GB568825A - Improvements in or relating to circuit arrangements for generating signals suitable for automatic frequency control - Google Patents

Abstract

568,825. Automatic frequency control. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 29, 1943, No. 15977. Convention date, Sept. 29, 1942. [Class 38 (iv)] Frequency drift in an oscillator is corrected by combining the drifting oscillations with stabilized oscillations of the same frequency and of greater amplitude, and passing the combined oscillations to an amplitude-modulation detector and to a frequency-modulation detector. When deviation occurs, each detector produces a beat frequency, and the two beat currents are combined in opposed rectifiers to produce a D.C. voltage which is used to correct the original frequency drift. As shown, oscillations from the main oscillator 12 are first heterodyned to a lower frequency by mixing with a fixed frequency from a source 10 in a mixer 20. This lower frequency is combined in the circuit 30 with oscillations of greater amplitude and of the same mean frequency from a fixed-frequency source 22. The combined wave is applied to the grid circuit of a valve 38, which operates by grid rectification to detect changes in amplitude, and which feeds the resultant beats through the coupling 80, 84 to the valve 90. The valve 38 also operates as an amplitude limiter removing amplitude modulation from its output which is fed to a frequency discriminator 40 of the kind described in Specification 489,094. The beats, which are present as a frequency modulation on the combined wave applied to the discriminator, appear across the resistances 48, 50 and are amplified in the valve 66, the output of which is applied in like phase to the cathode and anode respectively of diode rectifiers 74, 78. The output of the valve 90 is applied in phase opposition to the other electrodes of the diodes 74, 78, and a load resistance 100 is connected between the mid-point of secondary coil 92 and the earthed end of the secondary 70, the arrangement being such that the beat currents oppose each other in one rectifier and aid in the other. The D.C. voltage across the load 100 is smoothed by the network RC and is applied to the generator 12 to correct its frequency, e.g. in the manner described in Specification 504,766.