27mhz transmitter circuit

Posted on Feb 6, 2014

This is a design circuit for a simple 27MHz transmitter producing a carrier. The circuit can produces an unmodulated 27MHz signal and when picked up by a receiver. This is the figure of the circuit; The transmitter is a very simple crystal oscillator. The heart of the circuit is the tuned circuit consisting of the primary of the transformer and a

27mhz transmitter circuit
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10p capacitor. These two components oscillate when a voltage is applied to them. The frequency is adjusted by a ferrite slug in the centre of the coil until it is exactly the same as the crystal. The crystal will then maintain the frequency over a wide range of temperature and supply voltage fluctuations. The transistor is configured as a common emitter amplifier. It has a resistor on the emitter for biasing purposes but the 82p across the 390R effectively takes the emitter to the negative rail as far as the signal is concerned. The 390R resistor prevents a high current passing through the transistor as the resistance of the transformer is very low. The tuned circuit operates at exactly the third harmonic (also called the third overtone - an overtone is a multiple of a fundamental frequency) of the crystal so that the crystal will oscillate at its third overtone (27MHz) and in-turn, keep the frequency of the circuit stable. The transformer in the collector of the transistor performs two functions. 1. It matches the impedance of the transistor to the impedance of the antenna, and 2. Creates a resonant circuit at 27MHz to make sure the crystal oscillates at this frequency.

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