Patent GB-662133-A (GB)
Title: Improvements in or relating to electric control systems
IPC: H03C3/02
Filed: 1947-06-17
Abstract: 662,133. Automatic voltage and temperature control systems. ROBINSON, J. July 16, 1948 [June 17, 1947], No. 37186/46. Class 38 (iv). [Also in Group XL (c)] When a carrier wave is modulated in frequency or phase by a modulating-voltage, a multiplicity of side-frequencies or side-bands are produced, with frequency intervals equal to that of the modulating voltage. The amplitude of the pure carrier and of each of the sidebands varies according to the modulation index, i.e. the ratio of carrier frequency deviation to modulating frequency. The curves 12, 13, 14 of Fig. 1, show the manner in which the pure carrier 12 and the first and second pairs of side-bands 13, 14 vary in amplitude (ordinates) with change of modulation index (abscissµ). The invention in one form consists in stabilizing a voltage by using it as the modulating voltage, isolating the pure carrier or one of the resulting side-bands, and utilizing changes in amplitude thereof to regulate automatically the original voltage. In the voltageregulating circuit, Fig. 2, the source 15 which may be of radio-frequency or may be the supply mains, feeds a load 16 and supplies a modulating-voltage to the screen grid of a reactance valve 19 connected across the tuned circuit 18 of a carrier wave generator 17. Modulated output is taken by a coil 20 to an amplifier valve 21, from the anode of which one of the resulting frequencies (e.g. the pure carrier) is selected by a filter 22. Initial adjustments are made so that if the voltage across the load 16 is at its correct value, the modulating index is 2.4 at which point the pure carrier curve 12, Fig. 1, crosses the zero axis, there being therefore no output from the filter 22. If the voltage changes, output appears in one phase or the other, and is fed to a frequency-changer 23 to reduce the frequency to that of the source 15. It is then applied through a coupling 24 to restore the voltage to normal. For AVC purposes in a radio receiver, the initial voltage input may be such as to operate on the upper slope of the curve 12. The output from 23 provides positive feedback in the coil 24 and raises the modulation index until it approaches the zero point 2.4. The heterodyne source for the frequency-changer 23 may be provided by selecting an adjacent side-band from the output of the amplifier 21 and passing it through a voltage-limiter, Fig. 3 (not shown). Instead of feeding back A.C. energy to the coupling 24, the output from the filter 22 may be converted to a D.C. voltage which is used to regulate the inductance of a coil in circuit with the source 15; or the D.C. voltage may be arranged to bias an amplifier for the controlled A.C. voltage. The derivation of the D.C. voltage may be by mixing the A.C. output from filter 22 with the same output shifted in phase, and selecting the D.C. component from the resultant. Conditions other than voltage may be maintained at a datum value by generating an A.C. voltage whose amplitude varies with the condition, and by utilizing the output A.C. or D.C. voltage to restore the condition to datum value. For example, a resistance element in a furnace wall may be fed with A.C. and the voltage across it, dependent on temperature, may be used as the modulating-voltage, the output voltage being utilized to control the injection of fuel. In a modified AVC circuit, Fig. 4, the incoming wave is applied to an amplifier 29 having a tuned anode circuit 30 with a reactance valve 31 connected across it. The incoming wave is also fed through a frequencyreducing stage 34 to the grid of valve 31 so as to wobble the tuning of the circuit 30. A phase modulated output is taken off by the coil 32 to a filter 33 which selects the central band or a particular side-band, and feeds back through the transformer 28 to the input lead. The input to the stage 34 may be taken from the output of the filter 33, Fig. 5 (not shown). The required side-bands may be produced by pulse modulation instead of frequency or phase modulation.