VOLTAGE CONTROLLED OSCILLATOR

摘要

A voltage-controlled oscillator having an electrical means of frequency control which is able to operate from a supply voltage which is lower than is desirable to use with a varicap diode. The circuit is a modified Colpitt's oscillator. The oscillator comprises a pair of transistors whose base and collector electrodes are connected together, the base electrodes being connected to a supply rail (+Vcc) and the collector electrodes are connected to a tunable parallel LC resonant circuit. A frequency determining device e.g. a crystal is connected between a tapping of the resonant circuit and the emitter of one of the transistors to form a first feedback path. The crystal together with an integratable small value capacitor connected between the emitters of the transistors form a second feedback path. The phase shifts due to the first and second feedback paths are summed vectorially at the collectors of the two transistors. By varying the gain of each of the two transistors then the resultant phase shift, and thereby the frequency of oscillation, are varied.