Low noise frequency modulator using capacitance diode - is fitted with crystal controlled carrier frequency oscillator with transistor connected to crystal

摘要

A low noise transistor based frequency modulator uses a crystal controlled oscillator and capacitance diode modulating element which have good linearity and noise characteristics. The circuit is suitable for multi-channel communicaton systems and represents a development of the circuits. The basic oscillator circuit consists of a transistor (TS) connected in common base mode in series with a crystal (Q) in the emitter and tunable inductance (U) and capacitance (C1) in the collector. Feedback from collector to emitter is through a capacitance diode (D), the direct voltage operating conditions of which are determined by a potentiometer (R1) which can be set to give the most linear available circuit characteristic. The low impedance of the common emitter mode of the transistor gives rise to the advantage attained by this circuit configuration.