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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.
Circuit diagram of a ringoscillator. The output buffer
is added to decouple the oscillator from parasitic capacitances.
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.
© Bart J. Van Zeghbroeck, 1998