9.4 Ringoscillator

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A ringoscillator consists of an odd number of logic invertors which are connected in series as the output of one invertor is connected to the input of the next invertor. The output of the last invertor is connected to the input of the first one, thereby forming a closed loop. The number of invertors in the loop needs to be odd so that the circuits is unstable and oscillates. An even number of invertors in the loop would cause the circuit to be bistable. The circuit diagram is shown in the figure below.

ringosc.gif

Circuit diagram of a ringoscillator. The output buffer is added to decouple the oscillator from parasitic capacitances.

The ringoscillator oscillates by itself and the oscillation frequency equals half the inverse of the logic gate delay divided by the number of invertors in the loop.

The ringoscillator is not a useful logic circuit but rather an analysis tool. It provides an easy way to measure the invertor delay, the power dissipation and the power delay product of an invertor.