Patent GB-548191-A (GB)
Title: Improvements relating to the generation of modulated high frequency oscillations
IPC: H03C3/09
Filed: 1940-09-16
Abstract: 548,191. Automatic frequency control. GUANELLA, G. Sept. 16, 1940, No. 14254. Convention date, Sept. 14, 1939. [Class 38 (iv)] [Also in Groups XL and XXXVI] In a frequency modulated radio transmitter, undesired frequency variation is avoided by deriving a correcting voltage from a local radio receiver which combines the waves received from the transmitter with others received from a constant frequency oscillator the correcting voltage being fed back to the carrier generator to counteract undesired frequency deviation, and the time constant of the feedback circuit being of the same order as the periodicity of the signal or modulating current. As shown, the frequency-modulated transmitter comprises a retarding field oscillator valve V4 with Lecher wires L3 feeding the aerial P1, and Lecher wires L2 through which the D.C. voltages and the modulating voltages are applied to the electrodes. An oscillator V5 with a spherical shell G as oscillatory circuit acts as a frequency standard. Both oscillations are picked up by a local receiving aerial P2 and rectified at V9 to produce beat frequency voltages which are amplified at U and fed to a frequency discriminator comprising circuits C9, J9, C10, J10 tuned above and below the standard beat frequency. The voltages across these circuits are applied to rectifying triode V6, V7 and the difference between the voltages across their load resistances R11, R12 is applied across condenser C13 in the grid circuit of a valve V8. The original modulating voltage is applied across terminals 3, 4 also in the grid circuit of the valve V8. The two voltages act in opposition on the transformer T, the secondary coils of which are connected in the plate and grid supply circuits of the oscillator valve V4 to neutralize undesired frequency variations. Specification 504,766 is referred to. The Specification as open to inspection under Sect 91 also includes automatic frequency correction arrangements for amplitude-modulated transmitters. In a magnetron transmitter, Fig. 3 (Cancelled), the signal voltage is applied at terminals 1, 2 to the anodes of a magnetron M through the Lecher wires Ll forming the tuned oscillatory circuit. Two doublediode rectifiers DI D2 are connected across the wires L1 at points equidistant from a point K which is a voltage node when the oscillator is operating at the desired frequency. At this frequency equal D.C. voltages appearing across the load resistances R5, R6 are applied to the grids of amplifiers V1, V2, giving equal voltages on the anodes of these amplifiers and a null effect on the grid of the valve V3. Should the frequency vary, the current through the valve V3 and the magnetron coil F varies in such sense as to counteract the frequency deviation. In a further amplitude-modulating arrangement shown in Fig. 5 (Cancelled), the transmitting- aerial P3 is energized by the oscillatory circuit C14, J14 of a back-coupled triode valve V11, modulation being effected by applying the signal voltage at terminals 5, 6 across a choke Q8 in the anode circuit. A local receiving-aerial P4 feeds one grid of a mixing valve V13, the other grid of which is fed by a local heterodyne source V14 of standard frequency. The resulting beats are fed to a grid of a double-diodetetrode valve V 15 and the amplified beats pass through a frequency discriminating network of known form comprising coupled circuits J17, C17, J18, C18 tuned to the standard beat frequency. Any deviation in the frequency transmitted by the valve V 11 causes a deviation in the beat frequency, and the frequency discriminator co-operating with rectifiers V16, V17  produces a correcting voltage on the condenser C28. This is applied to the grid of a thermionic reactance valve V12, the anode-cathode path of which is connected in parallel with that of the transmitting valve V11. A condenser C19 is connected between anode and grid of the valve V12 and a resistance R25 between grid and cathode, giving the effect of a capacity variable with the voltage applied to the grid by the frequency discriminator, and arranged so that frequency deviation is corrected. Amplitude fluctuations in the beat currents are removed by an A.V.C. circuit comprising one of the diode rectifiers of the valve V15 fed from the anode of the valve and arranged to supply an AVC voltage to the inner grid of the valve. The other diode of the valve V15  rectifiers and beat-frequency oscillations and supplies them through a low-pass filter R19, C25 and an amplifier W as a negative feed-back to the original modulating voltage, thus reducing undesired amplitude modulation. This subject-matter does not appear in the Specification as accepted.