Crystal oscillator and transmitter operating at the third harmonic of the fifth overtone of a crystal

摘要

An improved short distance, battery powered, amplitude modulated RF transmitter for transmitting encoded security data. An oscillator stage provides a control crystal and filter which are feedback coupled between the emitter and base of a transistor to oscillate at the fifth overtone of the fundamental crystal frequency. A filter tuned to the third harmonic of the overtone frequency selects the carrier frequency which is coupled via a resistive attenuator to an amplifier/filter stage, before the carrier is amplitude modulated with encoded security data. The improved transmitter circuit compensates for crystals exhibiting variations in series resistance and provides increased output power and improved frequency stability to temperature and over time. A foil antenna stage is augmented with the plates of the battery power supply to further improve performance.