Patent GB-640221-A (GB)
Title: Improvements in or relating to high frequency control systems
IPC: H03L7/04
Filed: 1944-10-10
Abstract: 640,221. Automatic frequency control systems. SPERRY CORPORATION. Oct. 10, 1944, No. 19538. Convention date, April 22, 1943. [Class 38 (iv)] [Also in Groups XL (a) and XL (c)] In an automatic tuning control arrangement, scanning means responsive to actuation of the tuning means causes the tuning to scan a predetermined band until it comes within the range of a tuning error detector which then assumes control to maintain correct tuning. The invention is described with reference to various electromechanical means for tuning a velocitymodulated oscillator. Fig. 1 shows a velocity modulated oscillator, part of the output of which is extracted from cavity 103 and mixed in a unit 13 with the output of crystal-controlled generator 11 to produce a difference frequency which is amplified at 14 and passed to error detector and scanning unit 15 which controls a motor 17 by means of a control unit 16. The oscillator comprises two resonant cavities 77 and 78 electrically coupled by means of loop 109 and rigidly fixed to a flange 86 forming part of the main support 87. The end walls 79 and 82 of these cavities are flexible and are connected to plates 84 and 106 respectively which are urged together by springs, one of which (108) is shown. The separation of these plates is adjusted by arms 91 and 92 which are pivoted at 88 and 89 and are forced apart by struts 93 and 94 bearing against non- rotatable nut 95. Thus, by rotating the threaded extension of shaft 62 the size of the cavities and hence .the frequency of oscillation may be adjusted. Fig. 2 shows a control circuit for the motor. An amplifier 14 supplies the difference frequency to a discriminator comprising diodes 25, 26, output from which is amplified by valves 34, 35 and passed to balanced modulator 42, 43, A.C. supply is fed to the modulator via transformer 54 and the centre tap of winding 46 and the resulting A.C. output which in magnitude and sense is representative of the frequency error is amplified in valves 51, 52 and supplied through phasing condenser 111 to winding 57 of motor 17. Winding 59 is connected directly to the A.C. supply 53. The motor besides effecting tuning adjustment drives cam-operated switches 65, 66 which connect valve 71 alternately across the anode resistors 37, 38 of the modulator. This unbalances the modulator causing the motor to run in one direction until tuning limit is reached when the cam transfers the unbalance to the opposite valve reversing the direction of the motor. If a signal is present, a negative voltage is developed across resistor 73 in the error detector as the correct frequency is approached, which is sufficient to suppress the anode current of valve 71 and thus remove the unbalance, and the motor finally sets the frequency under the control of the error detector which assumes control of the modulator. Damping of the motor is obtained by passing D.C. through winding 57 via resistor 110. In an alternative arrangement (Fig. 6, not shown) a winding 57 replaces transformer 5 in the anode circuit of amplifier 48, 49 ; the D.C. for valve 52 being passed through the winding for motor damping. In another modification (Fig. 3, not shown) one of the stages of amplification in the control circuit is dispensed with and, in addition, the grid voltage for controlling sweep valve 71 is obtained by separately rectifying an output from amplifier 14. Furthermore, a small A.C. voltage is injected into the cathode of the sweep valve to reduce the impedance in the conducting condition. In the modification shown in Fig. 4 the higher frequency side of the discriminator characteristic is rendered ineffective by means of a rectifier 149 which is connected in its output circuit. A voltage is therefore applied to the grid of valve 153 which over the range of the discriminator depends on the magnitude of one direction of the error only and this controls the amount of A.C. supply, from source 156 connected in the cathode circuit, which is passed to amplifier 159, 161. The separation of flanges 84 and 85 of the oscillator is controlled by compression springs 168 and 169 and thermally expensive wires 171, 172. Valve 153 is normally conducting and A.C. is therefore fed through amplifier 159, 161 to heat the wires which expand until a catch 179 operates the toggle 180 of a switch 167 to disconnect the heating current, which, however, is reconnected when the wires have contracted sufficiently to reoperate the switch. If during the heating cycle the oscillator tunes through the correct frequency the discriminator applies a bias to valve 153 which as the error increases progressively reduces the A.C. heating supply. A position of equilibrium occurs . close to the correct frequency. In a further embodiment (Fig. 8) the separation of the flanges is controlled by electromagnets acting against springs 196, 197 and the damping is provided by dashpots 210. The coils of the electromagnets carry the anode and screen current of a valve 203 which is controlled by the output of the error detector. The operation is similar to that of the circuit of Fig. 4. It is also suggested that the circuit of Fig. 4 could be used to control a motor to adjust the separation of the flanges. Fig. 7 shows a method of obtaining a motor damping directly proportional to the voltage across field winding 57. A portion of this. voltage is rectified and passed to a D.C. amplifier valve 187 whose anode current flows through winding 57 to effect damping. Specifications 541,393 and 557,147, [both in Group XL (c)], are referred to.