GB1076831A - Improvements in and relating to variable reactive impedance devices - Google Patents

Abstract

1,076,831. Frequency modulation; transistor oscillator circuits. BRITISH BROADCASTING CORPORATION. Feb. 15, 1965 [Feb. 14, 1964], No. 6383/64. Headings H3R and H3T. An oscillator comprises a series resonant circuit 18, 19 in the emitter circuit of a transistor 16 feeding through transistors 21, 22 a centretapped inductor coupled with the coil 18 of the series tuned circuit, and also, through a further transistor 28, feeding back its input so that oscillations are generated of mean frequency determined by the circuit 18, 19, but, on modulation being applied in push-pull to the transistors 21, 22, the oppositely acting feedback from the two halves of the inductor 25 acts differentially on the tuned circuit 18, 19 to produce frequency modulation. One or more square law diode 41 and resistor 42 series circuits, using different types of diodes, may be connected across the modulation input to counteract centre frequency shift of the oscillations and second harmonic distortion. To eliminate centre-frequency shift the D.C. and A.C. impedance in the modulation circuit must be equal, and to this end the modulation is applied in series with resistors 43 shunted by capacitors 44. The modulation sensitivity may be made independent of modulation frequency by a capacitor 46 shunted across the modulation input by a switch 45.