Bredhurst Receiving and Transmitting Society

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For an oscillation to be generated, there has to be an amplifier with a loop gain in excess of 1.

This means that the random movement of electrons at the input of the amplifier is amplified, and then fed back to the input, so that the fed back signal is then even bigger, and what reaches the input again is bigger than the original random movement of electrons.

The tuned circuit is arranged such that the open loop gain of the amplifier exceeds 1 at resonance. Due to the tuned circuit being "lossy" when "off resonance", not enough is fed back. "On" resonance, little is lost, and enough gets back. The transistor amplifies this over and over until it is saturated. (i.e. fully turned on). At this point, the transistor cannot amplify and the signal is lost. No output signal = no input signal, so it all collapses. Because of the tuned circuit, what would otherwise be a square wave becomes a sine, because the tuned circuit filters out (as it is "lossy at the wrong frequency) all the harmonics. (A square wave is really a sine, with all the harmonics added to it).

So... Transistor amplifies over and over until saturated, no more output = no more input, none fed back, all collapses, then starts again. This gives a square wave and keeps "poking" energy into the tuned circuit, which removes the harmonics.

Think of a pendulum. You keep giving it a "poke" to keep it going, but it stores energy, and swings smoothly when you are not "poking" it.

Important point re oscillators. They need buffer amplifiers after them, to isolate them from the load. Otherwise, changes in load will cause changes in frequency output. They also need stabilised supplies, and a mechanically rigid construction. Temperature compensation would also be common for a transmitter VFO. (A capacitor with a temperature co-efficient would be used as part of the tuned circuit. When the circuit warms up, the oscillator could drift, but the compensating capacitor alters it's value as well, and is connected such to retune the oscillator to where it ought to be.