GB614066A - Frequency modulated oscillating system - Google Patents

Abstract

614,066. Frequency modulation; valve generating circuits. STANDARD TELEPHONES & CABLES, Ltd. June 28, 1946, No. 19413. Convention date, Sept. 15, 1945. [Classes 40 (v) and 40 (vi)] An oscillator for generating frequencymodulated waves, comprises an amplifier with an output tank circuit, a transmission line of electrical length exceeding one wavelength connecting the output circuit to an input phaseshifting network, and means to apply a signal wave to vary the phase-shift produced by this network. In Fig. 1, the output from an oscillator valve 10 is applied to a tank circuit resonant at mid-frequency, comprising coil 17 and condensers 23 to 26. Condenser 24 is a blocking condenser and condensers 25 and 26 form a potentiometer. Energy is supplied to a load circuit 30. For feedback purposes the output from point 31 is applied to a coaxial transmission line 33, the length of which preferably exceeds five wavelengths. Other types of transmission line, or wave-guide may be used in place of coaxial line 33. The other end of the transmission line is applied to input tank circuit 35, 39, 37, which is connected through a phase-shift network to the input of valve 10. The phase-shift network comprises a condenser 40 and a resistance 45 in series with the resistive impedance to earth provided by the circuit comprising valves 48 and 59, and also condenser 42 and resistance 44. The impedance provided by valves 48 and 59 is controlled by modulating signals applied to the control grid of valve 48 through transformer 54. The modulating signals may be at audio or video frequency. The arrangement is such that in the absence of modulating signals, the impedance at the input end of transmission line 33 is made equal to its characteristic impedance, so that no reflection occurs. When the impedance conditions change owing to the application of modulating signals, valves 48 and 59 effectively act as a variable resistance, introducing a phase change in the feedback circuit so that the oscillator immediately operates at another frequency, whereby the feedback is applied at 180 degrees and the electrical length of line 33 is such as to provide non-resonant operation. The arrangement may be regarded as a means for varying the electric length of the feedback line at modulating frequency. Short lengths of transmission lines may be provided in place of the capacity and inductance elements of the output tuned circuits as shown in Fig. 2, and short transmission lines may also be provided at the input tuned circuit stage (Fig. 3, not shown). Other modifications include the use of quarter-wave sleeves to assist in balancing the ends of transmission line 33, the use of a low-frequency alternating power supply, or of inductors in place of condensers 40 and 42. Valve 48 may also be used without valve 59.