18. Specification 624,692 is referred to.