Posted on Feb 5, 2014

An oscillator is a circuit that can generate a frequency source such as a sine wave, square wave or pulse train. An oscillator is fundamentally a combination of a frequency-sensitive circuit such as an LC circuit or a crystal and a negative resistance usually obtained by using an amplifier with positive feedback. Phase shift is the displacement of

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a waveform in time. For example, if a waveform is displaced by a complete wavelength. it is described as having a phase-shift of 360 °. If it is displaced by half a wavelength (i. e. 180 °) one wave will peak where the other is in a trough state and complete cancellation will result. If they are at any other angle of phase-shift, partial cancellation will result. A phase shift oscillator is a simple sine wave electronic oscillator. It contains an inverting amplifier, and a feedback filter which `shifts` the phase by 180 degrees at the oscillation frequency. The most common way of achieving this kind of filter is using 3 cascaded resistor-capacitor filters, at the oscillation frequency each filter produces a phase shift of 60 degrees and the whole filter circuit produces a phase shift of 180 degrees. Thus the total phase shift produced by the three RC networks is 180 °. Therefore at the specific frequency fo the total phase shift from the base of the transistor around the circuit and back to the base is 360 ° The mathematics for calculating the oscillation frequency and oscillation criteria for this circuit are surprisingly complex, due to each R-C stage loading the previous ones. The calculations are greatly simplified by setting all the resistors (except the -ve feedback resistor) and all the capacitors to the same values. In the diagram, if R1 = R2 = R3 = R, and C1 = C2 = C3 = C, then: The Hartley oscillator was invented...

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