Patent GB-668558-A (GB)
Title: Improvements in or relating to circuits for automatic frequency correction
IPC: H03L7/08
Filed: 1949-04-14
Abstract: 668,558. Automatic frequency control systems; inductances. PHILIPS ELECTRICAL, Ltd. April 14, 1949 [April 17, 1948; Sept. 20, 1948], Nos. 10192/49 and 10193/49. Classes 38 (ii) and 38 (iv). The frequency of an oscillator is automatically bought into agreement with that of a control oscillation by the combination of an electronic frequency corrector operative over a narrow range and a movable frequency corrector operative over a wider range; and the movable corrector is caused to scan its wider control range so long as there is a considerable difference in frequency between the two oscillations. As shown, the valve oscillator 1 and its tuned circuit 2 are associated with a reactance valve 11, the control grid of which is connected to the output terminal of a mixer circuit through a filter 31. The mixer comprises rectifiers 25, 26 fed in push-pull by the control oscillation 27 and in parallel by the anode output of the oscillator 1. The control voltage fed to the grid of the valve 11 operates to maintain frequency and phase agreement if there is no considerable frequency error. If, however, there is substantial frequency error, the voltage on the grid of the valve 11 is alternating, and the movable correcting device 17 ... 21 is brought into action. This comprises a metal plunger 18 with a magnetic centre plug 19 mounted in a liquid-filled tube 17 which is surrounded by two driving coils 20, 21 and a high-frequency controlled coil 24 connected in the tuned circuit 2 of the oscillator valve 1. The coil 20 is connected in the anode circuit of the reactance valve 11 in series with a relay coil 34 controlling contacts 33, 37; and the coil 21 is connected across the supply 9, 10 in series with resistors 22, 23 one of which is shunted by the relay contact 37. With the contacts 33, 37 in the position shown, current through the coil 20 moves the plunger to the extreme left. The contacts 33, 37 reverse their positions, cutting off the heater current for the valve 11 at 33 and shorting the resistor 23 at 37. The anode current falls slowly and the coil 21 drives the plunger towards the right. During this movement the wide control range is slowly scanned, but as the valve 11 is out of action, the correct tuning position is traversed without effect. Owing to the closure of contact 33 and opening of contact 37, the wide range is now scanned in the opposite direction, and as the correct tuning position is reached, a negative potential applied from the mixer 25, 26 to the grid of the valve 11 reduces the current through the coils 20 and 34 and arrests the scan. The valve 11 operates as a low-frequency generator whose period is dependent on thermal inertia of the heater circuit and may be of the order of 5 to 10 seconds. Alternatively change-over contacts may be operated by the plunger 18 as it reaches its limiting positions; or a multivibrator may be employed for generating the low frequency. In a modified circuit, Fig. 2 (not shown), the electronic control valve 11 is of triode type and is connected as a variable resistance leak in the grid circuit of the oscillator valve 1. The coil 20 and relay 34 are connected between HT + and the anode of the grid-leak valve, and the output of the mixer 25, 26 is connected to its grid. The grid condenser 42 of the oscillator is preferably of reduced capacity in order to facilitate the frequency control. Specifications 607,798, 627,407, 633,419 and U.S.A. Specification 2,044,749 are referred to.